ASK THE EXPERT

Douglas Kent, technical and research director at the Society for the Protection of Ancient Buildings, answers your renovation queries

QThe living room floor of our late-17th-century cottage is uneven, with some old flagstones and patches of concrete. We think the flags have been laid directly onto the earth, and we’ve recently lifted the damp-proof membrane to find damp patches. Is there a way of levelling the floor and incorporating a dampproof or thermal solution? Undulations add interest to old floors so unless they’re dangerous are best left and not ironed out. Isolated uneven stones can be rebedded in coarse sand or hydraulic lime mortar, or missing areas reinstated without re-laying the whole floor, although any concerns about instability in the subfloor should be investigated/ rectified first. Slight undulations can be reduced by laying suitable matting to avoid disturbing a floor.

Most old floors were laid directly onto earth or chalk. It sounds as if a modern, plastic damp-proof membrane (DPM) has been laid over your floor, which is inadvisable as it prevents the old stone floor ‘breathing’, trapping moisture in the flags. As with inserting a DPM below flags, it can also displace moisture into the base of a dry adjacent wall by restricting evaporation.

Instead, if a floor is genuinely damp, aim to tackle the cause by removing any rubber-backed carpets, improving drainage or controlling condensation. Allow the floor time to dry out. Residual salt deposits can be brushed off or poulticed out, but if you do decide to lift the old floor, take great care to re-lay it on a new base with insulation. Damage to the original flags is inevitable, but choose a ‘breathable’ material like insulated lime concrete, which is generally laid without a DPM.