FAQ – What is a box gutter?

This topic is covered quite extensively on the Roofing professionals website.

The image gallery above shows some typical box gutters found on tile and metal roofs in Sydney:

A box gutter may look like a box because of its ‘boxy’ shape. But the main reason it is a box gutter is because it is ‘boxed in’ on all sides. The picture below shows the layout of a typical box gutter on a metal roof.

typical box gutter layout

Typically, a box gutter is ‘trapped’ between two roofs that feed rainwater into it. The water is drained via downpipe nozzles in the box gutter or via a sump built into the box gutter. A good box gutter design will have falls along the length of the box gutter, sumps to collect water before it is fed into large downpipes. There will also be an overflow facility on the side or the end of the box gutter – so that water will overflow externally to the building, rather than flood into the roof cavity.

I recommend that colorbond should always be used for a box gutter. Some severe situations may call for colorbond “ULTRA” or even colorbond stainless. Box gutters get a lot of moist leaves and debris and these can shorten its life – so the rule of thumb is to have the box gutter material as good or better (in the corrosion department) than the roofing material. Sometimes I see architectural specifications nominating a colorbond roof with a zincalume box gutter. This combination can result in much higher maintenance costs for the roof. Why?

…Because, the box gutter will need replacement a lot sooner than the roof. And a box gutter replacement is quite expensive and time consuming – because it usually involves removal of the roof sheeting.

Since the box gutter is originally installed before the roof – the roof has to be removed to remove the old box gutter and to allow the new box gutter to be inserted.

Unfortunately, with flat roofs, larger complex roofs, parapet walls and other architectural building features, rainwater does not always drain out directly to the external perimeter of the roof. In these instances, box gutters are the only viable option. So, if your roof is not ‘straight forward’ – then you probably have a box gutter somewhere on it.

It pays to have box gutters maintained and regularly cleaned out – simply because a box gutter leak has much higher consequences that the eaves gutters along the edge of your roof leaking.

 

 

35 Responses to "FAQ – What is a box gutter?"
  1. Hi
    Looking at the “typical shape” of a box gutter in your iliistration, I note that there is no flashing shown, its simply a “u” shape…is this a normal design ? and if so what stops the water from a down pour overflowing back under the roof ?

    • James,

      It is quite normal for a box gutter to be a simple U shape. There is nothing stopping overflowing back under the roof.
      That is why it is important to have box gutter overflows and sufficient downpipe outlets to stop the water line approaching the top edge of the box gutter.
      Box gutters are not as good as eaves gutters because the overflows go back into the property rather than external.
      That is why particular care with design and maintenance (to prevent blockage of the drainage outlets) is critical when you have a box gutter on your roof.

      Jack

  2. WE have had a box gutter for new extension roof where it joins gable end of our existing building. However, the new extension bedroom corner wall has become very damp and new paint is now peeling off. There is no leak on roof tiling. I can only think the box gutter is tipping rainwater towards inside block wall although the external wall on both existing and new extn wall are also very wet. Any help would be appreciated to find out the cause, do I need the chippies back to sort it, or is it the roof tilers?

    • Jane,

      Sorry for the delay. We have just returned from a long Christmas holiday.

      The leak at the box gutter has nothing to do with the chippies. It would have been installed by the roof tilers (who are usually not very good at box gutter work). So, you should get the roof tilers back to check out the problem.

      Thanks,

      Jack

  3. Greetings Jack. Planning to construct a roof between twi existing buildings approximately 1.8 metres across from shed to shed. Intend to slope roff from left to right. Will be attaching the roof support frame to the walls of each shed, approximately two thirds up the side of each shed. Intending to install a box gutter. Despite a very significant eave on the existing shed, on the box gutter side. Will need to flash both shed walls to prevent leaking. Both shed walls are external walls, clad with colorbond sheeting. Would screws and silicone be sufficient? Any advice would be appreciated. thanks in advance.

    • Stuart,
      Sounds like a complicated little roof job.
      I assume your question is about how to waterproof the side of the box gutter to the colorbond metal walls of the shed.
      If this is the case, screws and silicone would be what we use. Yes, that will be OK.

