Property Clinic: How can we get rid of the damp below our bay window?

Q. The area under my 1930s front bay window has been damp since we moved in more than 30 years ago. We got it underpinned three years ago with the supervision of a professional engineer, and replaced the double-glazed windows and also the cast-iron down pipe . We also got a groove cut into the underside of the window sill.

It’s still damp, especially under the joins of each windowpane. The room is sunny, and well heated and ventilated.

I keep bleaching and repainting it cosmetically, but it returns within a month to its white fluffy and bubbling mould. The uneven plasterwork is a result of amateur plastering.

I hope you can help. It’s a mystery to all.

A. The work done to date was most likely necessary and should have been effective in keeping the water out. However, I note from the photograph you sent that, in both the wall and sill, one can still see old cracks which have been previously infilled. While these are unlikely to be of structural concern, water has the ability to find its way through the smallest of cracks. This is generally more exaggerated when a crack occurs underneath a window, as any rainwater hitting the window will cascade down the glass and can be directed towards the cracks.

I would further caution that bay walls tend to be relatively thin, so, in the event of water penetration through a crack, the water can easily work its way to the internal face of a wall.

To deal with this, the cracks should first be raked out, infilled and properly sealed, using a good-quality mastic sealant. You will, however, need to be careful that the crack is infilled and finished in keeping with the character and texture of the wall, as very often the act of repairing and infilling a crack can actually make it stand out and become more noticeable.

Second, and perhaps more interesting, looking at the photograph, it appears to me that the plaster is a “bonding” type, which is very often used in remedial works, particularly in angled and curved areas, as bonding plaster is soft and easier to apply. The problem, however, is that bonding plasters are hydroscopic and tend to absorb moisture. They can absorb moisture from a surrounding damp environment or, as in the case here, even if there is some minor water penetration through the cracks. Bonding plasters should never be used in areas that are prone to dampness or that may have suffered dampness prior to the remedial works.

Assuming that the plaster is a bonding plaster, the most effective remedy would be to have it removed and have the wall replastered using a cement sand scratch coat and undercoat with a gypsum plaster finish. Alternatively, the bonding plaster could be removed and the wall dry-lined with an insulated plasterboard, thus improving the thermal insulation standard at the same time.

While this is clearly a nuisance problem, it is resolvable provided that the defect is correctly diagnosed. For the avoidance of doubt, you should have the wall checked to verify the type of plaster used; no doubt your local chartered building surveyor will be able to assist you with this.

Val O'Brien is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland; scsi.ie