Could flat roofs prevent urban flooding?

11/05/2017 0 Comment(s)

In our last blog, we considered how you might be able to use flat roofing to your advantage – to recycle rainwater and reuse it within your home, therefore saving you money and helping the environment. Continuing along similar lines, this blog looks at another way in which flat roofing can benefit both the homeowner and local communities. The design of flat roofs can be modified slightly in order to collect and drain rainwater in a particular way, which would help to relieve flooding during periods of intense rainfall. What’s more, EPDM is the ideal material for the job.

Heavy rainfall and flooding events are becoming much more common, particularly in urban areas where there are fewer ways for water to be absorbed (i.e. trees, grass, bare soil). This means more chance of rainwater accumulating over built-up surfaces causing damage to buildings and cars. Between November 2016 and February 2017 there were five storms alone – the worst one being storm Doris, which caused up to 25mm of rain in some areas in just a few hours. With flooding on the rise, the best option seems to be integrating flood management strategies into existing building structures. 

One key area is in EPDM flat roofing. A certain amount of surface water build-up on flat roofs is normal during periods of heavy rain and should drain within a day or two. However, by adapting the design of EPDM flat roofing slightly, so water purposefully collects on the roof in a controlled manner, it reduces the amount of water running down streets and roads, therefore helping urban areas to cope with very heavy rainfall. Even storing rainwater to a depth of just a few inches can make a big difference to urban areas when implemented on a larger scale. EPDM flat roofing systems could store this rain water for a few hours, either slowly draining it back into water run-off systems (which will reduce the immediate volume of water) or using it within the house. 

Rainwater harvesting (RWH), which we discussed in a previous blog, is one example of this in action. The rainwater can be drained into the plumbing system of a house, so that each time the toilet flushes rainwater replenishes the water supply in the tank. Whilst this is slightly more costly than a temporary storage system, it will help you to cut down on your water bills in the long run and improve the energy efficiency of your house. It would be a sustainable investment to your property and a unique selling point if you ever chose to sell up.

New builds can obviously be built with this technology already integrated, but existing flat roof structures would not need to be changed massively – although it does require the roof to be completely watertight. Changes would include adding extra drainage systems to the roof, such as overflow drains for very intense rainfall, and adding a more complex drainage system if you wanted to implement a RWH system in your home. Before doing this you will need to ensure your roof is watertight and can deal with the added weight on the system. 


  • First measure the size of the flat roof, as this will significantly impact how much water you can collect. 
  • Check the roof angle and material. EPDM is great for these systems as its smooth rubber surface allows the water to drain into storage systems quickly and efficiently and the rubber is non-toxic. 
  • Calculate the size of your roof in square metres by multiplying the length of the roof by its width. Do not take into account any elevation changes, as it’s still the same surface size no matter of angles and slight sloping. 
  • The next step is to assess how much rain your area roughly receives every year; an easy way to do this is to multiply the total square metres of your roof (which you just calculated), with an average precipitation rate for your area – try looking here for these rates. Multiply this figure by the number of litres in a square metre to reveal how many litres every square metre will collect. This will help you to learn how much water you can expect to harvest. 
  • Once you know this, you can then calculate the weight of the water using an online calculator. If you doubt the carrying capacity of your flat roof, it is always best to seek professional advice. 

EPDM flat roofing could be a great way to prevent urban flooding. It’s relatively cheap for those who already have a flat roof or are considering one, and it can be used to harvest rainwater to minimise your carbon footprint. It’s also an unobtrusive method of combatting flooding – you would barely be able to tell the difference in appearance between flat roofing with and without this system installed. 

If you want any more advice on EPDM flat roofing, or would like to install an EPDM flat roof on a domestic or commercial property, look no further than Waterproof systems.

If you would like to find out more about RWH, see Nate Downey’s book: Harvest the Rain: How to Enrich your Life by Seeing Every Storm as a Resource. This Guardian Article is another useful resource too.

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