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Last edited 03 Feb 2021
Flood Resistant Construction
A flood resistant building is one that is designed to resist flood water ingress. That means that the building is designed to prevent flood water from entering through the walls, floor and any apertures. The deeper the flood water and the higher the velocity, the more difficult it is to keep water out. As water rises on the outside of the building it creates a force on the ground floor and outside walls including any windows and doors at that level.
Flood resistant buildings are typically constructed using concrete or steel and concrete but may also be made with masonry provided there is an impervious layer, such as water-resistant render or asphalt. Typically, frame buildings are more difficult to make flood resistant without a concrete or masonry layer due to the number of potential pathways for water around junctions. Masonry is generally permeable, as is concrete unless to a certain specification. Therefore water can seep through walls and floors unless designed properly. Cavity walls may need to be filled with water resistant insulation below the flood level to prevent the passage of water and to prevent contamination within the cavity.
The ground floor is a potential pathway for floodwater to enter, particularly if flood water remains present outside for a period of time. This is because the water will seek to reach an equilibrium inside and outside the building. If the pressure from the rising water is substantial it will apply an upward force to the floor potentially causing structural damage, water penetration or the floor to rise, particularly if light.
Concrete floors may need to be reinforced to prevent the risk of fracture from the water pressure. Beam and block floors are likely to require additional waterproofing to prevent water ingress. The membrane is also likely to need to be weighed down to prevent it being forced up by the water.
Where flood depths can be greater than a few hundred millimetres (in the order of 0.5m) it may become expensive to make a building resistant to floodwater. In this case it may be more cost effective to make a building resilient to flooding. This may also be more appropriate for existing buildings.
Where floodwater is likely to remain for several days, such as areas with relatively flat topography, it may be better to consider flood resilient construction, to reduce the reliance on the structural and waterproofing measures.
The key components of flood resistant construction are:
- Structurally and water-resistant superstructure
- Water resistant materials, including cavity insulation
- Impervious doors or flood guards and raised windows
- Seals to all incoming services
- Perimeter / sub-floor drainage, sump pumps and non-return valves
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