    • Tommy,

      Some houses have a hidden (box) gutter on the perimeter of the roof – instead of exposed guttering.The box gutters can be replaced by standard eaves gutters – but there is a lot of work involved with this exercise. So, it can get quite expensive.

  4. Hi,

    Looking at purchasing a property with box gutters and wood fascia. The wood fascia has some rot and some of the downpipes may be blocked with leaf litter. Would you know if the fascia replacement is expensive to fix. The roof itself is in good condition

    • Hi Brendan,

      We usually cover the ‘tired’ wooden fascia with a colorbond flashing to provide a maintenance free fascia. The actual box gutters may need replacement also.
      Hard to estimate a price. $5000 for the fascia work will be a good stab for your purposes.

      All the best.

      Jack Yuen

  5. recently we had a roofer come out and install a box gutter in between two buildings however it still leaks we have put two downpipes from each end of gutter but still no luck . can u help please?

    • Hi,
      Sorry, we are closing up for the Christmas holidays and do not return until 19th January.
      If you have not resolved the situation by then, contact me via the contact box.

    • Hi Arthur,

      The reason for a ‘pop’ outlet into a downpipe is to drain normal rain flows into the stormwater system. A side outlet is normally used as an overflow to drain excess water due to very heavy storms. This overflow will drop the water onto the ground and it will become another source of ‘surface run-off’

      If you block off the ‘pop’ and cut in a side outlet that drains all the rainwater onto the ground, it goes against most local council regulations that require rainwater collection from the roof to drain into the storm water system. You will also have a large volume of water consistently running off your roof and splashing onto the ground next to your wall. You will end up with a wet wall and a crater in the ground….

      Hope this helps.

      Jack

  6. Hi There ,

    I am just having a box gutter installed on our house that is being renovated at the moment .

    The roofer says that by Australian Standards the colourbond box gutter insert must sit on either a plywood or AC sheet base to be compliant .

    Is this correct ?

    Thanks

    • Hi Chris,
      I am not sure which Australian Standard is being referred to…
      But a standard is written based on safe and sensible ways of doing things. Box gutters of any decent width will be walked on by someone during its life.
      An unsupported box gutter will either collapse or buckle under load. That is why box gutters are always supported by gutter boards (ply, metal sheeting or otherwise)
      Hope this helps.

      Jack

  7. Our townhouse is 5 years old with a box gutter. After the recent heavy rains,the water came from the roof into the bedroom like a curtain waterfall down two walls, and also leaking down into the ground floor. After cutting open the ceiling, the water was found to be coming down between the wall and the brick outer wall. The plumber has put bitumen on the gutter joint, which has not helped, as the water leak has not stopped completely. Could you advise what needs to be done. Thanks..

    • Hi Alka,
      There are two types of leaks from box gutters.
      The slow leak: This is typical of a leaking joint (if the box gutter is fairly new) or a rust hole (if the box gutter is very old). So, the joint has to be re-sealed in the first case or the box gutter replaced n the second case.

      A sudden flood of water during very heavy rain: This is typical of an overflow situation. The outlet of the box gutter may be blocked by debris, or the box gutter outlet simply cannot discharge the water fast enough. If the box gutter has no overflow provisions, this causes a ‘waterfall’ over the sides of the box gutter – typically running down the walls.

      It sounds like you have an overflow situation. Check the outlet first to see if it is blocked.
      If the outlet is too small and there is no ‘box gutter overflow provision’ – then these have to be rectified.

      Hope this helps.

      Jack

  8. Hello.
    I have a 79 year house with galvanized box gutters, I noticed water is pounding in the middle. Is this normal for box gutters or should they be adjusted to run off better.

    • Hi Leo,

      In theory, all gutters should have a fall towards the outlet. In practice, a lot have no fall or too little fall. So, it is normal for some sort of ponding to happen…although not desirable.
      There are plenty of reasons why box gutters have little fall and the resultant ponding. These may prevent any easy adjustment to eliminate the ponding.
      A compromise is to leave the box gutter they way it is and just give it more regular cleaning to eliminate silt build up and premature rusting. Then when the box gutter gets towards the end of its life, look at ways to replace it with a colorbond box gutter with a fall…

      Hope this helps,

      Jack

  9. My sons home is only 4 years old and has a flat roof with box gutters when we had a bad storm mid April 2015 we had lots of hail very small built up and block the drains causing water to pour into house bedrooms plus. How can we correct this happening once again.

