Wall Dampness Treatment: Find the Source, Then Treat the Wall

Damp walls: find where the water is coming from

Damp patches, a tide mark near the floor, peeling paint, white powder and a musty smell are the usual signs of wall dampness. Wall dampness treatment starts with the source, because the same damp patch can come from four different places and each needs a different fix. Paint, putty or a coating applied before the source is stopped will blister and come off again. Five checks tell you which wall you have.

Find the right damp wall treatment

Match your wall to one route. Each route tells you what is done and takes you to its section.

Source first, wall second. Repainting the same damp wall every year buys a finish and never a fix.

Still unsure which cause you have? Compare rising damp, penetrating damp, condensation, leaks and salts side by side, or find your type of wall.

Damp wall treatment finder

Answer three questions for a starting recommendation, then confirm it with an Armstrong expert. Dampness is often misread, so where the answers do not fit one pattern, ask for a site assessment before buying material.

Rising damp or penetrating damp? How to diagnose a damp wall before you treat it

A tide mark, a dark patch and peeling paint can come from five causes, and each needs a different repair. Read the pattern first.

Why a moisture meter alone proves little

Salt makes a meter read high, so a high reading on a dry, salty wall is not proof of rising damp. Read at several heights: a steady fall with height that stops at a clear line supports rising damp, and a patchy pattern points to rain or a leak. For a doubtful wall, drilled samples can be tested for moisture and salt.

Rising damp treatment: restore the damp-proof course

Rising damp is groundwater drawn up through the base of a wall by the fine pores in brick and mortar. It shows as a band with a clear upper limit, usually 1 to 1.5 m above the floor, with salts at the top. A damp-proof course, or DPC, is the band built into the wall near the ground to stop it. In many houses the DPC is there but has been bridged.

1. 1
   Check for bridging first

   Look for soil, paving or a flower bed raised above the DPC, plinth protection that slopes towards the wall, an inside floor raised above the DPC, and plaster carried down over the DPC line. Removing the bridge is often the whole cure.Why: A DPC that is bridged does nothing. Removing the bridge is often the whole cure and is far simpler than any chemical treatment.
2. 2
   Rule out rain and leaks

   A moisture meter alone does not prove rising damp, because salt makes it read high on a dry wall. Check the pattern and rule out rain and plumbing.Why: A moisture meter reads high on a salty wall, so the pattern and the other sources have to be checked. Treating rain or a leak as rising damp wastes the work.
3. 3
   Form a new damp-proof course where it is missing

   Mark a continuous line along a mortar joint at least 150 mm above outside ground level. Cut a chase along it in short alternate sections so the wall stays supported, clean it out and pack it solid with RAINSEAL BXL, a ready-to-use acrylic blended bituminous grout for cracks and joints that bonds to brick and concrete and stays flexible.Why: Where no DPC exists, ground water keeps rising. A packed chase cuts that path.
4. 4
   Strip the salt-contaminated plaster

   Remove plaster to at least 1 m above the floor, or 500 mm above the highest salt line, whichever is higher.Why: Salt-laden plaster keeps drawing moisture from the air and breaks any finish over it, so it comes off well above the visible band.
5. 5
   Brush off the salts dry

   Use a stiff brush. Do not wash the wall, because water carries the salt back in.Why: Washing dissolves the salts and carries them back into the wall. Dry brushing removes them.
6. 6
   Replaster with a waterproofing compound

   Apply a bond coat of ROOFBOND SBR 401, then a sand and cement plaster dosed with CONPROOF 500 or CONPROOF LW 200. Do not use gypsum plaster or gypsum putty on a wall that has been damp.Why: The bond coat ties the new plaster to the old wall, and the compound makes the plaster resist the water that is left.
7. 7
   Let the wall dry

   A thick wall dries slowly, about one month for every 25 mm of thickness. Decorating too early is the main reason damp returns.Why: Thick masonry dries slowly. Decorating early traps the moisture and brings the damp back.
8. 8
   Seal the salts, then finish

   Apply RAPIDSTRONG SALT FLUSHER in two coats on the dried surface and leave it 24 hours. Then prime and paint.Why: The primer is the barrier between whatever salt remains and the new paint.

Wall seepage from rain, bathrooms, terraces and pipes: fix the source

When the water comes from outside the wall, the repair is made where the water starts. The damp wall is treated afterwards, once it has dried.

