SUBFLOOR AND BASEMENTS
When walls are below (or partly below) ground level, you are asking for water penetration problems! I have inspected literally hundreds of houses that have damp-related problems because walls below ground level are not adequately waterproofed.
I consider it poor building practice to have habitable rooms below ground level, with the external side of the wall contacting the earth. Best practice is to have a space of at least 1 metre, and a durable retaining wall such as reinforced concrete blockwork. Ideally, the outside ground level should be lower than the inside floor level and
slope away from the building towards a surface drainage system.
You can often smell a damp problem before you see it. The human nose has evolved to detect mould-like smells as a survival mechanism. Our Homo erectus ancestors instinctively knew that a dry cave was better for their health than a damp one. So sniff the air. Look for blistering paint low down on walls. Look for swelling of skirting boards, tea-coloured stains on walls and carpets. Peel back the carpet in the corner where the wall is below ground level and look for black- coloured water staining on the timber ‘smooth’ edge. If the ‘smooth’ edge is blackened with mould or even decaying, then this could indicate a dampness problem.
Surface water can run under the house without causing a problem, provided that it keeps on flowing away from the house and does not collect under the house or cause damage on the way through.
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