17 Aug How do flat roofs drain?
If you take a look at some of the many flat roofs we have installed or look at any other flat roof, you will find that some of them do not have traditional drainage systems in place.
This lack of guttering can be confusing at first glance – and you may wonder why they do not suffer from ponding.
But the way flat roofs drain is actually rather simple – although there are a few different methods flat roofs use to sufficiently drain. These are outlined below:
- Flat roofs are not 100% flat
It sounds so obvious, but it’s not always easy to tell that a flat roof isn’t 100% flat if you are looking at it from above. The general rule of thumb for sufficient drainage on a flat roof is a fall of 1% or 1/4” per 1”, however the ideal fall depends on the material. For example, aluminium and lead sheet have different fall requirements (leads needs slightly more).
- All flat roofs have drainage points
Even if you can’t see them, there will be drainage points. On large flat roofs an inner drainage system is preferable because it can be strategically placed on the roof and it will not freeze in the winter. On smaller flat roofs, it is always best practice to have run off points that lead directly into gutters. Water will run off due to the fall, as outlined above.
The importance of sufficient drainage on flat roofs
Even flat roofs with a hydrophobic material layered over them require sufficient drainage to perform efficiently in the long-term. And if a flat roof is submerged in water for a long period of time, it can cause many problems, such as:
- Overlapping can come apart and cause roof leaks;
- Tar-based products can deteriorate due to over-exposure to moisture;
- Leaves and debris can collect in the pond area, which can cause plant growth;
- Pond areas will freeze in the winter, which can cause stress to the roof;
- A roof can bow over time if water becomes deeper than an inch due to the weight.
If your flat roof suffers from ponding, the only way to solve your problem is to have a drainage point installed. But, this can be as expensive as a new roof, depending on the size of your roof and where the drainage point needs to be located.
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