Blown plaster caused by disintegrating brickwork

[Phil Addison]

Back to DIY after a long break and I'm easing in with what should be (have been!!) a simple job, redecorating the spare bedroom.

However, after steaming off the old wallpaper I discovered some very large swellings of the plaster. By large I mean about 9" to 12" diameter and protruded about an inch at the centre from the surrounding flat wall surface.

On investigation with hammer and chisel I found several soft bricks where the surface in contact with the plaster had disintegrated to powder, and this had expanded behind the plaster blowing it off. I've hacked out the softest stuff and after 1/4" to 1/2" deep reached reasonably sound brick, although still soft, i.e. I can scratch a line into it with the edge of a cold chisel.

The wall is the end wall of a Victorian extension, single skinned about 12" thick. So far, on a section of wall 8'6" high by 3' wide, I've uncovered 4 of these crumbly bricks. I suppose they are from a stock of sub-standard bricks that the Victorian builder used up, a few into each wall. The bricks of course are imperial 9.5" x 3.25".

The question now is what to do about it. I'm wondering about a) cut out the complete bricks and replace with good ones, b) Chisel off as much soft stuff as I can, and refill with cement mortar, c) Drill a few holes into them and inject silicone waterproofing.

Followed by a new scratch coat and plaster skim, having hacked off all loose old plaster, which probably means all of it!

Suggestions as to the root cause and how best to fix it gratefully received!

There are photos on my OneDrive site:

Phil

[Phil Addison]

Don't think so, there was no sign of damp on the wallpaper covering the blown bits.

The blown areas were over each of the exposed bricks in the inside photo. Also the soft crumbly bricks are a distinguishably different in colour, orange, from the others.

I've taken a photo now from the garden and annotated where the crumbly bricks are with corresponding annotations on the inside view.

The four blown areas were over the bricks numbered 1-4 on the inside view.

Phil

[tabbypurr]

It was common to use reject bricks for the inner leaf. I'd replace the bad bricks, or if I really was not willing maybe face the baduns with a bit of cardboard to permit expansion then cement over it. I can't see any point dampproofing a few bricks.

NT

[Phil Addison]

Thanks, that gives me encouragement. The idea behind the silicone waterproofing was that it might stabilise them, I don't think there is any damp in the wall. I realise waterproofer isn't ideal for this, some very thin searching glue might be better but I don't know of any such.

What's the best way to remove them should I go down that route?

Phil

[tabbypurr]

Very dilute pva. _very_ dilute. But if a brick is expanding with force, as it must be, I doubt watery pva would stop that.

Chisel em out, breaking them up in the process. I suspect I might choose the cardboard trick if the plaster layer's thick, adding some sort of netting into the cement.

NT

[Bill Wright]

If you don't have a drill with rotary stop Drill the mortar all around them with a 7mm drill bit. Use an SDS drill obviously. Drill holes as close to each other as you can get them. Then go round again drilling at an angle so the new holes cross several of the first ones. Eventually the brick will loosen. Bash it with a hammer. It will then be possible to lift it out.

If you do have a drill with rotary stop Break up the surrounding mortar with the drill then break the brick up with the drill.

Bill

[Phil Addison]

Yes, 1/4 to 1/2" of brick expanded to blow the plaster an inch, so I reckon I'll take them out.

OK. I do have an SDS with a nice selection of rotary-stop chisels. I wondered about the cheapo multi-tool I got from Aldi to cut round the mortar, but it only has a depth of about 1.5 inches, and I expect the mortar would knacker it.

I can't see how cardboard could absorb the 1" expansion I'm experiencing, and yes, the plaster is 3/4 to 1" thick - its that grey gritty stuff as per Victorian plaster work, with a thin skim. You mean netting to strenghen the cement against the expansion?

Phil

[Phil Addison]

Sounds good. See previous reply to NT.

Phil

[Phil Addison]

PS. Meant to ask for any tips for mortaring in the replacement bricks. I imagine it's tricky to persuade the mortar into the narrow space around the brick. I'm thinking I'll need to fix up some shims/spacers all round to centre the brick, then butter up the back face, and a very thin buttering all round the aperture. Push the brick in, then ram remaining mortar in with say a length of 1/4" x 1".

What mix would you use?

Phil

[harry]

Old bricks are notoriously crap and variable in quality compared with modern.

If its an outside wall, I would replace them. On solid walls, porous bricks conduct the moisture into the interior and modern impervious decorative finishes trap it and cause the "blowing".

Older interior finishes let the moisture escape to some extent. (stuff like wallpaper and "distemper").

You need a waterproof finish on the outside of the building if it's solid walls. It can be waterproof rendering/masonry paint or pointed up and transparent water repellent stuff if the brickwork is a "feature".

Consider exterior insulation.

[Bill Wright]

Stand the brick on bits of wood or whatever. Put some mortar (not too sloppy; not too stiff; trial and error) on a flat metal plate. the end of a big flat rectangular trowel is ideal. Flatten the mortar so it's like pastry that's been rolled out, the thickness to suit the distance between the old and new bricks minus a smidgeon. Hold the edge of the plate or trowel so it just goes a tiny way into the gap and is at the bottom of the said horizontal and as yet empty gap and push the mortar in with a small trowel or a paint scraper. Each bit you push in will be shaped like a chip (as in fish and chips). Periodically use a bit of wood to ram the mortar to the back. Tap the loose brick to consolidate. The vertical joints are more tricky! The same technique can work though, with care and a stiffer mix.

Bill

[tabbypurr]

No need, it's Victorian, it'll be almost powder by now. SDS straight through the brick at a steep angle, the mortar won't be difficult to pull off. Don't SDS straight on, it could push an outer brick out.

NT

[tabbypurr]

definitely, I didn't realise it was a 1" expansion

I doubt you'll need to touch the mortar.

no, no chance of that. If the expansion were far less, netting gives the mortar enough tensile strength to survive normal use in a thinnish layer, that's all.

NT

[tabbypurr]

Hold mortar on flat surface upto the brick, scrape it in with a flat bladed screwdriver. Prebutter as much as you can, but that won't work as well as you might hope.

1:1:6

NT

[tabbypurr]

harry talks nonsense again.

[Phil Addison]

Actually I did notice a bit of a bump in one place and wallpapered over it 25years ago, so it has been an ongoing deterioration, only now revealing what was going on under the wallpaper, in a corner hidden by a wardrobe.

Apart from your earlier input, which ignored pretty much everything I described and photographed, most of the replies here have been most helpful - as is typical of uk.diy.

Phil

[Phil Addison]

I thought the photos already gave all that information, were you not able to view them?

Phil

[tabbypurr]

jim as usual

[newshound]

Good description. You can also get trigger-operated "mortar guns". These need a fairly damp mix (I normally use lime mortar which is a bit more plastic anyway), these make it easy to fill the vertical gaps too.

I have random stone walls, rubble filled, and they are brilliant for filling deep voids and cracks.

[Graeme]

In message , newshound writes

Didn't know such things existed, but, firing up eBay, by the time I had typed M O R T, sure enough, mortar gun appeared in the dropdown list. Without selecting a category, I then had to wade through various Airfix Kit and Action Man listings :-)

Funnily enough, I had wondered whether a fairly sloppy mortar mix could be pushed through an empty caulk tube.