Work out which kind of damp you have
There are three, and confusing them is why so many repairs do not last.
**Rising damp.** Water from the ground travels up the wall. You recognise it because it appears low down, forms a roughly horizontal band about half a metre above the floor, and comes with white salts that blister and lift the paint. It is the classic problem in ground floors, basements and country houses.
**Penetrating damp.** Water gets in from outside through a crack, a failing roof or an open joint. It shows as a localised patch and gets worse when it rains.
**Condensation.** No water is getting in: it is vapour from inside the house condensing on cold surfaces. It appears in corners, behind furniture and on bathroom and kitchen ceilings, almost always with black mould. It is fixed by ventilating and insulating, not with a mortar.
A simple test to tell penetrating damp from condensation: tape a piece of clear plastic tightly over the patch and leave it a couple of days. If the face against the wall is wet, the water is coming from the wall. If the outer face is wet, it is condensation.
Repairing rising damp
This is the most common case, and the answer is a renovating render. These are porous mortars that let the moisture in the wall evaporate through their surface instead of trying to seal it in, and that hold the salts inside without breaking down.
The method:
- Hack off the affected finish back to brick or stone, and go at least 50 cm above where the stain stops. Not going high enough is the most repeated mistake: the damp carries on and reappears just above the repair.
- Brush the wall down thoroughly and remove all salts and loose material.
- Dampen the substrate without soaking it.
- Apply a levelling coat if the wall is uneven.
- Apply the renovating render at the thickness the manufacturer specifies, usually two centimetres, in one or two coats.
- Let it cure for the stated time before painting.
And one detail that decides whether the repair lasts: the final paint has to be breathable, either lime or silicate based. Put an emulsion over it and you have sealed the wall again, and the problem returns.
If you can also cut off the water rising from below, better still: there are resin injection treatments and damp-proofing creams that create a horizontal barrier at the base of the wall.
Repairing penetrating damp
Here the job is to close off the way in, not to treat the stain. Find the source — a crack, a broken gutter, a roof or an open joint — repair it, and then waterproof the area from outside. Repairing only from the inside just postpones the problem.
A repair mortar will do for stable cracks; if the crack moves, you need a flexible sealant or a reinforcing band.
Dealing with condensation
This is not a materials problem, it is an air problem. Ventilate daily even in cold weather, do not dry washing indoors without ventilation, fit extractors in the bathroom and kitchen, and pull furniture slightly away from cold walls. If it persists, the insulation of the wall needs improving: mould appears wherever the surface is cold.
Clean off any existing mould with a fungicidal product before painting. Paint straight over it and it grows back through the paint.
Common mistakes
- Painting over it without treating the cause.
- Hacking off too little height and leaving the repair just below where the damp reaches.
- Using an ordinary cement mortar, which saturates with salts and breaks down.
- Finishing with an emulsion paint.
- Treating condensation as if it were rising damp, and the other way round.
Come in and we will look at it
Tell us where the patch appears, how high it reaches and whether it gets worse when it rains, and we can nearly always tell you which type it is and what you need. We stock renovating renders, waterproofers, water-repellents and breathable paints.