Porous stone coping

[Kit Jackson]

Porous stone coping

In Weston-super-Mare, house built in 1894, solid stone walls about 15" thick with Bath stone around all the windows, doors and coping stones on the parapet. We are getting a lot of damp in the bedroom wall and ceiling where the front gable is. A couple of people have said that the Bath stone is porous and the water is getting through the coping stones at the top and soaking into the wall.

Toolstation do Everbuild, Thompson's and Bostick water seal. Does anybody have experience of any of these? Are they all pretty much the same stuff or are working in a fundementally different way?

Any other recommendations please?

[Colin Bignell]

I had a problem with home made concrete lintels where the angle iron reinforcement was starting to rust, because of damp penetration. I painted them with Sandtex exterior wall paint, which cured the problem.

[Animal]

in most cases this is entirely the wrong approach, and leads to more damp and stone spalling as the trapped water freezes. First thing is to look for all the usual causes of damp due to defective rainwater goods. If it's not that, it may be condensation, in which case painting the outside is the opposite of what's needed. Porous walls are normal, and not normally the cause of damp. Rain soaks in a bit & evaporates off afterwards.

[Brian]

If it has been there since 1894 why has it suddenly become porous?

Look for:

Leaking roof above. Gutter problem. Condensation.

[Kit Jackson]

Thanks for that information. I don't really want to change the colour of the stone and I was specifically told not the paint them because that stops moisture getting out but I'll have a look at the Sandtex web site.

[Kit Jackson]

Thanks for replying. I want something that will prevent the water getting in but allows it to evaporate out slowly. I don't think it's condensation from the inside as the water marks are in very specific positions. The water is soaking in a lot in places.

[Kit Jackson]

It hasn't suddenly become porous. We have had similar problems elsewhere in the house and more or less got rid of them by repointing etc. On the side of the house I've lifted off the coping stones, put a wide sheet of lead over the whole wall and put the stones back but that was in a position where I could sit in a little valley and work safely. The stones are very heavy but it's worked well. At the front here the wall is another 10 feet higher and completely exposed with a drop straight to the ground and I don't fancy working up there heaving lumps of stone around. That's why I am considering "painting" them with something.

So that's why I'm asking if anyone has experience of these stone sealants but I'm certainly open to other suggestions.

[Animal]

it doesn't work like that. Any coating bar lime increases dampness by greatly hampering evaporation.

[Colin Bignell]

If lead under the stones has worked elsewhere, have you considered treating the bricks below the coping stones with an injectable damp proof course?

[RJH]

Yes but. A coating will prevent water getting in in the first place. Which can be handy if the structure is relatively porous, or say north facing.

Wicking from inside would need to be taken into account, and that would depend on the use, ventilation, occupancy, type of coating (such as masonary paint, impermeable cladding etc.) and interior finish for example.

it's not an absolute - sometimes a coating works, sometimes it doesn't.

[RJH]

I'd find that unlikely - it's obviously gone over 100 years without being too much of an issue. I'd be more inclined to look at any mortar bedding or cracks if you've ruled out condensation, gutters and roofs/gulleys/parapet flashing.

Or it may be the type of stone has reached the end of its useful life? Not heard of this as a thing, but a decent local builder - especially one who lives in that type of house - might know. You could try a local forum like Nextdoor - if you can put up with the poor signal to noise ratio and data mining.

[Animal]

It's not possible for an exterior coating of any type to prevent water getting into a stone wall. Water gets in from both sides, outside from rain and more slowly from inside by vapour permeating through the wall. By blocking evaporation, an external coating does generally result in increasing the damp in the wall. Yes there are cases where it does work, but generally it's a counterproductive move, worsening damp, causing mould & significantly damaging the wall, so can not be recommended.

If a building has worked ok for 100 years and is now damp, the design/materials are not the problem. OP needs to understand what's going on and pick an appropriate repair.

There are plenty of people that make money from misinforming house owners that the owner needs the specialist services of some damp chemical or other, and plenty of people that seem to believe anything an expert (or scammer to be more realistic) says. That does not make such info factual.