Corrugated iron roof pitch wrong

What is the minimum pitch for corrugated iron roofing?

The most common disregard for engineering logic is the rule of minimum pitch for corrugated iron roofing.

If you have a metal roof on your house – there is a good chance that it is made of corrugated iron roofing. The corrugated profile dates back to 1820 and it is the most used metal roof profile in the world…

It is no wonder that most house builders use corrugated roof sheeting when there is a metal roof to be installed. Here is a horror story about attempts to repair leaks on a flat corrugated roof.

Corrugated roofing is a great material on traditional pitched roofs – but it has severe limitations when the pitch gets below 5 degrees. That is why all roofing manufacturers recommend the 5 degree pitch as the minimum slope and they will not provide any guarantees if it is installed flatter than this.

There are two main reasons why corrugated roof sheeting and low pitches do not mix:

  1. During heavy rain, the valleys of the corrugations fill up to the height of the crest or even above that. This means that at the side laps of every sheet, water overflows under the lap. And fills up the gap between adjacent roof sheets. Then it trickles over the edge of the roof sheet into the roof cavity.
  2. At the upstream end of the corrugated iron roof sheeting, the valleys are turned up to prevent water flowing backwards and running off the upstream end. During heavy rain (or if there is wind combined with rain), water fills up the valley and overflows the turn up. Fairly heavy leaking can result from this.

Flat skillion roofs are the most common way to increase the size of a traditional bungalow house and many builders and roofers simply slapped on some ‘corrugated iron’ for the roof. This is the cause of most of the roof leakage problems with skillion metal roofs.

Modern architecture has birthed the demand for low profile roofs and builders and roofers still have a problem in this modern era of simply not understanding the limitations of corrugated roofing. I see countless examples of corrugated roofing on brand new low pitched roofs. These are water leakage time bombs.
… and many roofers think they can fix these roofs with a bit of roofers silicone – until they find that roofers silicone is not always the way to do a roof repair properly.

Roof sheeting manufacturers have long since overcome the limitations of low pitched roofs by inventing special profiles like “Lysaght Trimdek”, ‘Lysaght Kliplok’, and other profiles. But people still cannot grasp that when the pitch of the roof starts getting low, the look of the roof profile is less important than the function…

The problems with corrugated roof sheeting and low pitched roofs must be the number one metal roof problem because of all my videos on my Youtube, the one below is the most viewed….

Corrugated roofs are not the only type of roof that suffer from leakage due to being installed at low pitches. The ‘trimdek’ and ‘Kliplok’ profiles can also suffer problems. For more information about problems that these types of roofs are prone to, start your roof repairs in Sydney journey at our HOME page.


Answer to a smart question: “As a roof gets longer, does the slope need to increase?”

The answer is: “YES. It has to do with how much water the roof is carrying. A longer roof had more water at the downstream edge. Also, what is not usually taken into account is the average rainfall for the area the roof has been installed.

Therefore, in dry areas, you can have a much longer corrugated roof that can be laid at 5 degrees.
If your roof is in Bathurst, NSW, you can have a 25m long roof at 5 degrees.
But if you live in Coffs Harbour, a corrugated roof at 5 degrees cannot be more than 10 metres long.”

If you are interested in other roof profiles and maximum lengths, look at the picture below (courtesy of Stramit book of Answers).
minimum roof pitch and rainfall intensity
Length of roof against roof profile and rainfall intensity

90 responses to “Corrugated iron roofing and roof pitch problems”

  1. Lee Dugald says:

    I would be very interested to find another option for an
    extension I am planning as there is a legal covenant on my house that does not permit the use or corrugated iron…
    Does anyone know if ‘corrugated’ would include trimdek or Kliplok? Is there any other options available other that iron for a 5 degree pitch?

  2. Jack Yuen says:

    Hi Lee,
    The first step is to question why the covenant does not permit ‘corrugated iron’. Once you know the basis for the convenant, you can start investigating the options.


  3. Andrew says:

    Hi Jack
    I am writing a report for an insurance about a low pitch roof, under 5 degrees. I have looked in the BCA and Australian Standards but can’t find the magical number eg the BCA states that …….Is it only the recommendations of the roofing manufacturers or do you know what the number might be eg AS 123.
    Thanks Andrew

  4. Jack Yuen says:

    Hi Andrew,

    Roofing manufacturers design and manufacture roof sheeting to suit various uses.
    Therefore the ‘terms of use’ will originate back to the manufacturer. Not The BCA of The Standards Association.

    All the roof sheeting manufacturers nominate a minimum pitch that their various roof profiles are suitable for. It will be in their product brochures.

    The most common evil (performed by so called roofers) is the use of corrugated roofing (all the manufacturers nominate a 5 degree minimum pitch for this profile) – whenever and wherever a metal roof is called for.
    I have a closet full of stories about flat corrugated roofs that leak….


  5. Matt says:

    Hi Jack,
    I have an existing ultra low (0.5 degree) pitched cliplok roof that is about 20years old and has never leaked as yet and am looking to replace it, however I’m unable to increase the pitch on the roof when replacing to 1degree (as required for trimdeck)due to other building constraints ?? I’m in Sydney, Any ideas ?

