How To Deal With Penetrating Damp

Penetrating damp is caused by water seeping through the walls (as opposed to rising damp, which is confined to the lower part of ground floor walls). You may see damp patches appear when strong winds drive rain against the wall of your house, and disappear when the weather improves.

Difficulty rating: Medium

These tasks may be tricky so will suit you if you’re experienced in DIY, or simply love a challenge.

Before you get started on any of our ‘how to’ guides, please take a moment to read through our DIY safety tips.

Step 1

Look out for damp patches on the ceiling around your chimney breast. If damp appears here, it could mean that flashing, which seals the joint between your chimney stack and roof, may have cracked or become dislodged. To fix this problem you’ll need to refit the loose flashing, or replace the damaged flashing.

Step 2

If a damp patch appears on your upstairs ceiling when there’s heavy rain you could have a broken or missing roof tile that is letting water through, so you may need to replace the tile.

Step 3

If the inner side of an external wall shows signs of widespread damp, which spreads even further in wet weather, this may mean older bricks have become porous or they are letting water penetrate through the wall. To fix this problem, replace or repoint the bricks and treat the area with exterior silicone water-repellent fluid.

Step 4

If your brickwork isn’t causing the problem, mortar on a wall tie in the cavity could be bridging the gap and allowing water to cross to the inner wall.

Step 5

If you notice damp patches around your windows, mortar might have fallen out of the gap between the wall and window frame. To fix this, seal gaps with a flexible frame sealant.

Step 6

Your exterior window sill should have a drip groove on the underside to stop rainwater from running under it and into the wall. If there’s a damp patch along the underside of your window frame, this groove may need to be cleared. If there isn’t a groove, glue and nail a hardwood strip (about 6mm square) to the underside of your wooden sill about 35mm from the front edge. This will stop water from reaching your wall.

Step 7

If there are signs of rot at the base of an external door, or damp patches on your floor just inside the door, you may need to repair your door or replace it. It could be that your door is water-tight yet in an exposed position; if this is the case, make the door more weatherproof by fitting a weatherboard.



Writer and expert