Our house has a raised patio - a tall one at 900mm - built onto the back of the house. You can more or less see it here:
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There is a limited damp problem inside the house as you'd expect from bridging the damp course like this.
I'd like to functionally retain and extend the patio, so I am thinking about replacing most of it and building a new block wall with an air gap between the patio and the house. Am I going to run into problems if I bridge the gap at the top of the wall with paving?
Yes. Resolution isn't good enough to be sure, but it looks like you have flemish bond 9" brick walls. The splash-back from rain hitting the patio and splashing onto the wall will probably be enough by itself to keep the wall damp, even if you have no penetrating damp from below the patio surface.
If you are cutting away the patio from the house during the rebuild, I would tank the outside of the wall from below the damp course to the top of the patio with a damp-proof membrane to prevent damp penetration below the surface, and then have an 18" waterproof render skirt applied along the patio to protect the wall from splash-back. Also, make sure the patio slopes slightly away from the house, so there is no puddle forming along the join. Another option to protect from splashback is a French drain - a waterproof channel about 12" wide between the patio and wall which is filled with pea shingle - the rain won't bounce off this.
The other thing I would be slightly concerned about is 900mm of soil or rubble being retained by the house wall which might not have been built as a retaining wall in the first place. Is there 900mm of nothing on the inside at that point, or is the inside ground level higher than the original outside ground level?
Indeed. Currently, there's about 4" of lead flashing above the patio as the only splashback protection. So, hopeless.
One of those doors you can see is hopelessly rotten at the bottom, which is no doubt triggered in a large part by all that splashback.
Useful thoughts, thanks. A French drain would certainly be doable.
It's a solid brick wall for the house itself and the lower foot is much thicker. However, it's a fair point and I'm sure the Victorian builders didn't plan for almost a metre of sand/hardcore mix pushing against the house.
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