Old House Know How: Damp

Damp

Damp is easily spotted once you know what to look out for

Penetrating damp may be spotted before even setting foot in a property by some scrutiny of the exterior. From the top down, check the chimney pointing and flashing. Look for cracked or slipped slates or tiles. If these block a valley, rainwater can be forced into the loft. Decay in soffits or eaves may be visible through any paintwork.

All rainwater goods should be drip-free at joints. One broken bracket can allow water to pool in the gutter, or to spilt under the slates onto the wall plate. A ‘hanging garden’ will often indicate this problem. Corrosion on the back of cast-iron downpipes may be keeping the wall damp –— is lichen or moss growing?

Decay in window frames can often allow copious amounts of water into the masonry. Look at the entry point of wiring — it should always ‘loop’ so that it penetrates the masonry at an upward angle. Cracks in the pointing feed moisture inwards. Cracked or damaged render is worse than no render at all. Masonry will always absorb an amount of rainwater, but this is usually evaporated by wind and sun. Poor render on a solid wall will hold trapped water against the wall, forcing it inwards.

Cavity walls in old properties may contain masonry debris from building works, which can bridge the cavity and allow transfer of moisture. (Dampness at low levels may be mistaken for rising damp.)

Inside the property, penetrating damp manifests itself by stained décor, ‘hollow’ plaster, peeling wallpaper, and decay in adjacent timbers. Salting is frequently visible. Sulphates, present in most traditional building materials, surface as a fluffy growth.

Determining the exact point of ingress is not always obvious, as moisture can track along fissures within the wall for some distance.

Remedy penetrating dampness with repair of external defects, and allow one month per one inch of wall thickness for full drying out.

If you re-render, remember that the dampness within the wall can then only dry out internally, so extend the drying period.

Pointing should always be a softer material than the masonry itself — designed to be sacrificial, and replaced periodically. Determine first that any cracks in pointing are not caused by wall tie failure or other structural movement.

If an exterior water repellent is to be used, ensure pointing is sound and masonry has no excessive fissures, or that the silicones in the water repellent can ‘line’ these defects, and actually guide water even deeper into the structure.

Problems caused by high external ground level can be alleviated by installing a French drain (gravel-filled channel).

Further reading:

• Read Part One: Electrics
• Read Part Two: Flooring
• Read Part Three: Windows
• Read Part Four: Roofing