  6. Jack Yuen says:

    Hi Matt,

    If you stick with Kliplok or equivalent profiles (thicker 0.48BMT)… you should be able to get by with 0.5 deg. But make sure that the turn-ups are done correctly also.


  7. Will says:

    Hi Jack,
    I have experienced major leaks in our new house (< 5 years old).
    It has a low profile corrugated roof in certain sections.
    The builder has been resisting helping with the repairs.
    We have got the insurance company involved and when they recently sent the assessor to our house, he almost immediately commented that the pitch of the roof was too low!
    When he measured the pitch it was 2.2 degrees, way less than the minimum 5 degrees you mentioned.
    The assessor told us this would be considered a building defect and to try and negotiate with the builder to repair.
    We went back to the builder and they said that building standards always change. They were claiming that with long stretches of roofing low pitches are ok.

    Can you please help clarify what is the minimum pitch for corrugated roofing, regardless of the length?

    We are extremely frustrated with all of this and are now considering taking it to fair trading to assist.

    What do you think?

  8. Jack Yuen says:

    Hi Will,

    The 5 degree pitch for corrugated roofs is not something that I made up. It is something that the roofing manufacturer’s have specified because they know the limitations of their products.
    The best document that I have come across is the one put out by Stramit. You will see that the longer the sheets, the more pitch is required (opposite to your builder’s claim). The minimum is 5 degrees – even for very short sheets.

    The link to this ‘roof slope design’ document is:

    Hope this helps.


  9. the legend 27 says:

    what is the maximum pitch of the corrugated iron roof

  10. Jack Yuen says:

    That is a strange question.
    You can install corrugated roofing vertically – which means that it becomes wall cladding……

  11. Roger Turner says:

    I am planning to knock down and rebuild my house. My neighbor has a covenant over the roof height of my house. The covenant has different height restrictions for “flat” and “pitched” roofs. Is there some sort of legal definition of what is a flat and a pitched roof, for example in the Building Code of Australia or legal ruling precedents. Some online information indicates flat roofs are less that 1.5 degrees slope and other info says less than 10 degrees. Some info says that a skillion roof (which normally would have significant slope and I would call a pitched roof) is a form of flat roof.

  12. Jack Yuen says:

    Hi Roger,
    I would define a ‘pitched roof’ as a hipped roof (google it). It would have ridge capping on the ridges.

    ‘Flat’ roofs have only one plane – with one angle. There are often no ridges on a ‘flat’ roof (but there are always exceptions).
    Skillion roofs are typical flat roofs.

    A lot of modern designs now have flat roofs throughout. (google flat roof designs – and look at the images).

    Hope this helps,


  13. Fel says:

    Hi jack ,
    Appreciate your help .

    Our insurance company is saying that ‘The pitch of the Trimdek roof is 2degrees which does not comply with code’ , however when I looked up the trimdeck specification it stated 2 is the minimum …
    Could the insurance company be wrong ? Appreciate your direction to the correct information.

    Also if my roofer did not install flasings , is that against the building regulations ?

    Thanks for your help

  14. Jack Yuen says:

    Hi Fei,
    The suitability of different roof profiles for minimum pitches is based on manufacturer’s recommendations. It is not a ‘CODE’ requirement.
    Bluescope Lysaght manufactures the ‘trimdek’ profile roof and their minimum pitch recommmendation for that profile is 2 degrees.
    So the insurance company is incorrect in their statement.
    Any roof should be flashed correctly. If the flashings are not done correctly, then the roof is still ‘unfinished’ – and if there are leaks, then it should be expected.
    Roofs that are installed to manufacturer’s recommendations and finished according to their recommendations will not leak.

    Hope this helps.


  15. Fel says:

    Thanks so much for your quick reply , it is very helpful . Will be sure to reach out for a quote if we need your assistance with fixing the issues

  16. Mel says:

    I have a query about battens. Our insurance company says our battens don’t meet Australian standards as the first batten down from the ridge capping is 1430mm and should be 900mm. Bottom batten spacing is 1200mm and supposedly should be 900mm. Somewhere in the paperwork, I see a reference to not suitable spacing ‘for this material’ referring to the wrongly used corrogated sheeting. The house is in south- east qld and the actual issue is the roofing the insurance company did after the floods. The roof pitch is 3 degrees. Anyway, I’m just trying to gain some insight into whether Trimdek and Kilplok will also have these batten standards?

  17. Jack Yuen says:

    Hi Mel,

    Roof sheeting manufacturers do not recommend installing corrugated roofing less than at a 5 degree pitch.
    They also have a recommended maximum span between batten supports for the different roof profiles and the two thicknesses of roofing material.
    If you want all the details, check out the pdf from Lysaght below:

    Hope this helps.


  18. Mel says:

    Thanks Jack. I looked at the link you referred to, and just have a question about page 9, trimdek .42. In the middle column under roofs, it says single end internal 1100 1300 1900. Trimdek .48 references1600 1850 2600. Are these figures the maximum batten spacings?