Damp walls by location: ground-floor, party, bathroom, wardrobe, basement and old masonry walls

The same cause shows differently on different walls. Find your wall, then follow the route it names.

Damp wall treatment from inside: coat, prime and paint

Once the source is stopped and the wall has dried, the inside face is treated so the finish holds. This is also the route where the outside of the wall cannot be reached.

1. 1
   Strip the wall

   Remove all loose paint, old putty, and any damp or crumbling plaster. Remove every previous coating where CONPROOF EP is to be used.Why: Coatings and paint only hold on a sound surface. Loose paint and crumbling plaster fail again and take the new work with them.
2. 2
   Repair the plaster

   Replaster damaged areas with a sand and cement plaster dosed with CONPROOF 500 over a bond coat. Cure it and let it dry.Why: A damp-proof coating needs a sound, even plaster to bond to. Curing and drying come first.
3. 3
   Treat the salts

   Where salts have shown, brush them off dry and apply RAPIDSTRONG SALT FLUSHER in two coats.Why: Salts that have shown will show again through a coating unless they are brushed off and sealed.
4. 4
   Apply the damp-proof coating

   Apply ROOFCOAT FLEXI 100 mixed with cement, or ROOFBOND RFX PLUS 210, in two coats, or CONPROOF EP in two brush coats 6 hours apart.Why: The coating stops moisture from the wall reaching the finish. Two coats close the pinholes.
5. 5
   Level with putty

   Apply RAINCARE WALL PUTTY in two coats of 1 to 2 mm, letting the first dry for 12 to 24 hours, and sand lightly.Why: The putty gives an even face for the primer and paint, and the drying time stops the second coat trapping moisture.
6. 6
   Prime

   Apply RAINCARE CP Primer and allow 4 hours.Why: The primer seals the putty and gives the paint an even, absorbent-controlled surface.
7. 7
   Paint with an anti-fungal emulsion

   SHINESHIELD Interior Emulsion and SHINESHIELD PREMIUM Interior Emulsion are anti-fungal and anti-algae interior emulsions.Why: On a wall that has been damp, fungus and algae return first. An anti-fungal paint is the last defence.

How long before you can repaint a damp wall? Drying time, finish and repainting

The wall sets the date, not the calendar. Once the source is stopped, masonry dries at roughly one month for every 25 mm of thickness: about 4 to 5 months for a 115 mm wall, around 9 months for 230 mm and around 14 months for 345 mm.

Ask the Armstrong applicator or distributor what moisture level is acceptable before the damp-proof coating and paint go on. To help the wall dry, stop the source, strip salty plaster and keep air moving.

Waiting times between Armstrong layers

These stated waits come on top of the drying time of the wall.

Choose the finish after treatment

• Inside: anti-fungal and anti-algae emulsions SHINESHIELD Interior Emulsion, SHINESHIELD PREMIUM Interior Emulsion and SHINESHIELD ULTIMATE Interior Emulsion over RAINCARE CP Primer. Clean off any fungus fully first.
• Outside face: prime with RAINCARE WP Exterior Primer, coat with RAINSEAL 200, a film that lets vapour escape, then SHINESHIELD Exterior Emulsion, SHINESHIELD PREMIUM Exterior Emulsion or SHINESHIELD ULTIMATE Exterior.
• Left visible: exposed brick and stone take RAINCARE WR or RAINCARE SWR instead of paint.

Damp proofing a new wall: the damp-proof course and plaster

Damp is easiest to prevent while the house is being built. Three things keep ground moisture and rain out of a new wall.

1. 1
   Build a damp-proof course at plinth level

   Lay a damp-proof course across the full thickness of every wall at plinth level, at least 150 mm above the finished outside ground. A cement mortar course dosed with CONPROOF LW 200 or CONPROOF No. 1 is the usual form, at 200 ml of CONPROOF LW 200 or 100 ml of CONPROOF No. 1 per 50 kg bag of cement.Why: A DPC across the full wall thickness stops ground moisture climbing into the walls for good.
2. 2
   Keep the DPC clear

   Do not carry plaster over the DPC line, and keep outside paving and soil below it.Why: Plaster or paving built over the DPC bridges it and undoes the work.
3. 3
   Add a waterproofing compound to the plaster

   Use CONPROOF 500 at 500 ml per 50 kg bag of cement, or CONPROOF No. 1 at 100 ml per bag, in the wall plaster.Why: The compound reduces the water paths through the plaster so less damp reaches the face.
4. 4
   Cure and prime

   Cure the plaster for at least 7 days. New plaster is alkaline, so prime with RAINCARE CP Primer before painting.Why: Cement needs moist curing to gain strength. New plaster is highly alkaline, and the primer protects the paint from it.
5. 5
   Protect the outside face

   Finish outside walls with an elastomeric waterproof coating. See external wall waterproofing.Why: Rain on the outside face is a major source of wall damp. An elastomeric coat keeps it out.

