Damp Walls: How to Treat and Repair Them

Repairs to walls, apart from dealing with holes and cracks before redecorating, are associated mainly with dampness, and damp conditions which arise from one of two sources — (a) from defects to exterior walls, allowing moisture to enter the structure, and (b) interior conditions leading to condensation of steam from hot water used in kitchens and bathrooms.

Damp Walls:

The construction of modern homes is such that every possible precaution is taken to prevent water entering the structure. A damp wall is not a ‘natural’ condition which the occupant has to put up with in wet weather; it is due to a defect that can and should be found and put right. The location and extent of the dampness usually indicates the source. Patches of damp showing through wallpaper covering chimney-breasts indicate a fault in the construction of the chimney, as also patches of damp on walls of adjoining rooms backing the fireplace. This may be due to decayed pointing of the joints between the bricks of the chimney-stack above the roof; the flashing inserted in the base of the chimney-stack just above the roof may have decayed; or there may be a slate or tile missing or cracked in the roof covering near the chimney.

It is possible, but not very likely, that water enters the chimney-pot, or the cement flashing capping the top bricks in the chimney-stack may have cracked. Another possible source of entry is a cracked exterior wall, where the chimney-breast is on an outside wall, due to subsidence, and another possibility is the loosening and dislodgement of a brick during sweeping the chimney.

If the patch of damp is small and isolated on a chimney-breast or backing wall this probably results from a single very porous brick in the breast. This may be cured by replacing the brick with a harder one, and it is a simple matter to chop away the plaster over the damp patch, clear the mortar round the defective brick with a thin brick chisel and old hacksaw blade, and extract the defective brick. The replacement brick should be hard and non-porous.

To test the absorbent properties of a brick, weigh it, place it in a bucket of water for 24 hours, and reweigh it; the wet weight should not be more than 10 per cent greater than the dry weight. The replacement brick is bedded into a mortar made of one part Portland cement and three parts sand, mixed stiffly with water. The cavity should be well wetted with water, also the replacement brick; the sides of the cavity should be ‘buttered’ with the mortar, spread with a trowel with the side thicknesses of the bed tapering from back to front, being thickest at the back of the cavity. The brick is then inserted, tapped firmly into place and any open parts of the joint filled with mortar. The plaster then needs to be refaced.

A damp patch descending under a windowsill indicates a fault in the sill or its bed. The sill may have rotted or shrunk, leaving a gap underneath through which water may enter, or the jointing between the window-frame and brickwork may have decayed and the remedy is simple to effect by renewing the joint and pointing. Damp stains ascending from the top of a window or doorway are also due to frame defects or decayed jointing.

Damp patches on walls descending from ceilings of top-floor rooms are almost certainly due to a break or hole in the roof covering, and these are easily located and repaired. Ascending damp patches on the base of walls of lower-floor rooms are due probably to defects in the damp-course, necessitating a builder’s repair. Odd small patches of damp on walls, for which there is no apparent reason, are usually due to a porous brick, to some obstruction in the cavity, which relays dampness from the outer wall, or — possibly — to a badly fitted metal tie from the inner wall to the outer wall. If the damp area is not extensive, the paper should be stripped and the plaster coated with a damp-proofing bituminous compound, of which there are several brands obtainable from local builder’s merchants.


Damp-proofing compounds should not be regarded as ‘cure-alls’ for damp wall defects; it is much better, and far more sensible, to get to the root of the trouble than to apply a surface treatment. This is especially necessary where damp stains adjoin woodwork; if the surface is sealed an apparent cure may be effected, but it is more than likely that the damp, unable to penetrate the sealer, will spread to attack the woodwork. To deliberately set up such a condition is asking for a great deal of latent trouble from wet and dry rot.

05. July 2011 by admin
Categories: Damp, Internal Repairs | Tags: , | Comments Off on Damp Walls: How to Treat and Repair Them


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