  19. Jack Yuen says:

    Hi Mel,

    Yes, the numbers are the maximum recommended batten spacings.


  20. Mel says:

    Thanks so much Jack!

  21. Mark Chapman says:

    Hi Jack
    We have a steep pitch corrugated roof. We recently had a roof added on over an entertaining area with shallow pitch. No surprises it leaks at the interface between the two. The builder was a friend (bad move) and he seems to be unable to fix it.
    Are there any solutions to this sort of problem? We live in Wollongong.

  22. Jack Yuen says:

    Hi Mark,

    If the shallow pitch roof has been laid with corrugated roofing – then you have the worst case. It means removing the corrugated roof with something like ‘kliplok’ roof sheeting which is designed for low pitch roofs.

    If your roof has been laid using the ‘trimdek’ or the ‘kliplok’ profile roofing – and it is still leaking…. then it just has not been installed correctly.
    Any good roofer in Wollongong should be able to rectify this.

    All the best.


  23. Charmian says:

    We have 20 solar panels on the corrugated roof which the insurance say is only 3 to 4 degrees pitch. Although the water in a storm came from the gutter end in my office and there were no internal leaks from the top end of the roof (7m into the house layout) which abuts a large heritage slate roof, the insurance assessor claimed the flashing seal near the slate caused problems too and water would be driven into that end too. Needless to say the assessor rejected any building claim saying it was maintenance and wrong building angle of the corrugated roof. Ie we need to install a whole new roof and relay the solar panels.
    I wonder if a strip of Kliplok roof could be placed between the slate roof and where the solar panels start – about 1m – placed on top of the existing corrugated roof and well sealed and flashed to remove the necessity of doing any movement of our recently installed solar panels

  24. Jack Yuen says:

    Hi Charmian,

    Firstly, if the leaks came in near the gutter section of the roof and no where else – then, the assessor is wrong about the leak originating from the joint between the slate and corrugated roofs. There is no need to consider the strip of Kliplok.

    Secondly, to completely satisfy the insurance company, your roof will need to be replaced with a suitable (eg. Kliplok) profile roof.

    Thirdly, if you want to take the insurance company out of the equation and just look at stopping the leak – then I suspect that the origin of the leak is at the laps of the corrugated roof near the gutter. When the corrugated roof is laid ‘flat’, one of the common leak paths is water travelling backwards along the lap and leaking into the house.
    There is a trick to overcome this – buy trimming the underlap shorter that the overlap where the roof sheet is discharging into the gutter. Any good roofer can give this a try for you.

    Hope this helps.


  25. Noelene says:

    Hi Jack
    I have just had my toof reroofed with kliplock on a.3 degree slope 16 × 10 long and an extension and gazebo with colorbond sheeting my roofer says he has finished the job but the joins are full of silicone whete metal gaps are and the colorbond sheets are scatched and some have already surface rust. On Gazebo corners dont meet up and sheets not secured appropriately 60cm from beam. Also whete they initially took the old sheets of the roof and left th their until a crane arrived they had render attached to it the results are there’s little bubbles of rust starting where this material was stored.
    Can you point me into the right direction where I can get some legal or professional advice on how to handle my problem and to deal with my roofer
    What authorities do I nred to go through
    I presume with what they ha e done I will have no warranty on my roof.
    This roof was not covered by insurance I had to pay insurance will not take responsibility for the any roofer’s work they told me this prior to job starting.

    Noelene Pensioner

  26. Jack Yuen says:

    Hi Noelene,

    Sorry to hear of your experiences.
    In NSW, we have ‘NSW Fair Trading’. You can contact them and they will guide you through a claims process.
    Other states should have similar departments who look after consumers.
    All the best.


  27. Shane Chaffey says:

    Hi Jack,
    I have just had some minor hail damage to clear sheeting on our back deck roof, put an insurance claim in offered payout as they said roof wrong pitch for corrugated iron. Fair enough but roof has been replaced twice before in 96 & 06 because of hail damage and nothing was said then why the change same insurance company different company doing quote as an aside how do I replace the decking roof with trim deck where it meets the roof correct pitch. Thank for your help

  28. Jack Yuen says:

    Hi Shane,
    When you replace the corrugated profile roof sheeting with the Trimdek “5-rib profile’ polycarbonate sheets, simply insert a ‘transition flashing’ between the new polycarbonate sheets and the existing (I presume corrugated) sheets for the rest of your roof.

    Hope this makes sense.


  29. shane says:

    Thanks Jack it does

  30. shane chaffey says:

    Hi Jack
    In reference to above NRMA offered a payout as building company would not offer under warranty as roof under pitched can this mean that in the future if we have another hailstorm, could NRMA come back and say we will not fix your roof as it is under pitched and your at fault and have standards changed in the past ten years as they have claimed they could have. cheers in advance

  31. Jack Yuen says:

    Hi Shane,

    There are two issues here.
    1) If the new polycarbonate roof gets hit by hail in the future, NRMA should not have a reason to exclude another claim – because the roof has been damaged by a storm. It would not matter even if you put another corrugated polycarbonate roof up (and it does not comply with the 5 degree requirement).
    I think that the insurance policy will cover any storm damage – whether it is built correctly or not.

