Basement and cellar waterproofing: diagnosis before treatment

Tide marks at the base of walls, white salt deposits, peeling paint, a musty smell and standing water point to different problems. Groundwater pressing through the walls and floor behaves differently from rain soaking a wall because a gutter overflows or the ground outside is higher than the floor inside. Condensation, from warm moist air meeting cold walls, often looks like rising damp and is cured with ventilation and heating rather than chemicals.

Look outside before anyone quotes for inside. Blocked gullies, cracked drains, leaking downpipes and garden beds banked against the wall are cheap to fix and account for a surprising number of damp cellars.

Many free damp surveys are carried out by companies that sell injected damp-proof courses, tanking or membranes. That does not make them dishonest, but the person diagnosing the problem earns from one particular answer. Readings from a handheld moisture meter on plaster are easily distorted by salts and are not proof of rising damp on their own.

For an older house, or before spending serious money, pay for a survey from a chartered building surveyor who does not sell treatments, or at least ask the specialist to explain how the cause was established. A good report names the source of the moisture, not just where it shows.

The British Standard for keeping water out of structures below ground is BS 8102. It describes three types of protection: a barrier applied to the structure (often called tanking, a cementitious or liquid coating or a bonded membrane), a structure that is itself watertight, such as specially designed concrete, and drained protection, where a studded cavity drain membrane lets water in behind it and leads it to a sump and pump.

Tanking holds water back and relies on the wall and the bond staying sound. Cavity drain systems accept that water will arrive and manage it, so the pump needs power, a backup and servicing. Excavating outside to waterproof the walls from the outside is the most thorough approach but is rarely practical for a terraced house. For habitable rooms, designers often combine two systems.

If you plan to use the cellar as a bedroom, office or living room, the waterproofing is part of a conversion that needs Building Regulations approval, which also covers fire escape, ventilation and ceiling height. Where the cellar is shared with a neighbour's wall or you dig down, the Party Wall etc. Act 1996 may apply in England and Wales.

Source: legislation.gov.uk

A damp-proof course is a layer near the base of a wall that stops moisture rising. Houses from the late Victorian period onwards usually have one; many older solid-walled houses do not, and have stayed dry for a century because the walls can breathe. Injecting a chemical DPC into such a wall, then covering it with cement render or gypsum plaster, can trap moisture instead of removing it.

Installing a damp-proof course usually needs neither planning permission nor Building Regulations approval, but a listed building will typically need listed building consent. Ask what the guarantee covers and who backs it if the company stops trading. Some firms offer an insurance-backed guarantee; read what it excludes.

• the diagnosed cause of the damp and how it was established
• the system used, with product names, and whether it follows BS 8102
• sump, pump, alarm and battery backup for drained systems, and the service interval
• what happens to existing plaster, skirting and floors, and the finish afterwards
• any outside work: drains, gullies, ground levels, gutters
• drying time before decoration and the guarantee terms

Your request is free for you. We check your phone number by text message and your email by link, then show a summary, without contact details, to waterproofing specialists in your area. Exactly one of them can buy it; it pays Offrano, and the request is taken out of our shop. We email you the specialist's name, address and phone number before it sees yours.

We cannot promise that a specialist will buy every request, and unsold requests are withdrawn after 21 days. You decide whether to accept any quote. Offrano only checks that buyers are registered businesses, so ask about the surveyor's qualifications, previous basement projects and who stands behind the guarantee.