How to Build in a Flood Zone

ABOVE: Julia Kendell and Andrew Hughes-Hallett have created an impressive low-impact, low-maintenance contemporary home on a floodplain site backing onto the River Thames (Read more about this self-build).

Any attempt to design flood protection into a house has to start with an assessment of the risk, i.e. the probable flood depth that the house will be subjected to. Houses likely to flood to a significant depth (500mm or more, somewhere just above your knees) need to adopt a very different strategy to those likely to get the occasional splashing. Deep-flood houses should not attempt to keep water out, as this could cause the house to collapse due to hydrostatic pressure. Instead, you have to design with resilient materials so that the damage is limited and the house is able to dry out afterwards. For shallow-flood homes, it makes sense to incorporate flood defences to keep the water at bay. There are many products on the market which improve on the simple sandbag. Companies such as Floodguards, Flood Ark and Floodgate (check out floodprotectionassoc.co.uk) sell a variety of flood-barrier products, which may perform well over a short period but do require some intervention by the householder. If you are out, the barriers won’t put themselves in place, so it’s not a good idea to rely on them solely, designing also with resilient materials. But what do we mean by resilient materials?

Foundations: Solid ground-bearing concrete slabs are the preferred option. Suspended floors may present problems as the subfloor space will need cleaning out after a flood. If you have to use a suspended floor, pay attention to the air bricks required for ventilation purposes: you can get products for capping these. Also consider incorporating a post-flood drain-off point.

Timber doesn’t withstand prolonged immersion in water. And plywoods, MDF and OSB like it even less. So it’s a good idea to avoid timber framed walls and products like SIPs (structural insulated panels).

Gypsum plasterboard: Cement and lime renders are preferable. If you do fit plaster - board, fit it landscape rather than portrait, so the bottom layer can be easily replaced.

Mineral wool and natural wool Insulation don’t like being immersed in water. Closedcell, plastic insulations, such as expanded polystyrene and polyurethane, are pretty much water-resistant and are preferable. Extruded polystyrene is particularly good.

Handmade bricks are very absorbent but engineering bricks are virtually waterproof, so use them up to likely flood levels.

Generally PVCu external doors are better than timber. Hollow-core timber doors, in particular, should be avoided. Patio and folding doorsets are best left out. Specify doors with good locking/sealing mechanisms. If possible, raise the thresholds (one must have a level threshold for wheelchair access).

Kitchens: Avoid fittings made with materials like MDF. Plastic or stainless steel are ideal; natural wood finishes will be preferable to manufactured wood products.

Place sockets and appliances as high as possible. Built-in ovens must be at eye level.

One nasty aspect of flooding is sewers and drains ‘surcharge’, causing their contents to back up into affected homes. However, drain manufacturers now produce one-way flap valves which will prevent backflow.

SHALLOW VS DEEP — TWO FLOOD DESIGN SCENARIOS

The Shallow-Flood House
The estimated flood depth is less than 200m with a probability of occurrence of just one in 100 years, so a water-exclusion strategy is recommended. Engineering bricks on the external face greatly enhance the house’s ability to prevent water ingress through the walls. Providing water is prevented from coming in through the doorways, using resilient engineering bricks would result in a flood depth of just 3mm inside the house after five hours (7mm after 12 hours).

The Deep-Flood House
The property is likely to be flooded to a high depth (over half a metre), so a waterentry strategy should be adopted to avoid structural instability and to allow water passage through the house. Materials that dry quickly while retaining their integrity should be specified. Standard cavity masonry wall construction (‘brick-concrete block-gypsum plasterboard’) is likely to be adequate if closed-cell insulation is used; it will take around five days to dry naturally.

Further Reading

• Everything You Need to Know About Foundations
• Is This the House of the Future?
• A Low Impact Self-build by the Thames