How to treat rising damp in walls and floors (UK guide)
Rising damp is the one type of damp that climbs. Ground moisture is drawn up through the pores of a wall or floor — like water up a sugar cube — because the original damp-proof course (DPC) or damp-proof membrane (DPM) is missing, bridged or has broken down. Because that moisture carries ground salts, rising damp leaves a distinctive salty, crumbly tide mark near the base of the wall.
Before you treat it, make sure that's what you have — low-level damp is often condensation or penetrating damp instead. Our diagnosis guide walks you through it.
The signs of rising damp: ground-floor walls only, a tide mark up to roughly a metre high, white salt deposits, flaking plaster and peeling paint near the skirting — and it's present whatever the weather.
Step 1: Confirm it — and rule out a bridged DPC
Sometimes there is a working DPC, but it's being “bridged” — soil, a raised path, render carried down over the DPC, or debris in the cavity lets moisture skip past it. Check the outside: the ground level should sit at least 150mm below the DPC, and nothing should bridge it. Fixing a bridge can solve the problem without any chemical treatment at all.
Step 2: Create a new damp-proof barrier in the wall
If the DPC has genuinely failed or was never there, you need to re-establish a barrier against upward moisture. The internal face of the affected wall is treated and sealed with a waterproof / damp-proof coating so ground moisture can no longer track up into the plaster and finish.
Our Waterproof Blue Liquid DPM brushes or rolls on to form a seamless, flexible damp-proof membrane on walls and floors — ideal for creating a continuous barrier where the original DPC/DPM has failed. It's also available in a 5kg size for smaller areas. The liquid application means it ties walls and floors together continuously, with no joints for moisture to find.
Step 3: Don't forget the floor
Rising damp isn't only a wall problem — a failed or absent floor DPM lets ground moisture up through the slab. A liquid DPM applied to the floor, lapped up and joined to the wall treatment at the perimeter, creates one continuous tanked barrier. Continuity at that wall-to-floor junction is what stops moisture simply going around the edge.
Step 4: Re-plaster correctly
Old plaster contaminated with ground salts will keep attracting moisture even after the damp is stopped, so it should be hacked off and replaced. On older, solid-walled and heritage buildings a breathable system is usually best, so the wall can still release moisture rather than being sealed tight. A breathable primer such as Lime Prime prepares backgrounds for lime and breathable finishes on these buildings.
What not to do
- Don't just replaster over salty plaster. The salts will pull moisture back and the damp will reappear.
- Don't ignore the outside. A bridged DPC or high ground level will defeat any internal treatment.
- Don't seal a heritage solid wall with a non-breathable system unless the whole build-up is designed for it — trapped moisture can migrate elsewhere.
Related reading
- Rising, penetrating or condensation damp? How to tell the difference
- How to stop penetrating damp through an external wall
- What causes damp walls and what is the solution
Frequently asked questions
How high does rising damp go?
Usually up to about a metre — moisture can only climb so far before gravity and evaporation balance out. Damp higher than that is more likely penetrating damp or condensation.
Do I always need a chemical DPC injection?
Not always. If the DPC is simply bridged (high ground, render over the DPC), removing the bridge can fix it. A new barrier is needed only where the DPC has genuinely failed or is absent.
Why does the plaster need replacing?
Ground salts soak into old plaster and keep drawing in moisture from the air. Stopping the damp isn't enough if the salt-laden plaster stays on the wall.
Dealing with rising damp in a wall or floor? Our technical team can specify the right liquid DPM and finish for your building.