Damp / Grease Patch

[LC1986]

Hello all,
Any advice on what this could be? It’s 1930s house and on outside walls. Only thoughts are a damp or patch.

The roof and gutting are in good shape, with minor repairs done a couple weeks back. First thought was water coming in to the cavity. There are no cracks on the outside render but some parts still appear wet (according to moisture tester). Weather has been good, some parts may have tried out but difficult to say.

Tried zinsser bin primer but still coming through!

 

[theknobs]

1930s houses don't usually have cavity walls, so any damp that lingers on the outside can percolate right through. If the house is rendered the damp could be getting in anywhere and creeping down to where you see it. You need to get a good look at the render all the way up.

 

[Oldguy]

1930s houses don't usually have cavity walls, so any damp that lingers on the outside can percolate right through. If the house is rendered the damp could be getting in anywhere and creeping down to where you see it. You need to get a good look at the render all the way up.

Many 1930's houses do actually have cavity walls. If so, it's quite possible that crumbled mortar or other debris has fallen down the cavity and got caught on the wall ties that link the two leaves of the wall. It can be cleared by hacking out a small piece of the inner or outer block or brick and removing it. I had that once where some of the mortar had partly perished and fallen down the cavity. I removed an end brick and scraped it all out.

 

[Paul Cumbria]

As per Oldguy's response above, most 1930's houses will have cavity walls. Mine was built in 1929 and has cavity walls. The top half is rendered and this is part of the problem I have had. Rendering of this age may not show visible cracks, but water can penetrate even the smallest crack which you may not see, especially if exposed to driving wind and rain - my worst affected wall faces into the prevailing weather. Once inside the cavity it can migrate large distances from the point of ingress, especially as stated by bridging in the cavity by fallen mortar or compromised wall ties. By far the worst culprit is cavity wall insulation - I had no choice but to have this removed as it was causing problems throughout the house; nothing as bad as shown in the photos though......

 

[Paul Cumbria]

Also, I should have added that trying to paint over a damp patch is a waste of time and money whatever paint you use, until the source has been rectified and the wall dried out properly. Even if you can find a true "waterproof" paint, all you will do is move the problem to another part of the wall.