    2) If the new polycarbonate roof leaks – then NRMA can say that it is not their responsibility. It is up to the owner to install building elements to an acceptable standard (nothing much has changed here in the past 10 years).

    Hope this makes sense.


  32. shane chaffey says:

    yes it does thanks but I worry also about what happens in relation to the rest of the roof which is corrugated and been replaced twice (am doing away with the two sections of poly carbonate)and under pitched if another hailstorm hits and NRMA has same assessor oh well suppose time will tel

  33. Blake says:

    Hi Jack
    I am having a similar issue regarding the Trimdek 2 degrees minimum slope not complying with the NCC (BCA).
    As you state the roofing manufacturers provide minimum roof pitches of 2 degrees (Lysaght – Trimdek and Stramit – Monoclad), even with long run lengths. I do not see any ambiguity in their documentation regarding the minimum roof pitch or any other limitations of their recommendations.
    However, the NCC 2016 Volume 2 states a minimum pitch of 3 degrees. This can be found in Figure Maximum span and fixing for metal sheet roofing. Diagram a. lists trapezoidal sheeting as having a minimum pitch – 3 degrees.
    Are you able to provide any comments regarding with difference between the code (NCC) and the manufacturers?

  34. Jack Yuen says:

    Hi Blake,
    It is a hard life writing a national building code…
    So many things to consider and so many things changing over time.

    My take on the whole thing is that the manufacturer of a product knows the most about their product.
    A code writer attempts to do a compromise with all the information available – and is always out of date…

    But you will notice that the code writers have an ‘out’.
    The code requirements are often just “Acceptable Construction Practice” ….and there are also “acceptable construction manuals” also.

    So, strictly by the book: (b) Metal roofing: AS 1562.1. (is an acceptable construction manual).

    If you go to the Lysaght “roofing and walling manual” (link below)… you will see that their roofing complies with this Australian standard.
    Therefore, their roof pitch recommendations are also deemed to comply…

    So, Trimdek at 2 degrees is OK.

    And this is a personal thing.
    I hate Trimdek at 2 degrees. It often leaks at the lap.
    I am more comfortable with 3 degrees or greater for this profile.
    (I am not saying that I know more than the manufacturer… just that I see more problems in the field than them)

    Hope this helps.


  35. Blake says:

    Thanks very much for your comments Jack and the very prompt response.

  36. Ash says:

    Hi Jack

    We are planning an extension at the rear of our house, however our title has a covenant requiring the roof on the dwelling needs to be tiled/slated. As a result we cannot go for an iron roof option.

    Our draftsman has drawn up a 5 deg pitch on the roof, and has told us there are limited tiles available in the market for a 5 deg pitch and that it was likely going to cost us a lot. He noted we may be better off creating a standard pitched roof rather than 5 deg, and using standard roof tiles.

    Can you recommend any tile/slate products we can look into for the 5 deg pitched roof, and do you know how expensive the products are likely to be compared to standard roof tiles? Any other suggestions/advice would be very helpful and greatly appreciated.


  37. Jack Yuen says:

    Hi Ash,

    I would not recommend any roof tiles or slates at a pitch of less than 20 degrees. If you are stuck with tiles, then you are best to design your extension with a 25 degree pitch.


  38. Hi Jack
    Great website and well considered responses to the questions under your article. Nice work. A quick search on under-pitched corrugated roofs (I am reporting on a roof in North Queensland) led me to your site.
    Best Regards
    Matt Cornell

  39. christine blamey says:

    We currently are designing our home.We have a height restriction so our roof will have a pitch of 3 degrees.We are hoping to run the roofing down the sides of two walls.Our builder is talking about the need to use specific profiles due to its pitch.We live in a coastal country town far south of Perth and experience long wet and windy winters. What would you suggest we consider.
    Thank you

  40. Jack Yuen says:

    Hi Christine,

    You do have a wide range of choices.
    There are quite a few profiles of roofing that will function well on a 3 degree pitch.
    And now you have a wider choice if you are considering walls also. The ‘matt’ finish colorbond that has been released is well suited for wall cladding – it is a bit more expensive though.
    Do a search on colorbond steel – matt colours and you will see examples.

    Your builder will guide you through the range of products and the relative costs…

    All the best.


  41. Terry says:

    Can you please tell me when the building code changed to a requirement of a 5 degree pitch on bullnose verandah roof.
    My house is 28 years old and has a verandah pitch of 3 degrees and the NRMA repairer will not warranty the job after a severe hailstorm.
    Thank you.

  42. Jack Yuen says:

    Hi Terry,

    I don’t think there has ever been a change from or to the 5 degree requirement for corrugated roofs (like corrugated bullnose roofs).
    It has always been 5 degrees.


  43. craig bennett says:

    Hi Jack,

    I’ve read most of these comments and you are dead on with your replies and advice. Some sad stories of contractors not doing it right.

