DANGER OF THE WEEK AWARD
Bad ‘Builders’

While working on another job we were approached by a neighbour to look at a suspected rising damp issue. She explained that someone had come in a couple of years earlier and plastered over the already occurring damp problem. So of course everything looked good for a short while and then the build up and surface salt started showing up, the wallpaper started pealing and the paint started blistering inside and out. You cannot stop damp with a bit of plaster and a lick of paint!

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Rising damp in walls is caused by a phenomenon called capillary action. Ground water is drawn vertically upwards through the fine pores in the masonry and the brick walls like a wick. Ground-level damp proof courses have been mandatory in all British buildings since the Public Health Act of 1875. Normally rising damp is prevented by the  waterproof damp-proof course (DPC) built into the walls. If however the DPC this becomes compromised water can rise up the wall. It’s been known to climb to heights of 1.5 metres or higher.

As ground water contains dissolved salts such as nitrates, chlorides and sulphates these will also be passed up the wall during the capillary action. They are then left behind as solid salts when the water evaporates, accumulating in the the wall and on its surface. You can see the resulting buildup in the photos above.

So the first step to repair this issue is to remove the source moisture. Sources of moisture can be leaks from in wall plumbing, rainwater gutters and downspouts. Also contributing to the problem can be poor ventilation through rooms and beneath floors. Many old floor vents have been closed off over the years. They should be restored if necessary. Outside  drainage should also be ensured and if necessary a new damp-proof course should be inserted into the affected area.

After the source of moisture has been removed some type of finish will need to be re-applied to the walls. If the damp was caused by condensation there is usually no need to remove plasterwork unless it has been significantly damaged. Clean it down and apply a coat of fungicidal paint that includes zinc oxychloride (ZOC) to inhibit mould growth. Once that has been completed a decorative finish can be applied.

To repair rising damp issues it is important that the plasterwork permeated by the salts is removed. If it is left in place they will continue to absorb moisture from the atmosphere and pass it back into the wall thus allowing the problem to continue and get worse. When re-plastering a plaster mix with a silicone-based waterproofing and fungicidal additive is suggested. The mix will resist the passage of hygroscopic salts and moisture.

Alternatively a layer of Platon plaster base can be fixed on top of the existing plasterwork, to which new rendering and plasterwork will be bonded. Platon plaster base is a clear, high-density polyethylene (HDPE) membrane sometimes referred to as tanking) which keeps the new plasterwork physically separated from the old hygroscopic masonry.

If the all important step of stopping the damp is skipped as it was with our homeowner then the problem will come back. It may not show for a few months or more but it will show up as soon as the moisture returns. If the moisture issue hasn’t been solved then the rising damp issue hasn’t been solved. Get a professional to sort it out for you. You get a warranty and the work is likely to last a very long time.

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