Choosing the Right Damp-Proof Course for Your Home

Start with the diagnosis, not the drill

Before anyone quotes you for a new damp-proof course, it is worth being certain that rising damp is genuinely the problem. Plenty of walls are drilled and injected every year for a fault that had nothing to do with a failed DPC. The classic signs of rising damp are a tide mark reaching roughly a metre up the wall, salty bloom on the plaster, crumbling skirting boards and a musty smell that never quite clears.

The common impostors are condensation, penetrating damp from leaking gutters or cracked render, bridged air bricks, and external ground levels that sit above the original course. Hygroscopic salts left behind by an old chimney or coal store can also pull moisture out of the air and mimic rising damp convincingly. A good surveyor will test with a moisture meter and back it up with a carbide or gravimetric test, then inspect the outside of the building before recommending anything.

One useful piece of history: most houses built after roughly 1875 already have a physical damp-proof course, usually slate, bitumen felt or a couple of courses of engineering brick. If that course has been bridged by a raised patio or a solid cement plinth, removing the bridging and letting the wall dry may solve the problem without any injection at all.

Chemical injection: the modern workhorse

A chemical DPC works by drilling a series of holes into the masonry, usually around 12mm diameter at 100mm to 150mm centres, about 150mm above external ground level, and injecting a siliconate or siloxane cream. The cream diffuses through the brick or mortar and lines the pores, breaking the capillary action that draws water upwards. On most brick and block walls it is the most cost-effective option available.

Work should follow BS 6576, the standard covering chemical injection damp-proofing. Ask your contractor whether they are drilling into the mortar joint or the brick itself, how deep the holes go, and whether thick walls need a second row or injection from both faces. On hard, dense stone it often needs pressure injection rather than cream.

• Best for: solid brick, cavity brickwork, lightweight block and reasonably porous stone.
• Weaker on: random rubble, flint, cob and very thick walls, where the cream cannot reach the full depth reliably.
• Expect: a 20 to 30 year guarantee, ideally backed by independent insurance rather than the installer's own promise.

Physical membranes and remedial barriers

Where injection will not work, or where the wall sits below ground, physical systems take over. A meshed cavity drainage membrane — studded plastic sheeting fixed to the wall — creates a drained gap between the masonry and the finished surface. Water runs down behind it and collects in a channel or sump at the base. This is the standard approach for basements, retaining walls and lower ground floors, and it copes with genuine water pressure as well as capillary rise.

Where salt contamination is heavy, a salt-retardant render or renovating plaster used with a membrane stops the salts blowing your new finish within a season. For new builds and extensions, the physical DPC is a different animal altogether: a membrane under the floor slab, a course in the walls and cavity trays above openings, all covered by Part C of the Building Regulations. The downside of membranes is that they cost you 25mm to 50mm of room width, and the detailing at the floor junction has to be right or the whole thing fails quietly.

Electro-osmotic systems: niche but useful

Electro-osmotic damp-proofing uses a small electrical current to drive moisture back down the wall. Passive versions rely on the natural electrical potential in the ground and use titanium or copper wires inserted at intervals. Active versions add a small power unit, with anodes in the wall and an earth electrode outside, maintaining a gentle current across the masonry.

These systems suit solid stone, rubble-filled and flint walls, and can be attractive in listed buildings where extensive drilling is contentious. Be realistic, though: the evidence base is thinner than for injection, results vary with the wall, and the installation has to be carried out by someone who genuinely specialises in it. It is not a cheap substitute for a chemical DPC on an ordinary cavity brick wall.

Matching the system to your property

• Victorian or Edwardian solid brick: check for bridging and blocked ventilation first, then chemical injection.
• Cavity brick, 1930s onwards: chemical injection, with a look at cavity trays and wall ties while you are at it.
• Stone, flint or cob: cavity drainage membrane or electro-osmotic, as injection is unreliable.
• Basement or lower ground floor: drained membrane with a sump and pump, plus tanking where appropriate.
• Listed or in a conservation area: consent first, minimal intervention, and no cement render near the base of the wall.

Making the right call

Whichever route you take, fix the easy things before the expensive ones. Clear the gutters, repair broken downpipes, lower external paving to at least two courses below the DPC, and unblock air bricks. Then insist on proper preparation: skirting removed, plaster cut back to around a metre, bridging render hacked off, and the wall allowed to dry before replastering. Get three written quotations that describe the specification rather than just the price, and read the guarantee carefully. Spend the diagnosis money first, and the right damp-proof course will follow naturally.