There is a large industry selling the idea that a house can be made watertight, and it cannot. Water finds the floor, the airbricks, the pipe penetrations and the drains, and it does all of that while your expensive door barrier is still holding perfectly.
What flood resilience genuinely buys is time and depth: hours to move things upstairs, and protection against the shallow events that are far more common than the deep ones. That is worth having and worth being honest about.
The honest position
The last two are the ones that matter most and cost the least. Everything above them is about reducing damage. Only those two are about not being hurt.
Resilient repair beats resistance in most flood-prone British houses: accepting the water will come in, and making the ground floor something that dries out and cleans up rather than something that has to be gutted. Tiled floors instead of carpet, plaster to a horizontal line with removable panels below, sockets at waist height, kitchen units on plastic legs rather than chipboard bases.
How water actually gets in
Roughly in the order it arrives:
- Under and around external doors. The obvious route, and the one everybody defends.
- Through airbricks, straight into the underfloor void of a suspended floor.
- Through service penetrations: the holes where the water main, the gas pipe, the electricity cable and the waste pipes enter the wall. These are rarely sealed properly.
- Up through the floor. On a suspended floor the void fills and comes up between the boards. On a solid floor, if the water table rises far enough, it comes through the slab and the joint at the wall.
- Backwards up the drains, out of the downstairs loo, the shower tray, the washing machine standpipe and the kitchen sink.
- Through the walls themselves, slowly, once the water has stood against them for hours. Brick is not waterproof.
Notice that only the first is a door.
Barriers
If you are in a flood risk area, barriers are worth having, and they need to be the right thing, stored where you can reach them, and rehearsed.
- Proper flood boards that fit brackets fixed permanently to the doorway, with a compressible seal. Fit the brackets now; deploy the board on the day.
- Demountable barriers and flood sacks that expand with water. Effective for shallow depths, quick to deploy, no fixing needed.
- Sandbags are the worst of the common options and the most famous. They leak steadily, they are extremely heavy, they need a lot of them to do anything, and once contaminated with floodwater they are hazardous waste. If sandbags are what you have, lay them in a pyramid with a sheet of polythene on the water side, tucked under the bottom row.
- A plastic sheet plus timber across a doorway, sealed with a heavy bead of mastic and weighted, is a real improvisation and better than nothing.
Store barriers where you can get them in the dark, in the rain, in five minutes. A barrier in a locked shed at the bottom of a garden that is already flooding is a barrier you do not have.
Rehearse fitting them once, dry, with a stopwatch. Everybody who might have to do it should have done it once.
Airbricks and service holes
Airbrick covers are cheap, and the smart ones are automatic: a float closes them when water arrives and they ventilate normally the rest of the time. Manual covers work and depend on somebody being home.
Seal the service penetrations with a flexible sealant or an expanding foam intended for the purpose. This is a twenty minute job with a torch, crawling round the outside of the house looking for the holes where pipes and cables go in, and almost nobody does it.
The drains coming back at you
This is the route people forget and it is often the one that beats them.
When the sewer or the surface water drain is surcharged, the water in it is under the same head of pressure that is pushing against your barrier. It travels back up your soil pipe and out of the lowest opening in the house, which is usually a ground floor toilet or a shower tray.
- Non-return valves fitted to the foul and surface water connections are the proper answer, and they are a drainage contractor's job.
- Toilet pan seals and inflatable drain bungs are the day-of version. A plug that expands into the pan or the waste, or a weighted board and a heavy object on the closed lid, both help.
- Plug the shower tray and the ground floor bath and sink. A standard plug plus something heavy on top.
- Block the washing machine standpipe.
Sewage coming back into the house is worse than river water, both for the clean-up and for the health risk, so this is worth attention even when the barrier is holding.
When to stop defending
Cut the power before the water arrives, not after. See the mains shut-offs. Once water is over a socket, that room is not safe to enter and the consumer unit may not be reachable safely either.
Living upstairs
If the ground floor goes, you may be upstairs for days.
- Water. Fill the bath and every container while the mains is still clean. Once flooding has affected the network, supply may be interrupted or subject to a boil notice.
- Warmth. The heating is almost certainly off. See make one warm room, which now applies upstairs.
- Food you do not have to cook, and a means of boiling water that is safe to use with ventilation.
- Charging. A power bank charged in advance is worth more than almost anything else in the house at this point.
- Sanitation. The drains are surcharged, so the toilet may not be usable. A bucket, bin liners and something absorbent. The composting toilet and waste with no collection pages cover the principle.
- Do not go down to look while water is standing over electrics.
Afterwards
- Photograph everything before you move it. Insurance will want it and you will not want to go back.
- Get the water out, then the silt, then wash down with detergent, then a disinfectant. See shock chlorination if any water store was affected: it must be emptied, cleaned and disinfected before use.
- Drying takes months, not weeks. A flooded solid floor and the walls above it hold a great deal of water. See damp and mould, and do not let anybody plaster or lay flooring until it is genuinely dry.
- Have the electrics and any gas appliance checked before they are used again.
- Take the airbrick covers off.