    Im a builder myself, have recently moved to the southern highlands, and am considering a bonnet roof, i quite like the look of the dual pitch, and being a chippy also, am willing to get the carpentry right.

    What id cant find online yet is what pitches are used for a bonnet roof..

    By eye, it looks like a 5-10 degree pitch for the lower section, and approx a 40 degree pitch for the upper…

    Have you come across many ? and do you remember what pitches were used ??

    Any help much appreciated

    Also, do you travel to the highlands for any of your jobs ? if so, I’ll keep you in mind for a price..

    And i’m happy to pay you for your advice and help this side of a sub-contract being agreed on

  44. Jack Yuen says:

    Hi Craig,
    Sorry, I do not travel to the highlands. It takes too long in Sydney traffic!
    I have not come across ‘bonnet’ roofs.
    But the basic roofing principles still apply. Do not use corrugated less than 5 degrees. You can mix and match profiles on different pitches by putting a transition flashing between the two.
    Hope this helps.


  45. Wesley says:

    Hi Jack,

    I have a rubbed low pitch roof that is exhibiting the upstream flow leakage you showed in your video. I have managed to overcome this. However at the run off end the water is travelling underneath and back up under. I have tried turning the lip down, however the ribbed design does not really allow it. It doesn’t bend too much until the metal rips and looks unsightly. FYI there is no gutter.



  46. Jack Yuen says:

    Hi Wes,
    You can try a drop of roofing silicone (applied to the underside of the valley) just where the end of the roof sheet is. This will ‘suck’ the ponding water onto the hanging silicone and drop it into the gutter.
    Alternatively, I have seen a small angle 20mm x 20mm attached to the underside of the edge of the roof sheet. This grabs the water and sends it dripping…

    Hope this helps.


  47. Max says:

    Hi Jack,

    Do you know if the Lysaght Custom Orb Accent 21 and Accent 35 can be installed at a pitch below 5 degrees?

    The Lysaght website states that they can be, but this is not consistent with Section of the NCC, which states that corrugated profile roofing must have a minimum pitch of 5 degrees, as you’ve indicated above.



  48. Jack Yuen says:

    Hi Max,

    The Accent series allow pitches to go to 3 or 2 degrees. This is because the corrugations are bigger (the standard corrugated shape only have a 16mm corrugation) and this allows for a larger ‘turn-up’ and the corrugations carry more water before it can flood the laps.
    NCC is usually behind the times because it is not quick enough to catch up with the latest products that roofing manufacturers develop.
    The Lysaght website recommendations for minimum pitch for their own ‘accent’ products are correct.

    Hope this helps.


  49. Janelle Kendrick says:

    Hi, I am currently having issues with my roof that is corrugated iron pitched roof with many valleys but a twenty year old extension which is on plans 5 degrees but the roofer says it is 2.5 degrees . Can you mix or for a better word join or slide under the differ profiles such as trim deck and can the Accent 21 or 35 be used and can it be used all over and is it a colourbond product. Thanks Janelle

  50. Jack Yuen says:

    Hi Janelle,

    A low pitched corrugated roof with ‘many valleys’ is a problem.
    The major issue is the low pitched valleys – more that the roof itself.
    In my experience, even low pitched kliplok roof with valleys are prone to leakage along the valleys.
    I would not contemplate trying to modify the existing roof.
    Find a good roofer to come up with some smart solutions to your particular problem.


  51. Michelle Kenny says:

    Hi Jack
    I have a pitched corrugated roof workers cottage with an insulated corrugated covered deck. The deck is pitched at 2.5 degrees. I have not had any problems with this product at this pitch since it was installed 20 years ago. I think the product was fairly new to market at the time. My problem is my insurer is replacing my roof due to hail damage. They have replaced the pitched roof and now the deck roof has arrived it is the wrong profile. I want it corrugated to match. They refuse or discuss. I have halted works. Any thoughts on a product in the insulated range which would work?


  52. Jack Yuen says:

    Hi Michelle,
    I don’t know of corrugated insulated product that would work on a 2.5 degree roof.
    I would recommend using the recommended ‘trimdek’ profile for your flat roof.


  53. Jess says:

    Hi Jack,

    We’re working on a new build at the moment and have some questions. The house will have 5° verandahs wrapping around much of the house, and colorbond custom orb was specified in the design which meets the manufacturers recommended 5° minimum pitch. Most of the verandah drops down about one cladding width from the main roofline, but a section at the front transitions from the main 28° roofline to the 5° verandah roofline without cladding between. The verandah also has a gabled section over the front door (so there’s a couple of valleys).

    Anyway, the roofer threw a spanner in the works today and suggested that we install trimdek for all the verandahs to avoid issues with leaks down the track as he doesn’t think custom orb will be good enough at 5° despite the manufacturers recommendation. As this is the front of our house and really visible from the street as we’re on a slope we’re really concerned about how it’ll look the have mixed roof profiles, and it seems unnecessary since custom orb is okay to 5°. What do you think? We’re thinking maybe a third option which would give us some peace of mind but also avoid the mixed profile look would be to use custom orb 21 for the whole roof as this is recommended to 3°. Would love your opinion on these options.


