Removing cement tanking and repointing

[benoliver999]

We moved into our place about a year ago, it's a ~180 year old red brick terrace. The house is on a hill that slopes down from the front to the back, so the lower-ground floor has the cellar at the front of the house (below the front door, buried underground), and the back of the house has the kitchen with a back door that opens onto the garden. The 'slope' is actually a tunnel between the two terraces, with a flying freehold overhead.

The other day we decided to peel off some flaking paintwork in the kitchen, with the goal of stripping it back and re-doing the paint job. It's never that simple is it?

We started peeling the paper off the exterior wall that's on the side of the house (the side with the slope on). All gypsum, but below about 1m it's clear that the damp-proof guys have been in and also added cement. This was done about 10 years ago from what I can tell on the paperwork. It's rock hard and the paint really doesn't want to come off.

Then we spotted damp patches in the gypsum above the cement layer - almost like it's wicked upwards from behind the cement. I stuck a chisel in and sure enough it went through like sand, and it wasn't much work to get rid of the wet gypsum. The brickwork behind it is quite degraded.




The brickwork behind the gypsum looks a little ropey



Outside, in the tunnel, a lot of the bricks look new-ish, but it's all been pointed with cement. Presumably this was done 10 years ago as part of the 'damp proofing'.

It seems to be like a classic case of hiding damp rather than tackling the problem. One of the air bricks was also sealed, which I've now unblocked.

I guess my questions are -

Is it worth trying to take the cement off or can I get away with just lime plastering the top half (and maybe getting the outside repointed in lime)? The cement is rock hard and it's not coming off easily. I'm worried about damaging the bricks.

If I manage to get it all off and replaster in lime, how urgently do I need to look into repointing the outside? Do I hang fire on the plaster until the repointing is done?

 

[Cubist]

It's not unusual for builders/renovators to use a hard cement mortar as the render coat that is later finished with a gypsum skim coat when repairing and internal wall. Aside from the fact that it is considerably less breathable than a compatible lime based product it can be a major problem to remove as it's a lot tougher than the bricks it coats. Much of the damage you're seeing is due in no small part to that. On balance though that's not your biggest problem as I think there are a couple of significant 'timing' challenges you need to take on board.

The first of these is that we are drawing toward the end of the weather conditions that best suit exterior works with lime mortars and it may not be advisable to start the raking out and re-pointing exercise to the exterior of that wall before next year. The second timing issue comes from the possibility that while raking out the hard cement mortar outdoors you may find, or cause, damage to some bricks that forces their replacement. While you're raking out the old mortar, depending on what tools and how heavy-handed you may be theres a risk that vibration transmitted through the wall may damage to the interior plasterwork.

I think the best sequence would be exterior first then interior but.... you may not want to leave the interior walls like that until next year?

As to whether to strip the cement render/plaster?If you're sure that the damp patches are indeed penetrating damp from outdoors then replacing the interior plasterwork with lime products will be of some help to the bricks in the wall but not necessarily the decor. You may be off better focusing on improving ventilation in that room if you cannot stop the water penetration.

 

[benoliver999]

Ah sorry I should be more clear - the bit I have removed is not cement, just gypsum. The sort of horizontal line you can see in the photos is where the cement goes up to.

 

[Cubist]

Thanks for the clarification but I'm taking that to mean that the damaged bricks you've found were under the plaster not the cement. Sadly, should you try to remove the cement render the chance of damaging other bricks will be considerably greater.

 

[paulc]

Sadly, should you try to remove the cement render the chance of damaging other bricks will be considerably greater.

But if one is replastering, does it matter too much if the surface of the brick is damaged ?
If anything, the rough surface left behind will give something for the new plaster to grip on to.

 

[benoliver999]

Well the situation has developed a little bit since I posted - with a little bit of teasing a sheet of cement came off the wall. Took a little brick but not tons. When I get a chance I'll have another go, just gently, to see if anything else is loose.

I'll ask around about repointing - as you say it's looking a bit late in the year for it. We can hold off on finishing the kitchen if we have to - the units etc are on a different wall and already in, so it's just a case of cleaning up and moving the pots and pans back in.

 

[paulc]

Well the situation has developed a little bit since I posted - with a little bit of teasing a sheet of cement came off the wall. Took a little brick but not tons. When I get a chance I'll have another go, just gently, to see if anything else is loose.

Please tell us you didn't use a wood chisel to chip the cement off....

 

[benoliver999]

Not cement no, but soft old plaster etc. We found a blunt chisel was working better than a bolster at not damaging the substrate. A sacrificial chisel, if you will.

 

[aockenden]

Please tell us you didn't use a wood chisel to chip the cement off....

Haha I’ve done that with old crap chisels laying around. Hell, sometimes I even use a nice screwdriver. I assault the cement with whatever is to hand!

 

[Cubist]

But if one is replastering, does it matter too much if the surface of the brick is damaged ?
If anything, the rough surface left behind will give something for the new plaster to grip on to.

If its just the surface then no, its not a problem. However, I've seen many such exercises where the damage to some bricks has been sufficient to force their replacement. That, in itself, is an easy enough job but theres an element of care needed during the render removal step as too many damaged bricks could compromise the structural integrity of the wall.