damaged parapet causing damp - roofer or stonemason?

[mg0685]

Hello! New owner of a Georgian terrace here, with lots to learn and two pressing questions. We have a large water stain outside on on our exterior brick wall, which our surveyor said came from "lack of drip detail" on our parapet/cornice (can't remember the word he used), causing rainwater to run down the wall.

1/ what trade do I call to repair that? A roofer, a stonemason, or someone else? And what do I ask them - I have no idea what a drip detail is or what I should ask them to do!

2/ this is causing damp inside on that same wall, in the exact place where the water stain is outside. I'm reading that lime plastering/limewash can help with that, however our upper floor isn't original, it was rebuilt after war damage, and as a result there is no lime plaster inside but instead what looks like a cement mortar (darkish grey, slightly shiny and rock solid). I don't have the funds to take that off right now. Would limewashing still offer some benefit (maybe as a mould preventative) or should I not bother?

 

[Cubist]

What kind of roof do you have behind that parapet wall? Do you have access to examine the rear face of the wall? Is there any possibility that water is pooling against its rear/hidden face?

The external render above the cornice is in pretty poor shape and there is a possibility that driven rain is being forced behind it where it can then cause damp patches to appear lower down the wall and both outdoors and indoors. The sooty staining on the outer face of the cornice is is due to water run-off during heavy rain carrying with it accumulations of airborne pollutants from vehicles. This also could suggest that driven rain pooling on the top face of the cornice and resting against the mortar at the foot of the parapet wall where, again, it could penetrate to cause the exterior and interior damp patches.

A rebate or channel is sometimes to be found under the lowest edge of such cornices but more usually beneath the cills of window and door frames. These act as a capillary break to encourage water to fall from the projection before the water can come into direct contact with the masonry - i.e. the drip detail your surveyor mentioned. Frankly I think your surveyors conclusion was wrong and misleading.

You will need a general builder to make any further investigations or repairs rather than a stone-mason or roofer. However, there is the possibility that the roofing materials behind the parapet wall is at the root of the problem in which case a roofer may indeed be more appropriate.

 

[malcolm]

Good point Cubist. Normally the Georgian houses had a shallow pitch roof falling to a gutter behind the parapet wall. You need a fair bit of water to cause the water staining so more likely the gutter is overflowing than it being the result of water falling on the top of the parapet.