What’s the story with the damp patch in our concrete wall?

Property Clinic: Damp has appeared on an internal wall in our 1930s home, but where is it coming from?

 

Text updated on Friday, July 27th

My house was built in the 1930s and is constructed from poured concrete. We added double glazing and some internal insulation on moving in around 10 years ago but have not made any major changes since. Over recent months, a damp patch has appeared on the lower half of an internal wall. It’s about 8-10cm in diameter and is towards the bottom of the wall but does not reach all the way to the floor. There is no evidence of damp on the floorboards on either side of the wall. There are radiators on both sides of the wall and no evidence of any leak or water escaping from either of these. What could possibly be causing this and how should we go about treating it?

I’ve completed a number of inspections on poured concrete buildings and it is a good form of construction. The principal is very simple, with formwork (shuttering) used as a mould for a structure into which fresh concrete is poured.

Once the concrete is set/hard the formwork/mould is removed and the new concrete wall remains.To support the formwork during construction and to create the mould/space into which the concrete is poured the formwork is usually pinned with steel bars or timber wedges running from either side.

The steel support bar/spacer will have a protective sleeve to allow for its removal once the wall is set/hard. The result of the support bar (a number of them will be in place within a section of wall) is a hole within the wall about 15mm-25mm in diameter. This form of construction can be utilised for both the internal and external walls.

The first step is to determine the internal wall construction as this will allow a greater understanding of the elements used originally and why the damp stain is manifesting. The description of the damp patch indicates a localised issue which is unlikely to be rising damp. You are right to check the heating system as it will be a process of elimination to ultimately determine the cause of the dampness.

If possible, taking moisture readings of the area will also help to determine if the damp patch is as a result of water penetration or cold bridging due to limited insulation. Your local chartered building surveyor would be able to assist with this. Once there is a better understanding of the cause of the issue a solution can be put in place to remedy it.

Andrew Ramsey is a chartered building and project management surveyor and member of the Society of Chartered Surveyors Ireland (SCSI)

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