  54. Jack Yuen says:

    Hi Jess,
    What is missing from your description is … where the gutters are?
    The water carrying capacity of custom orb corrugated sheets depends on the slope of the roof and the likely volume of water running on the roof.
    If the main roof have their own gutters, then the verandah roof just has to carry the direct rainfall that falls onto it. 5 degrees is good to handle this situation.
    At the front of the house, I assume that the main roof discharges directly onto the verandah roof. This will introduce additional water load. Depending on the amount of this additional water load, 5 degrees may still work.
    You just need a wider transition flashing between the two roofs to ensure that the turn-ups on the verandah roof are not flooded.
    The valleys on the gable could be a problem if too much of the main roof water is to be run into them. If the valleys just carry the verandah roof water, then the valleys will work.
    The potential problem with the valleys do not go away even if ‘trimdek’ is used.

    I have not had to use custom orb 21 yet. It has higher pricing and longer lead times. It is an option if you get stuck…

    Hope this helps,


  55. Ian says:

    Hello Jack

    Thanks for the very useful article and replies to comments

    Do you have any advice re profile and pitch for roofing under a deck?

    I have a deck that’s about 7.2m x 4.5m with 25% protected by a roof and the other 75% open to Brisbane weather

    I’m replacing corrugated iron under the deck that protects a deck below – it definitely didn’t have enough battens and I calculate the pitch as about 1:50 (1°) which would be much too flat for an open roof

    I’ve seen 1/4” per foot pitch (1:48) recommended in US-based YouTube videos that seem otherwise competent

    Car parking under the lower deck is protected by TrimDek which also is at about 1:50 pitch

    Would TrimDek be a better option under the top deck, rather than corro?

    Maybe increasing pitch to 1:30 (2°) regardless?

    Thanks for any advice you can offer

  56. Jack Yuen says:

    Hi Ian,

    In my experience, you can use ‘kliplok’ profile roof sheets for a 1 degree roof.
    Stay away from corrugated or trimdek roof sheets – they can leak.
    In Sydney, we experience condensation on the underside of low pitch roof sheets and water drips. Anti-condensation blanket and a ceiling will solve this problem.
    You may have less condensation problems in warmer Brisbane.

    Hope this helps,


  57. bobbi mcminn says:

    Hi jack –

    We are building a granny flat currently, it has a skillion roof- 5 degree’s, plans are drawn for use of standard custom orb. Our builder has recommended we change to trimdek for the reasons you have described above .

    We live in Alice Springs – rainfall is minimal but, then again rain happens!!

    At the risk of sounding superficial, we would like to stick with custom orb-
    I know I could get a deeper profile at an increase cost.

    In your experience have you seen leekage problems with roofs pitched at 5 degree? If we followed through with the use of custom orb on the roof and something was to go pear shaped with water leakage – given that the pitch is within the parameters defined by the manufacturer, who is accountable and what processes need to occur?

    Thank you

  58. Jack Yuen says:

    Hi Bobbi,

    If it is not a huge roof, then 5 degrees on a standard custom orb roof is acceptable and should not leak. The key is getting a good installer to make sure that the sheets are turned up properly and there is no insulation trapped between the laps.
    If this is done, it will be in good shape rather than go pear shaped.

    Hope this helps.


  59. Ann says:

    Hi Jack
    As of June 2020 is there still no BCA code/requirement re the minimum pitch for Trimdek?
    The manufacturer’s recommendation still stands at 2 degrees (Lysaght website still states that as of 19/6/20)
    I have just had an insurance assessor say the existing slope of 3 degrees on my trimdek garage roof does not comply with current standards?
    Appreciate your update on the situation.
    Many thanks

  60. Jack Yuen says:

    Hi Ann,

    The BCA does not often rule against manufacturer’s recommendations.
    In this case, the manufacturer’s recommendations still apply.
    ‘Trimdek’ is good for a 2 degree roof.
    Although I don’t like using it for this minimum pitch without some precautions.

    I have found in my experience that a lot of insurance assessors have no idea of what they are talking about when it comes to what cause roof leaks.


  61. Ann says:

    Appreciate your timely reply Jack.
    Kind regards

  62. Ranji Thomas says:

    Hi Jack

    I am planning on building home with an adjoined granny. The wall height of the main home is 2700mm. I would like to keep the granny height at 2400mm.

    The builders suggest the best solution is to have the granny and the main home at 2700mm with one single roof. Given the additional cost I would like to keep the granny at 2400mm itself. The granny would be 5.5m wide and run along the main home at 11m length.

    I proposed if we can have the granny with a separate metal roof at a 5 degree pitch, however builder states there is insufficient space for the roof angle, and possibly needs a 3000mm+ roof on the main home.

    Would you have any other alternative solution to this roofing problem?

  63. Jack Yuen says:

    Hi Ranji,

    I can’t quite picture what you are trying to do.
    If the builder is any good, he will have all the options.


