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How to Get Rid of Penetrating Damp

By Liz Ransome-Croker

We explain what could be causing your penetrating damp, from cracks in a wall to blocked cavity walls, and how to deal with them. 

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What's causing your penetrating damp, from cracks in external walls to leaking pipes, will affect how easy it is to solve.

This means you'll need to work out the cause of penetrating damp before you can treat it, or decide whether to get a professional in. 

In our survey more than half of people with a penetrating damp problem used a builder to deal with it, while 8% used a specialist damp company and 2% called in a surveyor. Some 22% dealt with it themselves.

Take a look at the gallery and read on below for a run down on what might be causing penetrating damp and how to fix it. 

Check gutters, roof, window frames and pipes for leaks

You'll need to work out what might be saturating your walls or ceiling with water, so cracks and defects are likely to be the culprit. Where the damp patch is will give you an indication of where to look first. For example, if it's at the height of your guttering, start by examining your gutters and downpipes for leaks or cracks. 

Other places to check include:

  • Your roof, including missing tiles, the pointing (the cement between tiles), flashing (where a roof meets the wall of a chimney) and roof ridge tiles (the triangular-shaped tiles on the very top of the roof). If any of these are loose or missing, they'll need fixing or re-pointing. 
  • Window frames to see if there are any gaps or cracks around them. If there are, these will need filling with a flexible frame sealant. Also check there is a 'drip groove' (which should stop water going onto the wall) under the windows. If there is one, make sure it's not filled in, or if there isn't one, try adding a hardwood strip (6mm square) about 35mm from the front edge to see if that solves the issue.
  • Cracks around door frames will need repairing. And if your external door doesn't have a weatherboard (which sits at the top and helps protect against excessive rain), it's worth thinking about fitting one.
  • Cracks in the external walls or missing pieces of mortar (the filling between bricks) could be the cause. If this is the case, make sure these are filled. In old buildings, lime and sand mixes are often used as these are more porous and so will take water away from the bricks. Speak to a builder for more advice on what would be best for your home.
  • Where you can access them, look at pipes in your home, particularly in the bathroom and kitchen, to see if they're leaking.

It may also be that old bricks have become porous and are allowing water to get into your home. Either get these replaced, or paint them with an exterior silicone water-repellent fluid or limewash, that will allow walls to still breathe. This could also be a good preventative measure, too, but seek professional advice first on what would work best for your home's age and type.  

In our survey nearly six in 10 people with a penetrating damp problem got their roof fixed, 34% mended cracks and leaks in their gutters and downpipes and 16% sorted gaps in their window frames and doors to deal with the problem. 

Clear wall cavities or add a damp-proof cavity tray

It is possible that penetrating damp is caused by problems with your cavity walls. Walls in modern homes (and many built since 1920) have a cavity between two walls, so that there is space for moisture penetrating through the exterior wall to evaporate before it gets to the inner wall. But there are a few reasons this could end up causing issues.

Where something 'interrupts' the wall, such as a window or pipework, you may have or need a cavity tray (see image below). These come in many different 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 in place, these may need to be repaired or added - you can find out what the cost is of having a cavity tray added to your wall by visiting our damp costs page.

There are also reasons why your cavity itself might be causing damp. It may be that the cavity has become filled with objects, such as broken mortar or bits of brick, which are taking in moisture from the outer wall and passing it to the inner wall. In these cases, the debris will need to be cleared out to stop the issue (see our damp costs page for more details).

Lastly, in some rare cases, cavity wall insulation can cause damp problems. This is usually only the case if your home is exposed to severe levels of wind-driven rain or your walls are poorly constructed or in very bad condition. You can find out more in our guide to wall insulation and damp problems.

* We surveyed 1,592 Which? members about damp in February 2015