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We live in a semi-basement flat and there are small patches (c.30cm sq) of wall in one area of the flat (sub-ground) which have just had the paint disintegrate off of the wall !! It's not peeling off exactly (although there are bits that have peeled), it looks in the main just like flour or sugar on the wall and the paint is just like dust on the floor !! We aren't sure what it is -- and how to treat it ... some of the wall is fine still - but the 'floury paint' patches are by the skirting board, but others are high up on the wall (near the boiler, although it's a combi) -- now the paint has come off in patches it's showing bare plaster that seems to have little tiny speckle holes in it --- I would like to redecorate the room, but I don't want to repaint over it if it is going to come back again !! Do I need a stain/damp/condensation paint maybe ? Any advice gratefully received !! Thank you :D

The sugary flour substance you talk about is salt. This is known as effloresence. This occures when moisture passes through the wall containing and picking up soluble salts such as sulphates. When the dampness evaporates the salts are left behind.

As this is a cellar area the dampness affecting the wall could be rising damp (as damp proof courses cannot function below ground) and lateral dampness possibly under hydrostatic pressure. In other words the damp is coming through the wall.

The solution here would be to 'tank' the wall, otherwise know as structural water proofing.

There are many methods available to remedy this problem. These have improved greatly in recent years. First of all forget brushing or painting something on the wall plaster. This will not work if you understand the science behind the problem. They will only temporarily mask a problem.

Hacking off wall plaster and appliing a cementisious slurry may help but these are not the best methods available although they are cheap. Cavity drainage systems are the norm these days.

Note: There is a british standard for water proofing areas below ground level - BS8102. 75% of specialists out there will not fully inform you of this and explain what it means. A number of them may never have heard of it.

If you are wanting to repair the problem properly seek professional advice from a CSSW surveyor. Ask the specialist what BS8102 is. If they dont know, send them packing. If they do, they should be able to provide you with a damp proofing system that is recognised as being up to a reasonable standard for its purpose. This means you are more likley to get a damp proofing system that works, therefore your guarantee will be worth something and you may be able to get an insurance backed guarantee for the work. Also having a system that complies with a set standard, or as closely as possible to the standard should mean that there is not room for argument should a surveyor question dampness if you sell your property.

Damp proofing areas below ground level to comply with BS8102 is expensive which is whay many people choose for a bodge repair by a cowboy builder or un qualified specialist and then they have the cheek to complain if it doesnt work!

Thank you so much for your helpful and very comprehensive reply -- WOW -- that was really appreciated.