Q. I own a semi-detached house built around the 1930s.  A few weeks ago I discovered that there was a small damp patch just above the skirting board in the lounge - the wall paper had begun to peel away and some salty deposit on the plaster. The wall in question is an internal load-bearing wall dividing the lounge and the kitchen.  The kitchen has a concrete floor. The lounge has a suspended wooden floor - underneath the floorboards there is a void of about 75cm with concrete at the bottom (top of the soil). I removed some floor boards near the damp patch and on inspection saw water present (2cm depth) just around that area. I suspect this water is rising up the wall and causing the damp. There is no evidence of dampness on the other side of the wall (i.e. in the kitchen). Where has this water come form? I though that it may be the water mains leaking so I arranged for the water supply to be turned off from the stop cock on the pavement out site. It is now over 4 weeks since the water has been turned off, but the water under the floor boards is still present. Could this water come from a neighbouring property with a leaky water mains? A friend has advised that it may be spring or ground water that is causing the problem. How can spring or ground water be 'diverted' or moved away from under the property (e.g. by putting sand or something on the concrete)? What is the best remedy and the cost involved?

Submitted by: AL
 

A. A 1930s property will have a damp proof course (DPC) in the walls and provided is being allowed to work correctly you could have a river beneath your property but damp should not rise past the DPC. Indeed I have surveyed numerous properties one of which did have a stream beneath the floor and others with water depths of between 10cm - 30cm of water subject to the varying height of the water table. So it is not unusual to have water beneath a property. It is essential however that the sub floor ventilation is very good indeed - 9"x6" efficient air bricks every 1.5m in all external walls. By efficient we mean square pattern cast aluminium or plastic louver type but NOT terra cotta type as these are only 20% efficient and have the ability to bridge a DPC

You are correct to investigate all other possible ways by which the water may have entered the property and no doubt you have checked the plumbing on the kitchen side of the wall. One point however, in 1930s properties the DPC was built in at a level to protect the timber floors - about 8" beneath the timber floor level but when they in filled to create the solid floor for the kitchen they often bridged this DPC so it is not unusual to have damp in the walls surrounding a solid floor in such a property irrespective of the water under the floor.

Your local water authority is able to test the water to see if it has come from a main because it contains their treatment chemicals - chlorine, fluoride etc. so perhaps this is worth a try. If everything draws a blank your friend could be right but do not worry - just make sure it is well ventilated.