    • Hi Chris,

      It sounds like the box gutter has no overflow protection. An overflow (large enough to pass lots of hail stones through) will provide the safety relief against this happening again.
      Depending on how the box gutter has been built, this may be a simple or very complicated solution….

      Hope this helps,

      Jack

  10. Hi Jake,
    Hope you are well.
    I recently replaced my old box gutter. However, the new one doesn’t have enough fall to let water flow to downpipe. The tradesman said that they cannot make enough fall as box gutter need to align with eave. My eave doesn’t have enough fall. Is this true?
    The new box gutter blocks old overflow point. The tradesman said that it is a more advanced system ( the front of new box gutter is lower than the back). No overflow point will be required as the water can over flow from the front of box gutter. Is this true?
    Thanks for your time.

    • Hi Ethan,

      It sounds like what you have described is a ‘FASCIA gutter’ – rather than a ‘box’ gutter.
      Or you have a box gutter around the perimeter of the roof just above the edge of the eaves. If you have this type of box gutter, then it can be installed with a fall to the down pipes because it is independent of the eaves.
      The front of this type of box gutter can be fabricated to be lower than the back and overflowing will occur over the front edge and no other overflow provisions are required.

      Hope this helps.

      Jack

  11. Hi,
    We have a box gutter on our new renovation. It is 9 metres long. The pitch is sloped slightly the wrong way at the beginning causing a few mm of water to stand there for many days after a rain. Is this bad for the gutter? Is this compliant?

    • Hi Maz,
      Ideally, a box gutter should drain to the outlet and not pond water. Ponding water can shorten the life of the box gutter – by causing premature rust.

      Hope this helps.

      Jack

  12. I have installed box gutter in a church .it was originally installed the same as previous box gutters .They were not happy that it held water so they wanted a fall on it .So I had to take out again and cut one side down and indoing so cut the turn down of to get a fall .My question is ,because I had to cut it down to fit it so it has fall it dosent have the turndown ontop edge , does it void there warranty?

    • Hi Paul,

      It depends on who has provided the warranty and what the nature of the warranty is. Is it a performance warranty (that the box gutter will not leak) or is it a method of construction warranty?
      In my experience, if the top of the side of the box gutter is hard up against the underside of the roof sheets. then it performs just as well as a box gutter that has a lip that folds back under the roof sheets.

      Hope this helps.

      JAck

  13. Hi Jack
    My son is building a house which has a surrounding parapet on the front and sides. They are using standard 1/2 round gutters of 150mm. Iits a flat colourbond roof and i am concerned these will not cope in a big downpour. Any ideas on this. The buikder says if it overflows it wont come in the house. What questions and warranties should we try and get from him. Many thank

  14. Thanks for the advice here, it is very useful. We are going to have an unavoidable box gutter about 8m long, with one down pipe of 100mm, draining a pop up roof that is about 8 x 5m. There are a lot of gum trees above and maintenance will be high. I’ve had conflicting advice about gutter guards. Do you recommend some sort of gutter guard or will this make cleaning more difficult?

    • Hi Robynne,
      Leafguards have limited use over most box gutters. The leafguard does not deflect leafs off the roof and the leaves just sit on the roof and the leafguard. Some leaves still get under the leafguard and into the box gutter. Then in order to clean the box gutter, you will have to remove the leafguard.
      I recommend investigating the possibility of ‘poking’ the outlet end of the box gutter through the side wall of the house and having the full box gutter discharging into a rainwater head mounted on the external wall. (this eliminates the regular blockage of the ‘tiny’ 100mm outlet).
      With regular cleaning of the box gutter, this box gutter setup will virtually eliminate overflowing of the box gutter.

      Hope this helps.

      Jack

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