Penetrating damp is water getting into a building from outside through a defect, such as leaking gutters, failed pointing, cracked render, a poor chimney flashing or a leaking pipe. It usually shows as damp patches that get worse after rain, and you stop it by finding and fixing the defect, then letting the wall dry out.
Penetrating damp is the one that gets missed most, because people look at the inside of the wall when the answer is nearly always on the outside. Water is coming in sideways or downwards through something that’s broken, blocked or badly detailed.
The common causes I find on Yorkshire houses are:
- Gutters and downpipes that are blocked, leaking at joints or overflowing, soaking the same patch of wall every time it rains.
- Pointing that has eroded or been redone in a hard cement mortar that cracks and traps water.
- Cracked or blown render, and render carried down over the DPC.
- Chimney stacks, flashings, flat roof edges and parapets.
- Window and door surrounds with failed sealant or sills with no proper drip.
- Plumbing leaks inside the building, which aren’t strictly penetrating damp but get mistaken for it all the time.
The tell-tale signs are damp patches that appear or spread after heavy or wind-driven rain, damp at any height including upstairs and around ceilings, and staining that lines up with something outside: a downpipe, a chimney breast, a window. On exposed elevations, typically the side that takes the weather, it can be more widespread.
The fix starts outside. Get up and look, or get someone to look: clear and repair the gutters, rake out and repoint with a suitable mortar, repair or remove failing render, sort the flashing. On sound brickwork that is just porous, a breathable masonry water repellent can help, but it isn’t a cure for cracks or failed joints and it isn’t right for every wall, so get advice first.
Once the water is stopped, the wall needs time to dry, and on a thick solid wall that can be months, not days. If the plaster has been badly damaged or is full of salts, it may need replacing. What it almost never needs is a chemical DPC injection. Injecting a wall that’s being soaked from a leaking gutter is money down the drain, literally.
When to call a specialist
If you've fixed the obvious defects and the damp is still there, or you can't safely get up to inspect gutters, chimneys and roofs, a specialist inspection will track down where the water is getting in. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
Frequently asked questions
How can I tell if damp is penetrating damp?
It usually gets worse after rain, can appear at any height, and often lines up with an external defect like a downpipe or chimney.
Can penetrating damp affect cavity walls?
Yes, for example where the cavity is bridged by debris or where insulation in an exposed wall carries water across.
How long does a wall take to dry out after penetrating damp?
It depends on the wall and the weather, but solid masonry can take many months once the source is fixed.