DIY Doctor UK DIY Forums

My first post on the forum so I hope I don't contravene any rules :D

A friend has aked me for help on some 'damp patches' on her walls. The two walls it affects are external walls in a second floor modernish flat. There are no signs of damp nor leaks in the loft/external guttering.

From what I can gather, the walls have been dot and dabbed and the 'damp patches' look like the areas that adhesive is usually applied (straight lines in a chessboard style). There are no similar appearing marks joining the noted patches and none on the stud walls and the marks are from the top of the wall to the foot.

The walls have been painted several times with various colours of emulsion but the marks are still there. I can't tell if the walls have been plastered but regardless, I've never seen these marks before.

Any idea what it could be causing it? How can the marks be removed/covered? I suggested using a primer or possibly skimming but that seemed excessive.

There has to be something causing this, it wont be condensation if its coming through the dabs behind the plasterboard, you need to figure out where its coming from and its usually from the gutters, skimming the wall wont help it will just come through that. You need to sort the problem really but if you just want to hide it then I used to roll gloss paint on the areas let it dry then paint as normal on top, this is just a bodge it and leg it option really but it will do the trick to hide it if shes selling the house!

Hi, I agree with Dan on this you really need to get it sorted or the problem will probably only get worse. If you want to hide it then I would do a similar thing to what Dan has said but mix some gloss and undercoat together about half and half and paint this on. By putting the undercoat in it will help the emulsion grip the walls better. Good luck with it. Shed.

Thanks for the replies but I can't see where the damp would stem from. There's no reason for any loft pipework to run near the affected walls and the guttering looked fine - I can't imagine it being caused by bad guttering anyway.

I'll try to get in the loft although not sure she has access.

Many thanks.

I would guess that the young lady is suffering from condensation.
Probably what is happening is, the outer walls are cold.

The gap between the plasterboard and the outer wall is acting as a warm air buffer.

The dabs are acting as a heat bridge between the warm air of the room and the outside air.

The water vapour in the room is attracted to the cold patches where it condenses and gradually moves, through the plasterboard, through the dabs and through the wall to the sky.

There are several ways to deal with this.
One raise the temperature of the room to enable the air to hold more water vapour and to warm the plasterboard to in turn warm the dabs, doing this will direct the water vapour to the next coldest spot probably the windows, where it will condense.

To open a window from time to time to let the water vapour out.

To buy and use a dehumidifier, this will provide a colder spot in the room, colder than the damp patches and the water vapour will condense into the dehumidifier bucket.

Keep the bathroom and kitchen doors closed, to keep the water vapour in those rooms after cooking and washing.

Fit and use extractor fans in the kitchen and bathroom to move the water vapour outside.

Note: We all breath out 100% water vapour full breath and we all sweat, this needs to go somewhere, usually it condenses onto the nearest cold wall or window.

Keeping the room temperature steady goes a long way to reducing condensation.

The worse thing you can do is, turn the heating off, or down. Warm air holds lots of water vapour, cold air drops its water vapour onto/into the nearest cold surface.