Penetrating damp problem

[JasperW]

Hi,

The very top section of a bay in our c1850 town house has had a damp problem for a number of years . We had both the bay flat roof and parapet replaced, this is the time line of the works

1. Flat roof replaced
2. Room plastered and decorated
3. Damp appeared
4. Parapet replaced
5 Damp problem better but still there

I am very happy with the work done on the roof and parapet and I am sure that the water is not coming from there now and is probably penetrating (or condensation?).

I have finally decided to bite the bullet and try and fix it. Obviously one issue is that the plaster has been subjected to damp and needs to be replaced. As you can see from the picture I have cut a small hole in the ceiling and hacked away the plaster to see what is underneath. Most of the wall has a cement render which is showing completely dry on my damp meter.

The render however stops about 10cm from the ceiling and above that is just normal plaster which is showing a reading of 15% on my meter. This backs on to a wooden beam providing support for the joists. This is showing a reading of about 4% on my meter.

My original plan was to render with waterproof cement and then re skim. I did this a couple of years ago with a damp patch in the basement and it has worked a treat. I know with old houses though that render can also trap damp in causing wood to rot so I am wondering if this is the best course of action now (It may also be why the render stopped before reaching the beam).

It may just be that replacing the damp plaster will be enough. I was also thinking that I could fit some coving and leave an air gap over the wood which would help air circulation.

Any thoughts or advice?

Jasper.

 

[vinn]

Photos of a larger context in the room, and external pics of the bay, and the bay roof - do you have any main roof lights to photo from?
Is there a wood lintel over the front bay window? You might have to remove more plaster to find out - remove the plaster at the bearing ends of the lintel.

 

[JasperW]

Attached are a couple more photos. Only thing to say about the roof shot is that the felt extended under the concrete cap stones which drain on to the flat roof. There is no gutter. I added the waterproof tape a few years later to see if it made any difference (it didn't). I have had the inspection hole in the ceiling open for a few weeks now and the brickwork in the roof space is completely dry, even after heavy rain.

 

[vinn]

Do you have cover flashing or is that simply the tape you added?
If you removed all the tape and cleaned up the moss and any puddles - could you post a close up of where the bay roof meets the parapet?
There are obvious water staining patterns on the material covering the parapet.

How is water drained from the bay roof?

The exposed beam is probably the wall plate? There are signs of moisture penetration on the exposed woodwork.

 

[JasperW]

The flat roof has the asphalt running up the side of the wall, the builders who repaired the parapet found that a lot of the mortar had crumbled so they rebuild the top couple of courses of brickwork. They then put felt over the brickwork which overlaps the asphalt before putting the coping stones on top. The flat roof drains from one corner into a gully. I took quite an interest when then did the work and certainly I cant really fault what they did. One of the problems is that the parapet work was done after the damp problem was discovered and I never got around to replacing the plaster. The water proof tape was put they by me about 5 years ago when I was worried that rain might be

Despite all the heavy rain we have been having the the exposed brick work above the beam seems completely dry and is not registering anything on my damp meter. What is interesting is that the 10cm strip of plaster above the render is showing @15% on the damp meter while the brick work behind is only 1-2%

What I am worried about is if I put a waterproof cement render next to the beam, will I trap water in the beam causing it to rot?

 

[JasperW]

I have also noticed that a couple of the bricks above the beam are crumbling and quite damp. Now wondering if I should replace them.

 

[^woody^]

Your damp meter is calibrated for timber. Any readings from any other material are not accurate.

If the timber is less than 20%, it is dry.

 

[JasperW]

Thanks, I had assumed it was basically dry. My real concern and the original reason for starting this thread is to find out if it is OK to but a waterproof render over a wooden beam? My in-laws have a very old timbered house and when they first bought it someone had put a cement render on the outside covering the beams which has cause some of then to start rotting. So I am rather nervous about the possibility of making the same mistake, OK in my case the render will be on the inside not the outside.

 

[catlad]

It just looks like its festering to me ie. large radiator no insulation and no ventilation.

 

[JasperW]

A bit of a follow up on this....

In the end I hacked back all the damp plaster back to the original render or brickwork then used waterproof cement to extend the existing render to above ceiling height. I then re plastered painted etc.

So far after several months of London rain there has been no sign of the damp returning so I am hoping that this has fixed it.

 

[vinn]

Thanks for coming back and saying what happened next. I hope it continues successfully.

 

[JasperW]

5 Years on and still no damp!