Treating a wet wall

I have a situation with a damp wall and wonder if anyone has found a straightforward way to treat it.

A large floor area has been created by pouring concrete - I suspect without a membrane - onto which internal dividing walls have been built using cement concrete blocks (parpaing). The roof covering is good, but the whole ensemble is very damp (floors, walls etc); luckily the old house against which it is poured is coping with the additional ground moisture , but the walls are forcing the damp to migrate upwards and into the roof timbers....not good.

Is there a surface treatment (or invasive technique) which can be applied to these walls to help the moisture trapped in the concrete to dissipate to air? Would applying a surface coat of lime plaster or hemp-lime mix achieve a satisfactory result?

Thanks for any pointers.


LoL, if your mother had taken that advice Brian we wouldn't be having this discussion!

John I just cannot see ground water getting up that high. Cement blocks are porous and Cement hold onto water, I would look more locally around that beam or suffer the issues later

just looked at the picture again but downloaded it to enlarge and lighten it a bit. The stains, as far as I can see them, round the beam are absolutely as close to what we had at ceiling level and also around beams as could be. This winter we are having four beams reinforced during attic conversion, having been told the damage to them and the rotten floorboards was termites. In fact, each of the four carpenters we had in to have a look and shock us with their devis said it was severe water damage. On looking closer, it would have been seepage running down the adjoining, more recent blue breeze block wall (an extension in the late 1960s) and finding its way under the roof into that corner. The point at which it originated was the fissures well before we had the zinc gully that the roofer had not seen and closed but actually made worse. All a bl**dy nuisance but ultimately an expensive one. As my father always said 'never trust a builder'. He was a builder!

Many thanks to you both for taking the time. Ground water migration is a constant concern, and for rainwater it can be a challenge to make sure it is safely dispersed, especially when concentrated into gutterings and downpipes. many of the older houses hereabouts don't have gutters, the rain falls to ground against the external walls, which usually have a mortar dressing to half metre height and apron at ground level....... after +/- 300 years the main house is functioning very well.

The beam in question has been cemented onto a concrete dividing wall, on the other side it is concelead behind insultaion and placo boards for a toilet and bathroom.

My thanks once again. John

I agree with you there John. We have looked into wet some time ago, the previous owner of the house put it down to a spring under the house rising. Complete nonsense. We had somebody who really knows help us find it. It turned out to be a roof gully that appear totally sound, lined with zinc to flow into the gutters. However, water was going under it and slowly seeping down through fissures so that it eventually found floor level and pooled on the ground with the walls wetter at the bottom than top. He had started by looking for weaknesses in the walls, we too are lime but put together with the local soil based mortar that eventually dissolves and can simply drain away but the houses stay up (over 260 years and still very sound) however it can let water. He found nothing, then looked at floors because damp damage had destroyed the ground floor beams and boards over time we have had cast concrete put in. He thought that might have been behind it, but in fact it was only allowing the pools to form on the floor. Because the roof had been done recently by reliable people it was the last suspect. However it was. The remedy was simple, up came the gully in question and this time back in and nothing since. The roofer did it under his warranty with only a couple of whimpers about it must be the house moving, etc. So, following John W's line, dismiss nothing and look for what you do not imagine it could possibly be as well.

The weight of the water won't let it climb too high hence the 1 metre idea, so surprised at 3m. Cement based products hold on to the damp. Even brick walls the bricks can be bone dry whilst the cement mortar can be very wet but even on my works on the river basin at St Pancras 1100mm was about the maximum we ever recorded and those were soft red bricks so acting much like limestone.

DPC into cement is ok, I use the silicone cream, far better than the liquid.

Looking at the photo it looks to be penetrating into the end of that beam? what kind of reading are you getting surrounding that? What is the otherside of the wall?

Thanks John. The roof is near-perfect (hats-off to the contractor),so I'm confident it isn't a leak (no stains to adjacent boards, for example. I'm suspicious of a 'capillary' motion - moisture can climb through granite to some considerable height, but what about cement-based concrete walls? I'll attach a picture of the most obvious stain.

The floor and walls are very damp.; the contiguous old (lime and schiste) farmhouse wall is coping well with the increased ground moisture - dissipating moisture to 'dry' at 3m height (according to my Protimeter): the blockwork walls are not coping so well..........

Is injecting a DPC into the block wall the way forward? The objective is to protect the roof structure from high humidity which could induce woodworm and rot attack if allowed to continue.

John, it sounds from your text that the water is climbing up the wall to the roof timbers?

That's not really likely as water can't go higher than a metre, the moisture must be coming from elsewhere, penetrating damp or from the roof downwards.