Penetrating Damp

The main requirement for a building in our climate is to prevent penetrating damp in the building’s envelope.  The envelope of the building must shed rainwater away from the building, most of the rain in Britain come from the south westerly direction. When a building is new penetration damp should be a rare occurrence, but over time, and erosion from the weather elements, defects begin to appear in buildings due to a lack of maintenance, which allows rain water to penetrate the structure. 

Penetrating damp in a house will lead to dry and wet rot in structural timbers, woodworm infestation and an unhealthy atmosphere. A survey should be carried out to find the extent of the damage caused.

Some signs of penetrating damp

  • Stain marks on external walls
  • visible damp patched on walls and ceilings
  • wet and crumbling plaster
  • Erosion of masonry

How the climate effects a building

The southwesterly elevations of buildings generally suffer most from rain penetration because this is the prevailing direction of the wind.  It can damage the fabric of the roof and chimneys, it carries leaves, which block gutters, downpipes, and drives rain into cracks in the masonry, gaps around windows and doors.  On the opposite side of the building it creates an under pressure which draws dampness into the building.  However, in built up urban area the local effect on the wind’s direction can be changed due to densely and shape of buildings.  In high rise buildings wind strengths and directions may change with height.  On coastal sites the movement of onshore wind has an added effect on buildings due to the salt carried in the air (salt damp).  This has major effects on buildings constructed using lime mortar. 

Ice, frost and snow (water in its solid state) can damage masonry pointing, create spalling on the face of bricks leaving them porous and subject to more rapped decay, and can crack renders.  Any flashings embedded in pointing attacked by frost can become loose and allow water penetration.  The frost action (combined with salts) on one side of a chimney over time may cause the joints in the stone/brickwork to open wider and cause the chimney to lean.

The heat from the sun can cause blistering of flat roof felting fabric leading to defects and penetration of rainwater and insulation failure.  The shrinkage of timber windows and door frames etc., away from the masonry, over stretch sealants and allow rain penetration.

Images of some period property’s suffering from penetrating damp

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Images of some modern property’s suffering from penetrating damp

Defects in rainwater disposal goods

Damp patches and sometimes surface mould inside buildings are often caused by water penetrating from faulty rainwater goods.  These are responsible for most wood rot in buildings and all abutting timbers must be investigated.

Typical defects are blocked downpipes, guttering with incorrect fall or a belly creating overspill, guttering standing beyond the drip of tiles or slates.

Cast iron rainwater goods are particularly prone to rust and rot. The backs of cast iron gutters rust with age often due to lack of paint. The backs of cast iron downpipes are rarely painted and rust and crack with age.  Also, frost can split rainwater down pipes.

Broken or missing shoes at the bottom of downpipes cause rainwater to be deflected onto the wall instead of away from the building.

Valley, parapet, and box gutters often cause wood rot problems due to defects in their surfaces or they become blocked and allowing water to escape over the flashings and into the building, this is a big cause of dry rot.  There supporting timber may warp, bend or affected by wood rot which will cause changes in falls or slopes creating ponds. The lead forming valleys and gutters will become thin and pitted due to pollution and acid carried in the rain water, erosion with age allowing the wood support beneath the gutter to become wet, this is a course of major wood rotting fungi infections.

Common repairs Scotland Act