by Mike G » 16 Jan 2022, 10:28
Great work Sam. That's done more to dry out your building than anything else you could have done. You should keep the ground level below the slab level all around, and if necessary, organise for any low areas to drain.
Right, the problem of concrete extending outside the walls....... Mortar won't stop water penetration. You'll actually see it run in if there is enough water externally. You should take action externally, not internally. The simplest course of action would be to paint some Black Jack/ Synthapruf over the junction, extending 3 or 4 inches out, and 3 or 4 inches up. It might work. Personally, I would be casting a concrete fillet all around. This would be a sloping concrete shelf a few inches up the wall down to the outside edge of the slab, or at least a foot away from the wall. Then belt-and-braces, you could Synthapruf that. Make sure that this fillet doesn't come within 6 inches of your DPC.
Some might suggest Synthapruf over the junction, then a concrete fillet over that, but I would be worried about the concrete adhering properly to the Synthapruf.
This fillet will almost certainly crack over the years, and you should repair this when that happens.
Rendering the blockwork would be a good solution, but you MUST stop above the DPC. Normally, alternatives would include battening out and applying feather edge boards or proprietry cladding. I don't think FE boards are appropriate in the Peaks, though.
Before you do any work to the blocks and slab, you must thoroughly clean. A pressure washer would be a good starting point. Personally, I think I would be leaving all of this work until the summer, to make sure I started with a completely dry structure. Obviously, any necessary work to the roof, gutters and downpipes can happen as soon as you like, and the sooner the better.
As for lining and insulation.....Yes, you should do it, but you must take care with your choice of insulation. If you use a board such as Celotex, there really should be a ventilated void outside of it if you want to do a proper job. This will mean cutting in airbricks, and spacing the Celotex off the wall. If you use Rockwool or similar, this can be hard up against the blockwork, but you need a greater thickness to achieve the same benefit as Celotex.
If you are going to insulate the roof (you should), then you need to ensure there is airflow across the top of the insulation below the roofing material. That means openings between both ends of each joist pair, and you'll need insect mesh. You can add a layer of Celotex below the rafters, too, and this can make a huge difference.