Corrugated iron roofing and roof pitch problems

The perils of using corrugated iron roofing on low pitched roofs…

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.

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.

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….

29 Responses to "Corrugated iron roofing and roof pitch problems"
  1. 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?

    • 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.


  2. 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

    • 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….


  3. 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 ?

    • 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.


  4. 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?

    • 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.


  5. 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.

    • 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,


  6. 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

    • 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.


      • 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

  7. Hi
    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?

  8. 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?

  9. 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.

    • 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.


  10. 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

    • 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.


  11. 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

    • 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.


  12. 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

    • 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.


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