Patch Repair Render

[jamesingram]

Hi,

I have a 2 storey side extension built around 1998. From the planning application I think it's a solid wall made with 250 Celcon Solar Blocks.

There is a localised damp patch inside the house on a downstairs external wall near the ceiling. The problem is not evident upstairs and there are no water pipes in that area of the house. On the outside of the wall in the same area as the damp there is a horizontal crack in the render which has some algae in in. When I tap the render near the crack it sounds hollow for about 20cm top and bottom of the crack, but when I took a hammer and chisel to it, even the hollow sounding stuff was quite well attached and took some elbow grease to get it off. I was expecting it to more or less fall off.

Once I'd got some of the render off, the blocks underneath seemed damp so I think the crack is most likley to be the problem. Let me know if you think that sounds unlikley though.

My question is, would you take off all the hollow-sounding render and re-render a strip about a foot wide, or just scrap out the crack with a chisel and fill with exterior filler?

Any other help or advice greatfully received as first time I've done this.

Thank you!
Jim

 

[motorbiking]

Have you checked the loft to see if there is a water leak from the roof running down in between the plaster and the block. I had something similar due to a poorly installed piece of membrane. It might be that your cracks etc. are a symptom not the cause.

 

[jamesingram]

I'll check that - thank-you!

 

[jamesingram]

Would expect damp higher up the wall if that was the case though? Wall is dry until ground floor.

 

[vinn]

Posting a photo of the outside might help?

In the meantime go around all your rendered walls and carefully examine the surfaces for any irregularities - little ridges or hairline cracks.

When did the damp first appear - is it constant or changing?

 

[motorbiking]

in my case it was running between the plasterboard and the block in the gap made by the dabs. foil back board meant it didn't show at all upstairs.

 

[jamesingram]

 

[jamesingram]

Thanks Vinn and Motorbiking. I've posted some images. The problem area is to the front of the house. The 2 bigger bits of the crack are where I've gouged in with a chisel to see if the blocks underneath were damp or if the render was loose. The render sounds hollow when knocked but takes some welly with a club hammer and chisel to come off.

I don't think it's coming from above as the plaster is straight onto the blocks so there's nothing for moisture to run behind.

I moved into the house 3 years ago and there was not damp noticeable or from the survey, although the cracks were present in the render. I had a loft conversion, ground floor extension and re-wiring and plumbing - a lot of work. It's possible the work loosened the render. About a year ago the damp was visible. I called the builder and he reckoned it was condensation due to defective cavity walls. I don't use the room much so was sorting it in my own time (2 toddlers on the go). It's got progressively worse though, there is mould inside and I can see the plasterboard in the ceiling is getting damp. Need to sort it before that gets worse and causes more damage.

Thank you!

Jim

 

[jamesingram]

Wall thickness is 29cm including plaster and render. I think that's too thin for cavity walling?

Even if not why would the blocks be damp if the problem was condensation?

Also - the problem has got worse over the summer and I would have thought condensation would be more of a problem in winter with heating and cold walls.

Thank you again!
Jim

 

[jamesingram]

The other thing I thought of was running a hosepipe into the gutter and seeing if any of it runs down the wall, exacerbating the problem.

 

[vinn]

Thanks for the photos & additional info.


Damp insulation in a cavity wall can cause all manner of difficulties but you claim to have 250mm blocks.
When you opened up the crack was there a further crack in the masonry behind the render?
Was there any external insulation behind the render?
How thick was the render?
Did you see blockwork?
Algae would indicate a difficulty thats been there for a while.

You have a "snub hip" & it has a particular kind of rafter framing - you say that you installed the dormer. Were the calcs and framing drawings for the dormer done by a SE? Dont be alarmed, I'm simply covering variables not spotting structural faults.

Why not post a photo of the inside damp area. You appear to have penetrating damp not condensation.

There are recent threads with photos of render cracks similar to yours.

Your water testing the gutter is good and photos could be taken up there - but if you are not used to ladder heights and dont have anyone to foot the ladder dont bother or get a professional in.

 

[jamesingram]

Cheers Vinn. Internal photo included.

• I could not see a further crack in the masonry behind the render.
• No external insulation behind the render
• Render is about 3cm thick.
• I could see the grey blockwork behind the render and it was these that felt a bit damp.
• We did have a structural engineer and architect for the loft extension and ground floor work.

Will read up on the recent threads and do a gutter test Saturday morning.

Thanks again!
Jim

 

[jamesingram]

Hi,

Checked the gutter and some mortar had fallen into the downspout with dirt building up behind it. In heavy rain it would shed water down the wall. Have removed the blockage now.

Drilled the wall to check it is solid and not cavity. It is a solid breeze block wall. The wall is wet inside so it's definitley penetrating damp and not condensation.

So, just need to fix the crack now. Should I hack off anything that sounds hollow even if it is stuck on quite firmly or just take off render that is not firmly attached and fix the crack even if it sounds hollow?

Thanks!
Jim

 

[jamesingram]

Hi,

Would be great if anyone can provide advice on this. On the outside of the wall in the same area as the damp there is a horizontal crack in the render. When I tap the render near the crack it sounds hollow for about 20cm top and bottom of the crack, but when I took a hammer and chisel to it, even the hollow sounding stuff was quite well attached and took some elbow grease to get it off. I was expecting it to more or less fall off.

My question is, would you take off all the hollow-sounding render and re-render a strip about a foot wide, or just scrap out the crack with a chisel and fill with exterior filler?

Thanks!
Jim

 

[vinn]

As your photos show, you obviously have a number of cracks in the render.
At first I thought the vertical line was a crack then I wondered if it was a movement joint now I'm thinking:
Does the 1998 extension tie-in at the vertical line in the photo?

If rubble was blocking the down pipe then check the gulley trap that the down pipe discharges into for debris.