A flood barrier forms the primary line of resistance at an opening, but the performance of that barrier is always linked to the condition of the surrounding construction. In practice, water does not try to pass through the centre of a sound brick. It follows the path of least resistance through mortar joints, small cracks and the interface between the brick and the mortar, particularly within the first 600 mm above external ground level where resistance measures are typically focused.
How water moves through masonry during a flood
Laboratory testing by Safeguard on a standard single skin masonry wall subjected to a 0.6 m head of water demonstrated how quickly water can pass through untreated construction. Flow rates were measured at around 12.5 litres per minute per square metre of wall. Across the perimeter of a typical house that equates to a very significant volume of water entering in a short period of time if no resistance measures are in place.
What is particularly interesting from the same testing is where the improvement comes from. A water repellent cream on its own provided some benefit at lower water levels, but the major change occurred once the mortar joints were repointed with a suitable additive and the brick to mortar interface was properly sealed. At that point the recorded flow through the wall reduced to almost zero even under higher flood depths.
This reinforces what we see on site. The weak point in many walls is not the brick itself but the mortar.
This is why, when we survey or install a flood barrier, we always look at the fabric immediately around the opening. Open joints, historic cracking, poorly bonded repairs and service penetrations can all act as seepage paths. Addressing these areas does not replace the need for a barrier. It allows the barrier to perform as intended.
Breathable water repellent treatments have been used for many years to reduce rainwater absorption into masonry. There are now newer enhanced waterproofing barrier coatings coming to market which can be used as part of this resistance zone. These provide a clear surface layer, offer crack bridging for fine cracking and are designed to work in conjunction with repointing and masonry cream systems to provide low level flood resistance up to the design height, typically around 600 mm above ground level. As with all resistance measures they should be considered as one element within a wider package rather than a standalone solution.
The effectiveness and suitability of any waterproof coating will depend on the wall construction and condition, and they are not always appropriate for solid wall or heritage properties where breathability and material compatibility must be carefully considered.
Guidance for customers measuring for a flood barrier
For customers who are measuring for a flood barrier themselves, a simple visual check of the wall condition around the opening is one of the most valuable steps in the process. Sound mortar joints, well sealed service entries and good quality pointing all contribute to the overall level of protection. For those who want a higher level of assurance, Flood Surveys include a detailed assessment of potential ingress points so that the barrier and the surrounding structure work together as a system.