Damp wall inspection checklist: what to check on a site visit

Use this list to prepare for a visit by an Armstrong applicator or distributor, or to judge one. A visit that skips these checks has guessed at the cause.

1. 1
   Map the damp

   Mark the edge of each patch, date it, measure its height from the floor and photograph it. Repeat after rain.Why: Dated marks and photographs show whether the damp is growing, seasonal or tied to rain, which points to the source.
2. 2
   Ground and drainage outside

   Check the damp-proof course (at least 150 mm above outside ground) for bridging by soil, paving, flower beds or plaster, and look for downpipes that discharge at the wall.Why: Raised soil, paving and poor drainage bring water against the wall and bridge the DPC.
3. 3
   The outside face

   Look at cracks at brick and RCC joints, sill slope and drip groove, chajja slope, frame gaps and the parapet top.Why: Cracks, open joints and flat sills let rain in. Finding them points straight to the fix.
4. 4
   The other side of the wall

   Check wet rooms, sinks, washing machines, overhead tanks, air-conditioner drain lines and the terrace above, and ask for a pressure test of the supply lines.Why: Leaks from pipes, tanks and wet rooms behind the wall look just like rising damp.
5. 5
   Plaster and salt

   Tap for hollow sound, check for gypsum plaster or putty, and note whether salt returns within days of dry brushing.Why: Hollow plaster, gypsum and returning salt tell you whether the plaster can stay or has to come off.
6. 6
   Wall type and thickness

   Measure thickness at a door opening and note any oil paint or tiles. Thickness sets the drying time.Why: Thickness sets the drying time, and oil paint or tiles trap moisture, which changes the method.
7. 7
   A written finding

   Ask for the cause, the height and the area in writing before any treatment is proposed.Why: A written cause, height and area stops the treatment being chosen before the problem is understood.

What not to do on a damp wall

• Do not tile over a damp wall. Moisture stays behind the tile.
• Do not render over salt. Fresh render over a salty wall takes the salts through. Strip the contaminated plaster, treat the salts with RAPIDSTRONG SALT FLUSHER and replaster with CONPROOF 500 over a bond coat.
• Do not use waterproof putty on the inside face against outside water. The repair belongs on the outside face.
• Do not seal a damp wall with an impervious paint. The moisture is pushed out higher up the wall.
• Do not strip a whole house for one patch. Remove plaster only where it is salt-contaminated.

Wall dampness treatment chemicals and products by job

How much material for 100 sq.ft of damp wall

Approximate quantities worked out from each product's stated coverage, for a wall 10 ft long treated to its full 10 ft height. Rough or porous walls use more. An Armstrong expert confirms the final quantity after seeing the site.

Standards for damp proofing walls

• IS 3067:1988 covers damp-proofing and waterproofing of buildings, including damp-proof courses at plinth level.
• CONPROOF LW 200, CONPROOF 500 and CONPROOF No. 1 are integral waterproofing compounds to IS 2645:2003.

Damp wall treatment material calculator

Enter the size of the damp wall and choose a coating to see how much material it needs. The calculator gives quantities only. Use the button at the end to talk to an Armstrong expert about your quantity.

Seven mistakes that bring wall dampness back

1. Painting over a damp wall. The paint traps the moisture and blisters within months.
2. Treating the wall and not the source. A leak from a bathroom, terrace or pipe has to be fixed there.
3. Leaving salty plaster on the wall. The salts keep drawing moisture from the air.
4. Using gypsum plaster or putty on a wall that has been damp. It holds salt and moisture.
5. Washing salts off with water. The salt dissolves and goes back into the wall.
6. Leaving the damp-proof course bridged. Raised soil or paving carries water round it.
7. Decorating before the wall has dried. A thick wall takes months.

Wall dampness treatment questions and answers