Penetrating damp, just as its name implies, is where water leaks into a building from outside the structure, through the walls or roof. Less common is where an internal leak from a water or drain pipe leaks into the structure of the building.Common causes of penetrating damp are:

• Porous Walls '� aging and crumbling bricks lose the ability to keep out rain penetration.
• Spalled Bricks & Degraded Mortar
• Bridged cavity or defective cavity tray
• Damage to stone and brickwork such as wall cracks allowing water to penetrate.
• Defective guttering and downpipes, roof damage, dilapidated or poorly installed windows.

As with rising damp and condensation problems, a thorough investigation is needed to establish the cause and decide on a cure, which may need some expert guidance.For example, a leaking internal water pipe can cause signs of damp some distance away from the actual leak - water can travel externally along a pipe.Look for lose or missing tiles on the roof, ridge pointing coming away, overflowing gutters or damaged downpipes and joints '� binoculars are useful for this.Examine the gutters and downpipes and the brickwork or other masonry around them for signs of water leakage and damage '� usually it will leave a greenish stain over time. Never allow these types of leaks to continue for long without treatment as they will inevitably cause expensive damage if left.Look for cracks in external walls and especially inspect the walls that get the most severe weather, driving rain will penetrate if the mortar is defective, soft or has gaps. Consider re-pointing and silicon sealing or even complete rendering of a problem wall.Check flashings on roof joins and valleys and the roof underfelt for any holes or deterioration. Particularly check flashing where a roof meets a chimney wall. If any of these are loose or missing, they'll need patching, re-fixing or re-pointing.Check all the window frames for any gaps or cracks or masonry distortion around them. These may need filling with a flexible frame sealant. Also check the window sills for a 'drip groove', which will stop water running down the walls.Cracks in the external walls or missing mortar will cause ingress of water over time. In old buildings, lime and sand mixes are often used. These are more porous and so will allow the building to 'breath'� and take away water from the bricks or stone. You may need professional advice for this.The cavity wall may need to be cleared of debris or a damp-proof cavity tray added. It is quite possible that penetrating damp is caused by problems with cavity walls, in which case again you may need professional help.Later built properties (post 1920s) will usually have a cavity between the inner and outer walls, so there is air space for moisture penetrating through the exterior wall to evaporate before it gets to the inner wall. However, a debris build up at the bottom, or faulty cavity wall insulation can bridge the gap causing water penetration form outside.Where the wall is 'interrupted' with a window or pipework for example, there needs to be a cavity tray. These come in various forms but effectively act as a way to drain any water away from the inner wall to the outside through 'weep' holes (holes in a part of the outer wall).If a cavity tray is defective, or there isn't one present, then it needs to be repaired or a new one installed. As with clearing out a cavity, this is a professional job.Keep an eye on all outside walls of a property for signs of water running down the brickwork or stonework leaving a tell-tall trail, as although this may not result in water penetration immediately, it will deteriorate the structure over time.