DIY Doctor UK DIY Forums

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:

https://drive.google.com/drive/folders/ ... 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

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

stoneyboy wrote: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

Thank you so much for your advice.

Based on the pictures of the vertical and horizontal gap would these be safe to fill? My main concern is with the vertical gap joining with our neighbours house. During construction of the house that gap was obviously left, so I wonder if it serves a purpose e.g. for air flow? Or maybe just a flaw in design. Could just the bottom section be partially filled?

The horizontal gap on the porch roof, that really is suspect. I can imagine water pooling on the roof and working its way down and back into the house somehow. It also seems to line up with where the water stain is appearing too.

Filling would seem the most sensible thing to do. You suggested a flexible building mastic - would this kind of filler work? https://www.homebase.co.uk/bartoline-ex ... g-_p508090 - would a different filler be needed for the vertical / horizontal gap?

I'm so greatful for the advice. Every time it rains I'm anxious its getting worse, but need to face repairing it!

Hi whiffrug
The building sealant I suggest is something like Everbuild Tecnic Silicone 825 which is a flexible product specifically intended for sealing movement joins in buildings. Fill the vertical and horizontal gaps.
Do not use the product you posted a link to.
You will still be left with a weak join between the flat roof and the rendering but try sealing the gap first.
Regards S