DIY Doctor

Penetrating damp / stain on joining wall just below porch roof

Postby whiffrug » Sat Aug 29, 2020 4:33 pm

We're trying to diagnose and prevent water getting into the property which is causing a stain to appear on the joining wall of our semi detached house

The wall is never wet or damp to the touch, but the stain has been gradually getting more noticeable, especially after heavy downpours.

Please see attached pictures on this google drive folder: ... sp=sharing

Inside - you'll see the stain on the joining property wall. The plaster is rough to the touch, but doesn't appear to be seperating from the wall at all

Porch Gap - there seems to be a strange gap all along the top of the porch which doesn't seem to serve much purpose, other than possibly allow a "channel" of water to go behind the exterior render? That appears to line up close to where the actual staining is on the inside of that wall


What could be the most likely cause? We are looking to fill the exterior render and add flashing to help prevent water seeping back into the property. Would it be sensible to fill the gap on the top of the porch?

Any advice would be much appreciated
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Simply Build It

Postby stoneyboy » Sun Aug 30, 2020 10:14 pm

Hi whiffrug
Fill the vertical and horizontal gap with a flexible building mastic. You will probably always have damp issues due to the nearly flat roof but fitting a flashing lapped under the rendering will help but seal the lower edge of the flashing to the flat roof.
Regards S
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