Should I put a water repellent on my outside walls?

Water repellents get sold as a general cure for damp walls and they are almost never that. They do one thing: reduce the amount of liquid rain the outer face takes up. If your wall is wet because of a gutter, a bridged barrier, a cracked render or condensation inside, a repellent changes nothing at all, and if the wall has a way for water to get in behind the treated face, the treatment makes the drying worse.

Safety

Step by step

1. Rule out every other cause first.Rainwater goods, pointing, render cracks, sills, flashings, ground level, cavity condition. A repellent on a wall with any of those is money spent covering up a fault.
2. Establish that the wall is genuinely rain-saturating.The pattern is damp that appears on the weather elevation during prolonged driving rain, over a broad area rather than at a point, in a house in an exposed location. That is exposure, and it is what a repellent addresses.
3. Understand the difference between the product types.A silane or siloxane cream or liquid penetrates the surface and makes the pores water-repellent while remaining vapour-open. A film-forming sealer or a masonry paint sits on the surface and reduces vapour transmission as well. Only the first is appropriate on masonry.
4. Check the wall is sound and dry before applying anything.Repoint failed joints, repair cracks, and let the wall dry properly. Sealing water into a wall is the failure mode, and applying to damp masonry also stops the product penetrating.
5. Do a test patch on an inconspicuous area.Colour change, darkening and patchiness are common and permanent. Some products change the appearance of certain bricks and stones substantially.
6. Treat the whole elevation, and generously at details.Untreated gaps concentrate water. Sills, reveals, string courses and the junction with the roof need particular attention because they are where water is directed.
7. Know the lifespan.These are not permanent. Manufacturers typically quote a period of years and the figure varies widely with product, substrate and exposure, so check the current data sheet rather than any general claim.
8. Reassess after a full winter.If the internal damp is unchanged after a wet season, the diagnosis was wrong and the money is gone. That is the commonest outcome when the cause was never exposure.

Tips

• External wall insulation with a render finish solves both the exposure and the heat loss at once, and on a badly exposed wall it is often the better use of money.
• A repellent will not bridge a crack. Anything wider than a hairline needs repairing first.
• On a listed building or in a conservation area, treatments to the external face may need consent. Ask before you order the product.

Common mistakes

• Using masonry paint as a waterproofer on an old solid wall — It forms a film that slows evaporation far more than it slows absorption, because water gets in behind it at every crack, sill and junction. The wall then holds water it used to shed, and the paint flakes as it tries to leave.
• Applying a repellent to a wall with a bridged damp course or a leaking gutter — Neither of those puts water in through the face of the brickwork, so the treatment cannot intercept it — and the treatment slows the drying of the water that is already there.

Questions people ask