Building in a flood zone
Architect Julian Owen takes a look at what design elements can help to reduce the risk of damage caused by flooding
e right design elements could fend off flooding and protect your property when nature does its worst. Julian Owen shares his top tips for reducing the likelihood of property damage
Many people would like to live by the sea or a river, attracted by the scenic views and nearby opportunities for boating, fishing and walking. However, one downside to residing in these locations is the threat of water ingress. Some of the UK’S most picturesque towns and villages have appeared on the news as possessions are lost, homes ruined and lives put in danger by flooding. And events like this aren’t one-offs, as it looks like the frequency of flooding is set to increase in the coming years due to climate change. This means future dangers have to be considered if you’re planning to self-build or extend in an area that’s at risk.
About 5.5 million homes in the UK are in danger of being flooded. That’s approximately 15% of properties built near to rivers, the sea or areas of extreme surface water run-off caused by local ground conditions – the latter of which are often in places that aren’t actually near to open water.
If you’re thinking of self-building or altering a house in an area identified as a flood zone then the risks need to be confronted early. This is likely to be an influential issue for your local planning department when they consider your application, and failure to comply with the stipulations of the Environment Agency usually results in a refusal.
There are ways of addressing the concerns of planners, especially if you’re extending a house in a lower risk area. In these cases, keeping the floor level the same as the existing building and including measures that will minimise potential water damage to the structure should suffice.
Getting approval for a completely new house, however, can be more challenging. As well as tackling the risk of the building flooding in the design, you may also have to prove that the local flood plain will not be compromised by the new property. These are areas of land that are designed to collect floodwater and prevent it from flowing onto higher ground, and their storage capacity can be reduced if they’re heavily developed and covered in buildings.
Resistance & resilience
If you can gain planning permission and are able to buy adequate insurance to build in a flood risk zone, there are two steps to tackling the problem. Firstly, work to prevent the water from entering the house and, secondly, minimise any resulting damage if it does. Experts call these flood resistance and resilience, and the measures taken are known as water exclusion and entry strategies.
The Association of British Insurers says that protecting a house against shallow flash floods costs around £2,000£6,000, but keeping water out during prolonged flooding requires more work and can come in between £20,000 and £40,000. If you can afford it, resistance and water exclusion are the best long-term strategies.
Fitting an early warning system in the form of a sensor and alarm is a sensible precaution. Preferably, these
should be positioned outside at a lower level than the ground floor. They work to alert you if a flood is expected, thus giving you enough time to put temporary measures in place and move some possessions upstairs. But, obviously, you’ll need to be in the house at the time for this to work.
The simplest water exclusion measure is to build the ground floor above the maximum level that any flood water is likely to reach. However, this is often impractical and planners may not approve of the building being disproportionately higher than neighbouring properties.
Where the potential flood level is below 600mm, there are a number of relatively simple ways of sealing off the inside and keeping a new house more or less dry. A common option is to use tanking; a continuous waterproof layer buried within the wall and floor construction that prevents floodwater from seeping into rooms. However, there are still plenty of routes for water to get in, not least through a home’s doors and windows. Fenestration can be sealed at a low level using hinged flood barriers that can be quickly swung or slotted in place in the event of a flood warning. Airbricks are potentially vulnerable, but they can be fitted with temporary covers, automatic seals that drop into place, or permanent snorkels that will allow air to flow via an outlet at a higher level.
One of the most unpleasant side effects of a flood is the potential release and circulation of raw sewage inside the house; this can be avoided by fitting non-return valves to all the drainage routes. Like many of these preventative measures, this is a simple DIY job provided the drains are easily accessible by manholes. All potential entry points via service pipes below the flood level have to be checked and tightly sealed. Bear in mind that electricity and water are not a good mix, so external meters will need to have the protection of waterproof cabinets.
High risk areas
Where the flood level could reach above 600mm, it’s more difficult to provide effective protection because the deeper the water, the more forceful the pressure pushing onto the property. Windows and doors can be specially reinforced using strong materials and seals, but even these have height limitations above which they will fail.
Another more extreme solution is to construct the house on stilts, lifting it above the predicted flood level. A recent innovation is to make the stilts extendable to allow the house to rise and float when the water is high, dropping back down as it recedes. This technique has been very successful in other countries, such as Holland, but it hasn’t been used on many new houses in the UK.
Above: Baca Architects designed and built The Amphibious House, which can rise up in its dock and float if flooding occurs
This flood gate can be pulled across to prevent water from seeping into the house through the door
Below: The Floating House by Carl Turner Architects is a lightweight cross-laminated timber (CLT) design that can be supported on a variety of substructures to suit flood-risk locations, including piles or even a fully floating pontoon