Replastering 1900 End Terrace Damp Cavity Walls

Starl

A bit of background: we have recently started renovating our first house. It is a damp end terrace in the South West, and we bought it knowing there were a few issues, including: damp at ground level, missing pointing, and corroded cavity wall ties.

The wall ties and lime pointing are sorted and we are lowering the ground level, but we now need advice on plastering as we have seen many different views. Originally, the whole house was plastered in what appears to be black ash mortar (like the house was built with) and lime. Due to high ground levels, placement of joists bridging the bitumen DPC and blocking the ventilation from air bricks, some of the plaster became very damp and is now not in a good state. Around the bay window and in the corner with a leaking gutter, there are visible signs of salts up to about 30cm in height (photos attached). This is where we need some advice on the different possibilities.

Firstly, is it ever advisable to hack off the original plaster all around the external walls to, say, a height of 1m? The plaster above this level seems to be in a much better condition. We were initially thinking of plastering the entirety in lime, but spending this on a small end terrace on a tight budget, we cannot afford it.

We are aware that there are many plastering options, some of which should not be used on a damp wall, even though it is a cavity wall (with a very small cavity). We have considered using a DPM kit (like the Permagard mesh) with some form of plaster/board over it. Would this work, and what would you recommend?

Thanks!

FreeBear

While you have the floor up, get some insulation in between the joists - I'd suggest nailing some battens on either side of the joists 25mm down and then cutting up a sheet of Celotex/Kingspan to fill the space (25mm sheet). Seal any gaps with a thin bead of expanding foam, and you'll end up with a nice warm floor.

On the walls, I'd also consider insulating with Celotex/Kingspan and finish off with plasterboard after taking all the old plaster off. 75mm will meet the current building regs requirement for refurbishment without sacrificing too much floor space. If you are worried about damp, use a warm batten method with an air gap between insulation & brickwork.

plumb1_2

Fit permaguard or wykamol mesh, then spot and dab with insulated plasterboard 
ya a asFB says insulate the floor also

BUFF

Starl said:

We are aware that there are many plastering options, some of which should not be used on a damp wall, even though it is a cavity wall (with a very small cavity).

Is it really a cavity rather than a solid wall & lath & plaster? If so that would be unusual for a 1900 terrace imo.
You might find this a worthwhile watch as a starter (which will rapidly lead you on a spiral of investigation, no doubt). https://www.youtube.com/watch?v=kWPN7cxlg1U

stuart45

How small is the cavity?

Starl

Thank you, all, for your advice.

The cavity is about 30mm, but does vary a bit - perhaps a bit of an experimental construction given its age.

We were definitely thinking of floor insulation, so thanks for confirming it would be a good idea.

Regarding the wall insulation, we would love to have it but were a bit concerned about insulation because of the damp, though understand building regs require it if we replaster the whole room. Sadly, I think warm batten plus the 75mm insulation would be beyond what we can afford if we replaster all walls.

The permagard or wykamol mesh - could that be placed on the wall and plastered over with lime or similar? And just on the bottom 1m (ish)? We're not really sure what that yellow stuff is on the bottom half metre - perhaps some form of tanking.

stuart45

A lot of those type of walls weren't really built as cavity walls, but  solid walls built in stretcher bond. Normally on a garden wall the cavity, known as the wall joint, is 10mm, but when used in houses the size of the wall joint was increased to make it easier to build the inside skin so you could get your fingers in between the outer skin and your fingers holding the brick. 
It was never intended as a cavity wall, which back in the late 1800's was 2-3 inches.
I've worked on properties like that where the cavity is half full of mortar. In theory it should be full, but the bricklayers often didn't fill the wall joint as it might cause the brickwork to belly out.

edgex

Have you got a picture of the wall outside?

Starl

That's really interesting to hear about those stretcher bond solid walls. Given that there were several corroded wall ties - and we have just had some new ones installed - I hope that ours are not solid walls!

I am attaching a photo of the gable end wall.

stuart45

Solid walls can have wall ties in stretcher bond. Your gable wall has got courses of headers higher up, so that bit at least should be solid. In theory all the joints in a solid wall should be full.

plumb1_2

Looks like the coping stones on top of the roof may require re pointing