Flood Damage: Drying Out Your Home Safely

Speed matters after a flood or major leak, but so does doing it the right way — particularly in an older property, where drying too aggressively can cause as many problems as the water itself.

Safety comes first

• Only turn off electricity and gas if it's safe to do so
• Treat flood water as contaminated — wear gloves and wash your hands thoroughly after contact
• Don't start repairs before your insurer's specialist has assessed the damage
• Never run a petrol or diesel generator indoors

Why older buildings need a gentler approach

A solid-wall Victorian or Edwardian property has a naturally higher baseline moisture level than a modern cavity-wall house — that's simply how breathable, traditional construction behaves. This matters after a flood: aggressive dehumidification, applied too fast, can actually cause secondary damage to older building fabric. The generally preferred approach for heritage properties is good ventilation combined with gentle heat, rather than maximum-output drying equipment run at full pace from day one.

Refrigerant or desiccant — which dehumidifier?

This distinction matters more than it might seem. A refrigerant unit, which many people default to, simply won't perform well in a cold, unheated room in the depths of winter — exactly when a lot of UK flood damage happens. A desiccant dehumidifier keeps working effectively in those conditions.

What the water leaves behind after it has gone

Flood water is not clean water, and the part that matters most for the building is invisible. Ground water and drainage water carry dissolved nitrates and chlorides, and as the wall dries those salts are drawn to the surface and stay in the plaster permanently. They are hygroscopic: they pull moisture back out of the air whenever humidity rises, on a cycle that has nothing to do with the weather, the drying programme or the damp-proof course.

This is why a property can be professionally dried, redecorated, signed off — and then develop a damp patch in the same place two winters later. It is also why that patch is so often diagnosed as rising damp when the real cause is salt contamination from a flood years earlier. A moisture meter reads salts as moisture, and will sit at the top of its scale on a wall holding no free water at all. The full mechanism is set out in our guide to why damp gets misdiagnosed.

The remedy is not another damp-proof course. Where salt contamination is confirmed by nitrate and chloride analysis, the contaminated plaster has to come off and be replaced with a salt-retardant system. Skipping that step is the single most common reason damp returns after treatment.

Timber is the expensive part

Water leaves plaster faster than it leaves structural timber, and a ground floor that felt dry underfoot months ago can still be holding enough moisture in the joists to support decay. Coniophora puteana — wet rot — needs sustained moisture, and a subfloor void that flooded and then had its airbricks left blocked by silt or debris provides exactly that. The void has to be inspected and its ventilation restored, not assumed to have dried with the rest of the house. Our guide to damp in floors and subfloor voids explains what to look for.

Dry rot is the less likely but more serious outcome, because Serpula lacrymans can travel through masonry to reach timber well away from the original wet area. Either way, the sequence is the same: establish the moisture source, remove it, then deal with the timber. Treatment applied before the water is stopped is a short-term measure at best.

Documentation matters more than you expect

Insurers, future buyers and their surveyors will all ask about a flood, and a dated written record of what was found, how the property was dried and what was done to the fabric is worth having. If you sell later, a buyer's surveyor who sees historic salt staining with no accompanying documentation will assume the worst; a report showing the contamination was identified and the plaster correctly replaced settles the question. What an unresolved damp issue costs at the point of sale is covered in our guide to the real cost of getting damp wrong.