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Question

I live in a semi-detached house that was built in the 1970s. The heavy rain of recent months has resulted in the accumulation of water in the sub-floor void. I am concerned the accumulating water will lead to rising damp and I want to know how the water is getting in and how to stop it. I am not sure where to begin.

The floor is comprised of timber with joists. There is about 1ft of sub-floor space, and I’m almost certain there’s a concrete oversite. I reckon the sub-floor oversite is about 5 inches (127mm) lower than ground level.

Any advice on where I should begin would be greatly appreciated

Answer

The floor construction you mention is called a suspended timber ground floor. The floor consists of a series of timber joists arranged parallel to each other, supported at each end on small sleeper walls, as well as at intervals along their length with honeycomb sleeper walls and wall plates (wall plates are long, horizontal timbers resting on the honeycomb wall supporting the parallel joists).

The honeycomb walls are constructed in such a way as to allow a continuous passage of air to flow beneath the suspended joists. To give access to this air, air bricks are also built into the outer structural walls of the building. This constant flow of air is necessary to keep the timber dry and prevent the possibility of dry or wet rot occurring.

Incorporated into the construction of the sleeper and honeycomb walls is a damp-proof course (DPC) which acts as a physical barrier to prevent dampness rising up through the blockwork to the timber floor. DPC is normally inserted below the floor wall plate about 220mm above the “concrete oversite” (the concrete fill below the floor) and so rising damp is unlikely to occur within the honeycomb walls. A DPC should also be included within the perimeter walls of the property above external ground level at a minimum of 150mm “splash-back height”.

In your case, if the external floor level is constructed 127mm above the internal level, there is a possibility of water breaching the DPC resulting in rising damp to the external wall and potentially seepage through the perimeter wall (subject to construction) which would, as you suggest, accumulate in the sub floor void.

The principle behind ground-floor construction is that the internal sub-floor level is higher than the external ground level to limit the possibility of water seepage.

You should also explore the potential for other water sources, ie to ensure there is no leaking pipework or blocked gully traps that may be attributable to water within the void.

I would recommend that the floor void is monitored over a period of four months to determine if the water within the void will evaporate.

Subject to monitoring it may be necessary to reduce the ground level around the building in line with the sub-floor “concrete oversite” .

Andrew Ramsey is a Chartered Building Surveyor and Chartered Project Management Surveyor and is chairperson of the Building Surveying Professional Group of the Society of Chartered Surveyors Ireland (MediKids).

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