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penetrating damp??? help!!

Postby leeanne.reed » Sat Feb 02, 2008 8:30 pm

i have a wet patch on both external and internal wall of a solid wall house, both outside ind inside were rendered with cement,. when we noticed the wet patch on the inside wall *(aprox 5' up the wall) we removed all plaster and internal rendering down to the bare brick which was damp, and noticed there was a wet patch on the external wall in the same place too! nothing seems to dry it out, any ideas on how to tackle this problem???????
please help!!
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Postby LCL » Mon Feb 04, 2008 7:09 pm


Your note mentions two very important factors. 1. Solid wall and 2. Cement.

Old solid walls are designed so that the outer section of wall becomes wet during bad weather which then evapourates when the air pressure outside is less than that of the inside. The application of a cement render is preventing any such evaporation occuring. Some might say; but if it stops water getting out, why has it not stopped it getting into the brickwork. There are a number of reasons for this.

1. Inherent moisture in the wall when the cement render was applied externally.

2. Fine cracks in the render offer a point for moisture to get in and

3. Moisture vapour from cooking, inhabitants etc will migrate to cooler areas by natural convection. Solid walls are such cooler areas and can attrach the moisture vapour from inside the property.

The issue could also be due to a plumbing defect or penetrating damp from say a dripping gutter.

I would suggest the render is hacked off externally and internally, preferably in summer of after a few warm weeks with no rain. Leave the outside for as long as possible, saturated masonry takes about a day / mm to dry out properly. Re-render the external wall with a lime:sand mixture. No cement what so ever. Re-finish inside with a lime : sand render with a skim finish. This should allow the wall to adequately breathe.

Dampness in old properties needs to be managed, not cured. Some will say to install a chemical DPC. These are very, very rarely required. They often rely on a waterproof internal render, and hence such 'guarantees' are not normally available without the 'special render', I wonder why???

Hope this helps.

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Postby leeanne.reed » Tue Feb 05, 2008 1:07 pm

thanks lcl

will try what you suggested!!
it is the wall at the side of the house which has no guttering or pipes running around it so it must be a prob with the render!!
there are salt deposits on the internal brickwork, so does this need to be treated with a salt nutrilizer before re-rendering???

thanks again, your advice is much appriciated!
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Postby matchmade » Sat Feb 16, 2008 5:51 pm

Render on an old house is generally not a good idea. The Victorians built their houses to "breathe", with lime pointing between the bricks as a sacrificial layer which needs to be renewed periodically. Then along came modern cements, which can't breathe - result, disaster. All you're doing with concrete render is sealing in the moisture, which leads to condensation and ultimately severely damages the house.
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