Penetrating Damp Treatment in a Mid Terrace House, Clapham Junction SW11
Rainwater running straight down an exposed brick wall had quietly soaked its way through to the plaster inside. Here’s how the survey traced a damp patch back to an outside roof drainage problem, and what it took to dry the wall out for good.
Project at a Glance
The Brief
The client got in touch about a mid-terrace house on Boutflower Road, Clapham Junction, after noticing damp showing on an internal wall. Our surveyor, Sam Fitzgerald, went out to have a look at the affected room and the external elevation behind it, taking moisture readings on the plaster and checking the brickwork and drainage outside for anything that might explain it.
Damp on an internal wall can point in a few directions: rising damp, condensation, water getting in from outside, and it’s rarely obvious which one you’re looking at just from the state of the wall. This one turned out to be coming from outside.
What the Survey Found
The moisture readings on the internal plaster came back high, so this was active damp rather than old staining. Stepping outside explained why. The roof drainage in this area wasn’t sending rainwater where it should, so instead it was running straight down the face of the brickwork. To make matters worse, that brickwork was porous and had never been given any protective treatment, so rather than shedding the water, it was soaking it up.
Over time, that had left the wall properly saturated, and with nowhere else for the moisture to go, it had worked its way through to the inside, damaging plaster and finishes across an area of roughly 4m². There were no musty smells, and no sign of rising damp or condensation either, this was a clean case of penetrating damp: water getting in through the external structure rather than rising from the ground or forming from moisture in the air. By this point the plaster itself was damp and salt-contaminated, and it wasn’t going to dry out or sit stable without some proper work.
The Treatment Plan
Since the cause was really sitting outside as much as inside, the plan dealt with both ends of it rather than just patching up the room.
Inside, all the damp, salt-affected plaster across the 4m² area was to come off back to the brickwork, clearing out material that would otherwise just keep hold of moisture. A specialist waterproof base coat render would go on next to create a barrier, stopping any moisture still in the wall from bleeding back through into the new finish, and then a final skim would bring the wall back to a smooth surface ready for decorating.
Outside, the brickwork was to be cleaned and treated with Stormdry breathable masonry protection cream. Stormdry stops rainwater soaking into the brick in the first place, while still letting the wall breathe and get rid of any moisture already sitting inside it. That external step is really the part that fixes the actual problem; skip it, and the wall would likely just start soaking through again down the line, however good the inside looked.
The Work Carried Out
Carl Shoreman spent 2 days on site carrying out the treatment: hacking off the affected plaster, applying the waterproof render system, and re-finishing the wall ready for decoration. Nothing unexpected came up, and the job stuck to what had been agreed.
The Result
The job was completed on 29th January 2026. The client was on site at completion, was walked around by Carl to see the finished work, and confirmed everything had been done as agreed and left safe and tidy. Waste from the job was cleared and photographed for collection. It was signed off on the day and later approved by our Head Contractor.
Worth taking away from this one: the fix for a damp patch inside a room doesn’t always live inside that room. Re-plastering deals with the damage that’s already happened, but it’s treating the brickwork outside with Stormdry that actually stops the water getting back in.
Rising Damp FAQs
What is penetrating damp?
Penetrating damp involves moisture that has entered the building through its exterior parts such as walls, roof and windows. Penetrating damp differs from rising damp because it does not involve moisture that rises through the ground, and condensation because it is associated with moisture in the air.
How do you tell the difference between penetrating damp and rising damp?
Penetrating damp is associated with some specific problem outside the building and may be found at any level on the wall, while rising damp will occur below the damp proof course. Moisture readings alongside a look at the external wall are what settle it.
What does Stormdry treatment involve?
Stormdry is a breathable masonry protection cream applied to cleaned external brickwork. It reduces the amount of water absorbed by the brick, but at the same time allows any moisture contained in the wall to evaporate, which is important in order not to trap moisture.
Is there a need to strip the plaster off, or is it sufficient to just re-decorate it?
Once the plaster has become thoroughly soaked with water and the salts from the penetration of moisture into it, it will be impossible to thoroughly dry the plaster and redecorate it. It is necessary to bring the wall back to its original condition and plaster it with waterproof rendering.
Do you cover Clapham Junction and South West London for damp surveys?
Yes, we carry out damp surveys and penetrating damp treatment across Clapham Junction and the wider South West London area.
Got a damp patch that won’t dry out?
Our qualified surveyors diagnose the real cause and give you a fixed-price quote. The survey fee is deducted from the cost of work if you go ahead.
Survey carried out by Michael Young, waterproofing surveyor at Advanced Damp. Project completed in Fulham, SW6.
Sam Fitzgerald is the founder of Advanced Damp and a working damp and structural waterproofing surveyor with over 12 years of experience diagnosing and treating moisture problems in residential and commercial properties across London and the South East. He is FODCS certified (a Federation of Damp Certified Surveyor) and a member of the Federation of Damp. His hands-on work covers rising damp, penetrating damp, condensation, basement waterproofing to BS 8102, and thermal imaging surveys, and he has personally completed over 400 damp surveys across properties of every type and age. Everything he writes draws directly from jobs he has carried out. He does not write about treatments, products or causes he has not dealt with first-hand in the field.