  64. Jason Tranter says:

    Thanks for the information you provide!

    I want to build a simple 4m x 8m structure with Bondor Insulroof as the roof for an off-grid, off the radar project. They recommend 5 degrees, or 2 degrees for their Solarspan product. (Insulroof has a sandwich of polystyrene between the corrugated iron and a metal finish which becomes the ceiling.)

    Because of the challenge of creating a skillion roof on the simple steel structure, I would like to lay the roof flat. They will be single sheets of iron but in 1 m widths where they clip together. I had ‘hoped’ that it would cope with heavy rain.

    When I asked them about the 5 degree requirement they cited building codes, the weight of the roof in heavy rain, and what might happen with heavy rain and high wind. I would have thought that the rain would pour off the ends of the roof even if it were flat, and that heavy rain/wind would still be a problem with a 5 degree fall..

    Am I crazy to consider a flat roof?

  65. Jack Yuen says:

    Hi Jason,
    If you want a leaking roof – then install the corrugated insulroof flat.
    There are reasons that corrugated roofs need to be laid at a minimum of 5 degrees.
    …Mainly leaking at the side laps and at the upstream end.
    Stay with the manufacturer’s recommendations and you will be safe.


  66. Russ says:

    Hi Jack,
    My shed has a corrugated roof with only 3.43 degree pitch over 5 metres. I’m not having any problems at the turned up upstream end but water is entering at the downstream end and wicking through the insulation blanket. I have no roof lining as yet! There is 2 obvious problems, one is the incorrect pitch and one is the overhang which ranges from 40 to 50mm. Can you suggest a possible fix short of replacing with kliplock roof?

  67. Jack Yuen says:

    Hi Russ,
    If you unscrew the sheets at the laps, you will see that the water is travelling (wicking) up the underlap. This is usually worse when the roof sheet that is lapped is slightly longer than the adjacent sheet that is on top. More wicking happens in this instance.
    Cut a large corner of the underlap away and this will stop the water wicking up the lap.
    Also, the insulation should not extend past the second half of the gutter batten. Otherwise it will suck water up from the gutter.

    Hope this helps.


  68. Russ says:

    Thankyou for replying so quickly. Should I dab a bit of silicone on the underside of the valleys as well as I noted you suggested to another question. Also is there a product you can add to the end of the corrugated to help in this instance?
    Once again Thankyou for your reply

  69. Jack Yuen says:

    Hi Russ,

    If the back flow is under the roof sheets and not just at the laps, a small continuous angle can be pop riveted to the underside of the troughs to create a drip ledge.


  70. Phil Wilding says:

    The surface tension particularly when oxidised corrigate iron draws water under the sheet and unless it has enough slope the water wil travel considerable Distance under the sheet. Thats why you need a slope to get the water to flow down hill into a gutter or bed of pebles if you dont mind the spashing. I would increase the slope to 7deg and turn up the ends of the corrogation bottoms to prevent wind driving water up there.
    I have just built a toilet out of a metal garden shed that has a flat roof with a tight channel each end touching the bottom of the corrogations and that seems to prevent normal rain getting under the roof.

  71. Phil Wilding says:

    The channel is actually a “C” Section so it only touches the top and bottom tip of the corrogations and allows water to flow in the channel.

  72. Ram Singh says:

    Hi Jack

    I have a small section of metal (custom-orb) at 4.5 degree pitch. The sarking is continued from the tile roof to end of metal roof.
    As there is no requirement for sarking to be provided under meat roof, would this still be considered a non-complying roof pitch?

  73. Jack Yuen says:

    Hi Ram,

    Compliance is a touchy issue. It depends on which side you are on.
    The roofing manufacturers ‘recommend’ 5 degrees as the minimum pitch for the corrugated profile – because they are confident that their product will perform at this recommendation.
    Other factors like the length of the roof, the rainfall intensities expected, whether there are downpipes from higher roofs etc are somehow taken into account in this recommendation.
    The requirement for ‘sarking’ mainly applies to low pitched tile roofs. Not for metal roofs.
    On a 5 degree roof, sarking is useless because it will pond water (between the battens) instead of draining to the gutter.

    I would be happy with a 4.5 degree corrugated roof.

    Hope this helps.


  74. Mark says:

    G’day Jack

    I am roofing a small verandah with corrugated tin roofing, I have got the point where the tin roofing is to meet on a right angle, what do I do to join them, a valley doesn’t seem to look right as it pushes the tin roofing to a different height.
    Thanks Mark.

  75. Jack Yuen says:

    Hi Mark,

    I assume that the verandah roof will be a ‘L’ shaped roof with a single pitch.
    It will meet at a 45 degree mitre corner – and the most common way to do this corner is to put in a valley.


