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Rain Coming Through A Solid Brick Wall
A damp patch appears on an internal wall during heavy driven rain and dries out again afterwards.
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What's happening?
Damp that arrives with the weather and leaves when the wall dries is penetrating damp, and it behaves nothing like rising damp — it is high up as often as low down, it follows the wind, and it appears on the elevation that faces the prevailing rain. On a solid wall the water crosses through the masonry itself, which is why Approved Document C treats solid walls differently from cavity walls and points to BS 8104 for assessing how exposed a wall is to wind-driven rain. The usual route is the vertical perpend joints. Approved Document C is explicit that masonry should be laid on a full bed of mortar with the cross joints substantially and continuously filled, and a wall built with the perps only buttered at the front has a line of small voids running right through it.
What usually causes it?
- Perpend joints never fully filled when the wall was built, leaving voids straight through the masonry
- Recessed or eroded pointing holding water on the brick edges instead of shedding it
- Spalled brick faces exposing the porous core, which absorbs far more water than the fired skin did
- Cracked render or a failed coating on a rendered solid wall, letting water in and stopping it drying out
- Missing or failed cavity tray, cill DPC or lintel tray above an opening, so water tracks inwards at that point
- Blocked or overflowing gutter, downpipe or hopper repeatedly soaking one strip of wall
- Severe exposure — a gable or an unsheltered south-westerly elevation taking the full driven rain
Is it dangerous?
The damp itself is a health and fabric problem rather than a structural one, but leave it and it compounds. A wall that is repeatedly saturated loses mortar and brick faces faster, wets any embedded timber such as joist ends or a bonding timber, and grows mould on the internal finish. It also makes the room genuinely harder and more expensive to heat, because wet masonry conducts heat much better than dry masonry. The point to act is when the patch is reappearing every wet spell rather than shrinking, or when internal plaster has begun to blow.
Can I fix it myself?
The diagnosis, which is most of the value. Go outside during heavy rain with the wind behind it and watch where water actually goes — gutter joints overflowing, a downpipe running down the wall, a cill throwing water back, render cracked at a corner. Note whether the internal patch is highest at a particular point, because water enters above where it shows. Clearing gutters and downpipes is worthwhile and often solves it outright. What does not work is a waterproof masonry sealer: it slows drying more than it stops entry, and a solid wall that cannot dry outward holds the water it already has.
When should you call a bricklayer?
Call a bricklayer where the pointing has failed, where perp joints need cutting out and filling properly, or where brick faces have spalled on the weather elevation. Call a builder where the fault is a missing cavity tray or lintel DPC, or where render needs stripping and replacing. If the wall is solid and severely exposed, ask about the whole strategy for that elevation rather than patching the wettest square metre.
What will the bricklayer actually do?
- Inspect the weather elevation in the wet, tracing the water path rather than treating the internal stain
- Cut out and repoint failed joints, filling the perpends fully through the joint rather than buttering the face
- Replace spalled bricks so the wall presents a fired face to the weather again
- Reinstate or install cavity trays, cill and lintel damp proof courses with stop ends where water is tracking in at an opening
- Repair or renew cracked render in a mix that lets the wall keep drying outwards
- Refer the rainwater goods, roof detail or internal replastering to the trade that owns it
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