Flood resistance

Provided by Robert Barker: co-director of Baca Architects


What, Why, When, How, Extras


What is it?

Building resistance is the process of making a building resistant to flood damage, either by taking the building out of contact with floodwaters or by making the building resistant to any potential damage resulting from contact with floodwaters. This process involves the construction of a building in such a way to prevent floodwater entering the building and damaging its fabric and allow flood victims some degree of protection from flooding. These measures can prevent water getting into a property, particularly for shallower floods or allow time for the householder to move valuable possessions upstairs.


Why use it?

Floodwater can be disruptive and can cause structural and cosmetic damage to a building, which can be expensive to repair. Floodwater can also carry contaminants creating additional health risks. With low flood depths it may be possible to prevent significant damage to a building and may allow continued use of a building during a flood. It may also be beneficial to install some flood resistant measures to properties located behind flood defences, to protect them in the event that the defences are overtopped or fail.


When to use it?

Ideally buildings should be placed so as to avoid flood-risk. However, when it is not possible to do so, such as if the building already exists or it replaces an existing building then measures to reduce the effects of flooding must be considered. Typically resistance measures may be used when the flood depth is less than 600mm but subject to the characteristics of the flood such as speed and debris. Consideration should be given to the effect that climate change may have on the potential depth of flooding. In some locations, buildings not currently at risk of flooding, maybe in the future.


How to use it?

Key points:


  • New buildings should be located out of flood-risk where possible.
  • Flood resistance typically requires human intervention to implement actions to protect the building, this means that ample warning time needs to be available to set up defences.
  • Consider how waterproof the building fabric is (type of frame, type of infill panels, floors, walls…).
  • Floodwater can rise up from the ground, not just enter through doors, windows or other openings.
  • It is necessary to determine the structural integrity of the building, deep water can cause structural damage.
  • Use certified waterproof sealants and materials to dry proof the building.
  • Consider pipes in and out of the building as potential paths for water.


Design procedure:


  • Step 1: Identify how deep the floodwater may come against/within the building and identify if this will change in the future, as a result of climate change. This is not always as easy to identify as it sounds. Consult your local council for more details.
  • Step 2: Identify if it is possible to elevate a proposed building above the flood level or to elevate it sufficiently to reduce the depth of floodwater likely to occur to below 300mm.
Flood resistant elevated building

Buildings are elevated above the flood level. (Click image to enlarge)

  • Step 3: Typically if the depth of water is greater than 600mm then resilience will be a more appropriate design measure. Additionally if there are a lot of openings in the building then it may be preferable to allow water into the property rather than try to waterproof every junction.
  • Step 4: Identify all of the openings around the exterior of the building, and determine how wide are they? Proprietary door guard products are typically designed for single domestic doors. Consider if it is possible or appropriate to reduce the number of potential openings / water pathways into the building.
Flood door barrier

Products exist to prevent water entering doorways and gardens. (Click image to enlarge)


  • Step 5: Identify any additional pathways that water could enter through such as drains, undercrofts, air bricks etc. During a flood a differential in water pressure can be created in WCs resulting in water spewing out of WCs. Determine which corresponding product is suitable for which area of the building (see below).
Flood backflow valves

Internal and external backflow valves prevent water entering from the sewers. (Click image to enlarge)

  • Step 6: Make sure every possible entrance is protected, if you miss something, the floodwater will find it! Good workmanship is essential to avoid holes in construction, which could form flood pathways.


Related strategies

Masonry Building orientation Flood-risk: building placement
SUDS Flood resistance


Conflicting strategies

Strawbale Inclusive design

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