Do I Need a Damp Proof Course Injection?

An injected DPC is a proper treatment when it’s the right treatment. Where a wall genuinely has no effective damp proof course and moisture is rising from the ground, a modern DPC cream injected into the mortar course, done properly with the right replastering, works. I install them. What I don’t do is recommend one for every damp patch at skirting level.

Before anyone drills a hole, these should be checked:

• Bridging. Is the outside ground, a path, a patio or a flowerbed at or above the DPC level? Building guidance generally looks for the DPC to sit at least 150mm above the outside ground. Is render carried down over it? Is the cavity full of mortar droppings and rubble at the base? Clear the bridge and an existing DPC can often do its job again.
• Leaks. Central heating pipes in the floor, waste pipes, a leaking shower tray, a dripping outside tap.
• Penetrating damp from gutters, downpipes and pointing.
• Condensation, especially behind furniture and on cold walls.
• Old salty plaster. Plaster contaminated with ground salts can keep pulling moisture from the air and reading ‘damp’ even after the original problem has stopped.

If rising damp is confirmed, the injection is only half the job. The salts already in the wall and plaster don’t disappear when the new DPC goes in, so the old plaster normally needs removing beyond the affected area and replacing with a salt-resistant replastering system. Skip that, and the wall can stay damp-looking and the decorations will keep failing, which is when people decide ‘damp proofing doesn’t work’.

A few things to ask any company quoting you:

• What evidence shows it’s rising damp and not something else?
• Have the outside ground levels and cavity been checked?
• What replastering system is included, and how far up and along the wall?
• What guarantee is offered, and is there an insurance-backed option?