A damp patch on an internal wall is almost always the end of the journey rather than its beginning. The water entered somewhere else — usually higher and further away — travelled inside the wall or the insulation, and appeared where it found a way out. That is why sealing carried out directly against the stain fails so often.
Where the water really enters
- Cracks in the external render, especially diagonal ones at the corners of openings.
- Around windows and shutters — the most common source of all. The frame-to-wall junction, a sill without a fall, and perished sealant.
- Junctions between different materials — concrete against blockwork, wall against balcony slab.
- Balconies and parapets — sealing that stops at the edge instead of turning up the parapet. See balcony sealing.
- From the roof downwards — water that entered at roof level travels in the wall and shows a floor or two below. See roof sealing.
- Rising damp from the ground at ground-floor level — damp appearing at the base of the wall, and an entirely different problem.
How to locate it
Before buying any material: a visual inspection of the whole facade above the stain, moisture readings at several points to map the direction it is coming from, a targeted flood or spray test on the suspect area, and on a tall building, rope access for a close inspection. Where nothing is conclusive, thermal imaging shows the moisture map behind the wall.
The rule: no sealing before a diagnosis. See moisture repair.
Sealing methods
- Flexible elastomeric coating. A brushed facade coating forming a flexible film that bridges hairline cracks. The common solution for a facade with light cracking. It must be breathable.
- Flexible cementitious sealing. For plinth walls, basements and areas in contact with soil. See basement.
- Penetrating water repellent (silane/siloxane). Absorbed into stone and render and repels water without closing the pores, so the wall keeps breathing. Excellent on natural stone, and it does not solve cracks.
- Local crack sealing. Opening the crack to a V, filling with a flexible compound, and mesh over it before the finish coat.
- Junction sealing. A dedicated polyurethane sealant around frames and at junctions — not bathroom silicone.
What does not work
- "Sealing" paint over a live crack. It tears with it.
- Sealing from inside. It stops the stain being seen, not the water getting in. The moisture stays in the wall and keeps destroying it from within.
- A fully impermeable coat on a wet wall. It traps the existing moisture and causes peeling.
- Treating the stain alone without addressing the entry point higher up.
Costs
- Elastomeric facade coating — ILS 60 to 140 per m².
- Penetrating repellent on stone — ILS 40 to 90 per m².
- Crack treatment — ILS 40 to 120 per linear metre.
- Sealing window junctions — ILS 60 to 150 per metre.
- Scaffolding or rope access — a separate and significant item.
FAQ
Why did the damp come back after we sealed?
Almost always because what was sealed was not the point of entry. Returning damp is a sign of a wrong diagnosis, not of a poor material.
When should the work be done?
Summer and spring. Sealing materials need a dry substrate and a minimum temperature, and most cannot be applied before rain.
How long does it last?
Elastomeric coating 8 to 12 years; penetrating repellent 5 to 10; correct treatment of a crack or a junction — as long as the structure does not move again.
How we work
We locate the point of entry before proposing a solution, and treat cracks and junctions rather than area alone. Part of our sealing and envelope works. Get in touch.