Water roofs lower pressure on sewer system

Water, it's a valuable commodity. But too much water is a flood! That's why we're testing how we can decelerate the drainage of rainwater in a few stores.


As Belgium's largest retailer, we manage a large paved area in the form of buildings and car parks. In an ideal world, the largest possible part of that surface is water permeable so that rainwater can easily infiltrate into the subsurface. If this is not possible, we often look at subsurface storage as a solution not to cause an overload on the sewer system.

This is not ideal for the building process: opening up and repaving a car park during the renovation of a store is time-consuming. This also goes for new-build projects, where building a car park with subsurface storage logically takes more time than building one without. On top of that, the rest of the construction site is also delayed when the car park is impassable for a longer period of time. Reason enough to look for alternatives!


Holding water

An interesting alternative for subsurface storage is to retain rainwater on the roof. Project manager Johan Pyl: "In Ath, we had the opportunity to test a water roof quite easily. The store was enlarged, which resulted in additional roof surface. Perfect to test the concept. Simply put, the water is drained in a delayed manner by installing a pinch pipe. Only when the water reaches a certain volume is it drained at its normal capacity. As a result, not all the water during heavy rainfall ends up in the sewer system at once, but we spread this peak over a longer time."

Compared to a normal roof, a water roof doesn't require a lot of extra work. Johan: "However, the study phase takes a bit longer. This extra preparation mainly concerns structural research: how much additional weight can the roof bear? On the construction site, the impact on work time is limited, partly because the activities take place on one (small) location. This means the rest of the site experiences little to no impact due to the installation of the water roof."

Combination with green roof

In Ninove, we combined a water roof with a green roof. Johan: "We were required to do this by the municipality because some apartments overlook the roof. The roof in Ninove is a bit more complex than the one in Ath. We use a type of bubble foil for example, that creates a kind of maze. This increases the distance the water needs to cover before it is drained."

Part of the water is of course also consumed by the plants. The rest of it stays on the roof. The entire system not only lowers the pressure on the sewer system in case of heavy rainfall, but it also has its use during dry spells. We can see that green roofs often struggle at such times. The plants in Ninove are able to survive such situations much more easily thanks to the additional water storage.


No standard solution

Will all our roofs be filled with water soon? It won't come to that. Johan: "The best solution is always infiltration. In this project, we are mainly targeting small renovations and extensions of stores, where opening up the car park would have too much of an impact. Also, these systems can't always be applied to existing and larger roofs."


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