  76. Helen says:

    We are getting our roof replaced through an insurance claim after a severe hail storm. Our old roof has never leaked in the 31 years since our house was built. We have a back verandah that joins on to the back of our house creating a join with a change of pitch. Our old trimdeck house roof overlapped the verandah roof then very neatly went all the way down to the verandah roof and was trimmed off to match the profile (same method used as used along the edge of ridgecapping). The builder said this method no longer met code and refused to redo the roof in the same way. Instead he has used a transition flashing that completely exposes the whole raw corrugated edge of our trimdeck house roof. It only overlaps the flashing by approximately 75 mm. We live in Central Queensland in a cyclone rated area and we receive quite a lot of rain and wind. We know that this overlap is not going to be sufficient to keep rain and moisture out of our roof space. They do not intend putting any trim or flashing over the open trimdeck. Any suggestions of how we would be able to make this watertight with what has been installed?

  77. Helen says:

    A slight correction to my description of our old roof – where the house roof overlap met the verandah roof it was not trimmed like ridge capping, it actually neatly married up because of same profile.

  78. Jack Yuen says:

    Hi Helen,
    From your descriptions, I assume that the old roof did not have a transition flashing and the new roof now has a transition flashing.
    A transition flashing is the correct way of detailing the joint.
    If the ends of the sheets are turned up properly on the verandah roof, and the transition flashing is of adequate width, then it all should work well to keep the rainwater out.


  79. Charlie says:

    Hi Jack, Do you undertake re-roofing works? The current roof aged 20 years, is colorbond corrugated, spans 10m with 200mm header flashings at the ridge, has a pitch of 4.5 degrees, and have about 12 penetrations with short lead dektiles throughout its breadth of 150m. The roofing is supported by a system of timber battens and frames on double-bricked walls. Can re-roofing with trimdek on the existing corrugated roofing work?

  80. Jack Yuen says:

    Hi Charlie,
    WE do re-roofing in Sydney only.
    Go to our Home page and reach out to us from there.


  81. alex fior says:

    Hi recommended pitch is 5 deg for corro.
    is there a point when the length of the run becomes the issue at 5 deg
    say 17m long run sheets on a mono shed at the lower end of the run profile will be struggling to discharge the water from the roof and may capillary in.
    What I’m trying to ask is is there a maximum length with corro when used at 5 deg.

  82. Jack Yuen says:

    Hi Alex,

    Good question.
    240mm/hr rainfall intensity is your answer for 17m long roof. If you live in an area that has rainfall more than this, then you have to go more than 5 degrees.
    I have uploaded a picture (courtesy of Stramit book of answers) that has a table listing rainfall intensities and lengths of roofs in different roof profiles at the bottom of this page (above the start of the comments).
    Hope this helps.

  83. KL says:

    Is there a way to calculate what the minimum slope should be for a 12 metre corrugated roof in Lismore, NSW?

  84. Jack Yuen says:

    Hi KL,

    Lismore’s rainfall intensity is 270mm/hr and the longest corrugated run of roof is 15 metres.
    Therefore a 12m roof can be installed safely at 5 degrees.
    Note that just because a 12m roof is less than the 15m on the table, does not mean that a corrugated roof can be installed less than 5 degrees.
    5 degrees is the minimum fall – no matter the length of roof.
    There are other considerations than make this 5 degree rule essential.


  85. Izzy says:

    We have a fairly low pitch corrugated roof in our patio area, which causes a lot of water to not drip off the edge into the gutter but instead track back underneath the sheets and run down the inside of the fascia and onto the deck.
    Is there a way to fox this? Should install the corrugated-shaped foam, slot flashing under the sheets or bend/kink the valleys down? Any other workarounds?
    Many thanks in advance, Izzy

  86. Jack Yuen says:

    Hi Izzy,

    I have seen some low pitched corrugated roofs with a tiny angle flashing rivetted to the underside of the roof sheets along the ends. This catches the water dribbling along the underside of the valleys and drop the drips into the gutter.
    You will need to rivet every valley to the angle.
    Hope this helps.


  87. Andrew O'Dowd says:

    Hi Jack,

    How are you able to warrant a concave or underslung custom orb roof when the minimum pitch is 5 degree as per the manufacturers recommendations, or is this a recommendation, not a requirement?

  88. Jack Yuen says:

    Hi Andrew,

    The 5 degrees is a ‘rule of thumb’.
    The pitch requirement changes according to the rainfall intensity where the roof is installed, the length of the roof, direction of wind, etc.
    A ‘sprung (curved) roof’ is better described by a radius than a pitch. The maximum radius for corrugated is 10m.
    A convex roof will have zero pitch at its peak, and therefore the laps need to be screwed and sealed until the pitch reaches 5 degrees.
    A concave roof has its critical section on the downstream portion of the roof sheet. This is where the minimum pitch requirement applies.

    Hope this all makes sense.


  89. Tonya says:


    I would like to turn the bottom edge of my corrugated sheets down to prevent water wicking up underneath and wetting my eaves, however the sheets run about 5cm over the gutter leaving very little room to get in there. What’s the best tool to do this after the gutter has been attached?


  90. Jack Yuen says:

    Hi Tonya,
    Corrugated roofing does not turn down well. The edges will tear.
    The best method I have seen around is to attach a small angle to the underside of the edge of the sheet at the gutter. This will stop the drip from travelling along the underside of the roof sheets.


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