Bridging dampness

Bridging dampness is where dampness crosses over the damp proof course in the base of an external wall, usually due to high external ground levels. This dampness can then damage internal timbers and wall surfaces. The building regulations recommend that there is a minimum vertical distance of 150mm between the level of the damp proof course in the base of an external wall and the top of the external ground level below. This distance is considered sufficient to prevent the possibility of dampness crossing over the damp proof course. However, when property owners lay new hard paving near external walls, they will often lay the new paving directly on to the top of the existing paving/ground level. Over time different owners will change the external ground levels resulting in the external ground level rising relative to the damp proof course in the wall. The raised ground levels can also interfere with the operation of airbricks/sub floor vents, resulting in sub floor ventilation being reduced or removed altogether. Sub floor ventilation is vital in older buildings with untreated timber floors, as it prevents condensation dampness affecting the timbers. The combination of bridging dampness and reduced sub floor ventilation, can cause significant rot/decay to affect the internal sub floor and other timbers in older buildings, as the timbers are unlikely to have been pre-treated to guard against rot/fungal growth or wood boring insect infestation. To remedy this defect, the external ground levels need to be lowered or alternatively a so called French drain can be used. It may also be necessary to replace existing sub floor vents or install additional vents (one for every linear meter of external wall is recommended).

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