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These are my back steps - two horizontal slabs and then the steps (which you can't see).
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I have a damp spot on the interior of the left hand wall - you can see it's looking a bit green and mossy on the bricks. Reckon the cause of this could be rain water falling down the gap between the two slabs into the cavity below, do you reckon it needs a good sealing up? If so, what with - do you use mortar or some sort of polymer sealant?
The slabs are sitting over space (inaccessible without knocking though my cellar wall), there's no solid foundation under them.
Are the gutters knackered above it?
Re point or shuffle slabs over a bit and put a French drain in to get the water away.
Check the levels - are the slabs positioned higher than the interior floor or above any DPC? Sealing it up might not help if that’s the case, you’re better off digging it out and lowering the path height.
Where is the damp proof course in relation to the slabs?
That could be rain splash back splashing off of the paving too.
Don't know if it has a damp proof course tbh - house is about 100 years old. Are they usually visible? House is nice and dry in general, just this one damp area.
Digging the whole thing out and just having some steps straight down from the door is something to think about in the longer term - don't think we'd miss the current design much.
Our terrace of similar age had a slate damp proof course I think. If the slabs are within 2-3 courses of the internal floor level that's probably causing the problem. You could just dig/cut out a gully so there is a 6 inch gap to let the wall breath.
Hve you done anything to the path before you took that photo?
It looks to me you have a line of drips.. vegetated crack and a vague line of dust less dust (where the drips lnd) then another dusty strip.
Gutter is a good shout.
The non cowboy damp guys look at outside levels, so reduce if you can,
Try using granno (1-6mm granite) as a fill, once settled it's essentially waterproof and can be added to if if runs into a cavity with rain
Another thing to consider is this the internal wall could just be colder and condensation is forming rather than damp penetrating, is air allowed to freely circulate?
Couldn't you just put a french drain against the building and then relay the slabs to fall away?
The amount of green on the wall, and the way the face of the some of the brick/stone has spalled away suggests that the wall is damp well above the slabs. Are you sure there's no water running down the wall from leaking gutters etc when it rains?
Exactly. the dampcourse may or may not be there but that wall is wet and there is definite signs of water splashing quite high up the wall and if the slabs are suspended like you suggest the green staining is quite high up the wall from ground level where has all that grit come?. putting a dpc in isn't going to help if its getting drenched above it. And french drains etc would be fixing the symptoms not the cause.
Thanks for all the suggestions, v helpful.
The guttering is in good order, we had some roof work done last year and checked them (all that detritus in the photo is just from under an identical mat to the one on the right). Both exterior walls look damp either side (photo below), so it seems to be a problem around ground level.
Think I'll shufty the slab back in place and seal it and see if that makes any difference, and if not I'll get someone qualified in to take a look.
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I'd say that wasnt damp more staining from splash back
What's directly above it?
IMHO it's rain splash in a sheltered area and so moss grows to the level shown
Moss creates it's own habitat so it grows more.
It's not rising damp
I'd treat with a gentle moss killer and not too vigorously clean and observe during the next lot of heavy rain
infact you can clearly see in the bttom right corner as it curves away from the step that its not green and not getting wet.