Guide to flat roofing drainage systems and EPDM

With a new structure in the design stages or part-completed, you need to know that your blueprint is going to work. If you are considering an EPDM flat roof for all or part of your building work, then a key question is how water will drain away from the flat surface.

By its very definition, a flat roof has much less of a slope for water to run off than a pitched roof system. This creates something of a challenge when architects and site managers are putting together plans for their project, as they do not want pools of water gathering in places and creating the possibility of a bulge or tear in the roofing material.

In spite of this, a huge number of flat roofs are still planned and delivered each day around the world. The reason for this is that technology has advanced to such a degree that the benefits of flat roofs can be enjoyed without this concern.

Flat roofs

If you were told that one of the worries when it comes to roofs is the danger of water and other debris gathering, then you would probably avoid flat roofs altogether – surely a pitched roof would be better? That way, water and debris would use its own momentum to fall down the roofing material and into the gutters.

However, this would mean the end of a flat roof’s advantages: it generally works out much cheaper at the point of construction than a pitched roof and a simpler design can be used for the building overall, due to the foundations required for pitching. On top of this, for the room underneath, you are not losing any space to the slope of the pitched roof.

Part of the reason why the flat roof has become a far more viable option in recent years is the emergence of EPDM (otherwise known as Ethylene Propylene Diene Monomer), as this means that the rubber membrane within the roofing material is far tougher and has a much longer lifetime. As a result, wear and tear becomes less of an issue – and attention can turn to drainage.

Drainage – What are your options?

This very much depends on the way it is being used. If is a small building with one flat roof, or a room with a flat roof on the side of a larger building, then best practice is to introduce a slight pitch to it – preferably a fall of about one in 80 from the back towards the gutter, so that water will naturally glide down the roof and into the gutters on one or more sides.

As long as the EPDM is properly installed, then the waterproof surface will encourage liquid to enter the gutters, but an important point to note is that an overhang at the gutter edge is required. By adding this overhang, the water flows across the flat roof and goes directly into your drainage system. If you are not sure how to add this overhang, look into Sure Edge Trim, which provides a perfect run-off from flat roofs.

Of course, this simple flat roof will not be appropriate for all applications; sometimes a flat roof may be surrounded by pitched roofs, a wall leading to a higher-level flat roof or some other obstruction. As a result, there is no clear side to act as the gutter edge and this raises the prospect of water simply pooling.

The answer is to look into getting an angled roof drain or top drain, which act as a kind of plug hole for water to fall down. So while the roof can still be pitched slightly, it is aimed at a smaller target. In both looks and application it is like altering the target from a golf green to the pin itself – but it works.

How to ensure a long life-time

The installation of the drainage system is of course only the first step towards a top long-term solution; during the roof’s lifetime, a whole host of debris is likely to fall and get caught in the gutters. Whether you have a gutter edge, roof drain or top drain, this is something to keep an eye on.

Essentially, all you need to do is regularly inspect your drains and clear any debris that might have built up – this will most likely be twigs or branches but could easily come about due to a wayward plastic bag or something completely random! In terms of how often to do this, this is difficult to judge and will vary according to the building’s location. For example, if your garage roof is surrounded by large trees then a build-up of twigs may become noticeable after a matter of weeks or months, whereas a more secluded building may simply require a twice-yearly check.

In terms of maintaining the EPDM layer, all you are advised to do is clean it occasionally to prevent any aesthetic issues such as ‘chalking’ (reaction to weathering) or mould. It is also important to keep sharp objects away from the material, so that you avoid tearing. Other than that, your EPDM membrane should retain its waterproof properties for many years, with your chosen drainage system taking water away from the flat roof day after day.