Marianne Suhr: Damp busting mission

The project: West End House, a Grade-II listed, late Georgian brick house with four bedrooms, undergoing conservation and repair
This month's challenge: To diagnose the causes of damp in the house and prevent the problem from coming back.

When the phone rang a year last May, I wasn’t expecting a complete stranger to ask me if I would be available to advise on damp problems in her home. Karen and her husband David had just moved to a late Georgian house, and she’d heard about my reputation for sorting out old house problems.

She has told me since that she was expecting me to be a lady in her sixties with grey hair in a bun, so she was a little surprised when I turned up with my toddler Max. Since that initial visit, Max and I have been regular visitors, watching with interest as Karen and David have lovingly restored their beautiful home.

Pinpointing the problem
West End House had been built without a dampproof course, as most houses before the mid 1800s would have been. But that was not the cause of the problem. I suspect low-level damp may have started when the original, suspended ground floor was replaced with concrete. The original floor had been constructed with timber boards laid on wooden joists, with a sub floor void beneath. We know this because the cast iron grilles that ventilated the void are still visible.

The ground level appears to have been raised externally at some point, causing the joist ends to rot and requiring them to be replaced. I suspect the suspended floor was replaced in the 1970s with a concrete slab, laid on a plastic sheet. Any ground moisture under the plastic would have pushed to the edge and wicked up the brick walls.

It was also apparent that the house had broken drains running close to the back wall, cracked stone window sills and rotten timber frames, as well as poor pointing. Two or three decades ago the damp problem had been exacerbated by rendering the lower part of the outside wall with a hard cement render, and ‘tanking’ the inside wall, again with a hard plaster. But this had only made matters worse by pushing the damp even further up the wall. That later internal tanking plaster was now bubbling, as the trapped moisture behind was trying to get out.

Finding a solution
My diagnosis was cement induced damp. Modern treatment couldn’t help; the hard cement render and internal tanking had to be removed to allow the wall to breathe and dry out. But at the same time, all the building defects had to be resolved.

The windows needed careful repair, the drain was to be replaced and re-routed and the external ground levels needed lowering. I also suggested removing the concrete path that went up to the wall and replacing it with a gravel filled trench to provide an evaporation zone at the base of the wall. The removal of the cement render caused a lot of damage to the face of the brick, so it was replaced with a traditional ‘breathable’ lime render, and internally the bubbling tanking plaster was replaced with lime plaster.

Of course, it will take a little time for the house’s damp patch to disappear completely. But by addressing the cause, rather than simply treating the symptoms, it won’t come back again.

ABOVE (clockwise from left): A trench is dug to reduce the ground level and increase evaporation at the base of the wall; the tanking plaster inside the room has been hacked away; internally, the tanking plaster is replaced with a breathable lime plaster.

Reno expertise: DIY damp diagnosis
Damp is usually caused by one of the following four things, and can be relatively easily diagnosed.

• Penetrating Damp This is the most common cause and is due to building defects such as leaking rainwater gutters and downpipes and cracks in walls – basically anything that causes water to work through from the outside to the inside. It is usually localised and causes inside finishes to bubble up or discolour.
• Condensation This is caused by water vapour in the air, condensing on cold surfaces. You often find it in steamy bathrooms, or behind cupboards on external walls where there isn’t much air movement. It is identified by black spots of mould and can generally be resolved by improving ventilation.
• Rising damp Very often misdiagnosed, it is often caused by an external ground level that has built up over time and is higher than the internal floor level. It is remedied simply by reducing the external level, letting the wall breathe and forming a gravel trench around the outside. A cracked drain near to the wall is also very regularly misdiagnosed as rising damp, so get a drain survey at the same time to make sure.
• Plumbing leaks Leaking shower trays can cause tremendous damage to old houses, creating the perfect conditions for dry rot. The soil pipe around the back of the toilet bowl is also a common cause of leaks, so keep a regular check on any sanitary appliances. Pinhole leaks in old copper radiator pipes can also fester unnoticed.

Eco tip: Insulating floors
If you have a suspended timber ground floor that is letting in the draughts and causing your fuel bills to rocket, don’t resort to a concrete floor slab – this could have a knock on effect and create damp in your walls. Instead, insulate beneath the floorboards and keep the historic fabric.

Get a carpenter to lift your existing boards, then lay a breathable membrane, sagging between the joists, to support an insulation quilt such as Thermafleece sheep’s wool (017684 86285; secondnatureuk.com). Once fitted, relay the original boards on top. Remember, it’s critical to ventilate a suspended timber floor or the wood will rot, so maintain the airflow through the grilles.

WORDS & PHOTOGRAPHS MARIANNE SUHR
Featured in the July 2010 issue of Period Living