Diagnosing Damp Problems

[Nigel Watts]

Following some subsidence caused by an over-thirsty Eucalyptus tree a number of cracks opened up in our rear two storey extension (below).

The tree was got rid off, the cracks then shrank somewhat and after they had stabilised they were filled.

After the rain of the recent days the upper room has a strong smell of damp plaster. Above it is a flat roof with parapet walls. There is no damp discolouration anywhere, but the side of the room most effected by the cracking has built-in cupboards so cant be fully inspected. Modern plasters and finishes were used. Most of the exterior wall is 9 inch brickwork with no cavity, faced on the inside with expanded polystyrene-backed plasterboard.

I would like to diagnose and fix the problem. Any ideas? Should I buy a damp meter?

 

[robgil]

have yuou checked the flashing on that lean-to?

 

[christianb]

Damp meters are pointless. They only measure electrical conductivity which varies more with the material than its moisture content.

I'll wager 10p of my wife's money that the problem lies with that equally pointless thing: the flat roof. They're going to leak sooner or later. And "now" in your case, I suspect.

 

[Nigel Watts]

I think the flashing where the parapet joins the main house may well be the problem. This is where the crack was. It has been filled with what looks like a cement fillet. A lead flashing over the top perhaps?

As for the flat roof we don't have much option. The house originally had small flat-roofed extension here which we got permission to extend by about four feet. Being grade II and in a conservation area we are not allowed to any further up or sideways. The flat roof has new roofing felt on it. This clearly wont last for ever, but I think it is in reasonable nick.

 

[Matt Green]

Damp meters are pointless. They only measure electrical conductivity which varies more with the material than its moisture content.

If the material is constant (modern plaster), then the damp meter should show a pattern of relative conductivity or resistivity depending on the meter. Taking readings around the perimeter of the wall/ceiling junction and looking for patterns of dampness doesn't seem like waste of time if there is a suspected leak that warrants repair.

M.

 

[Flyfisher]

I think the flashing where the parapet joins the main house may well be the problem. This is where the crack was. It has been filled with what looks like a cement fillet. A lead flashing over the top perhaps?

We once had a house with a flat roof that started to leak at the joint with the house. I used a 'stick-on' flashing that looked like lead from a distance. Hardly a traditional 'period repair' (it was a 70s house and I was younger and less-wise!) but it was quick, functional and durable.

I can't remember what it was called but a quick search found this flashing tape, which looks like the stuff.

With all the rain we're currently having, I'd want to stop the leak as a matter of priority and worry about aesthetics in due course - not that you'll see anything along the top of that roof anyway.

 

[philpjuk100]

I used this at a prievious property where roof tiles butted up against a wall and it was there for about five years until we left with no problems,I think that a "brush on" primer was used first.phil.

 

[Penners]

Flat roofs are notorious for leaking. And a cement fillet at a critical joint sounds like a recipe for problems at some time or another. Flashing of some sort may be the answer, unless the possibility exists for water to pool at the bottom edge of the flashing and creep underneath.