Masonry waterproofing and sealing

What sealing does and does not do

A masonry sealer is a breathable water repellent. It sits in the surface of the brick and stone and stops liquid water soaking in, while still letting vapour out. What it does well is reduce how much water a sound wall absorbs during driving rain, which means less freeze damage and a longer life for the mortar joints.

What it does not do is bridge a gap, fill a crack, or hold a failed joint together. It is a finishing coat on a repair, not a repair.

Why order of operations is the whole job

Here is the failure mode we get called out to. A wall has open joints and is letting water in. Someone seals the outside face. The sealer does not close the open joints — water still gets in through them — but it does slow the wall's ability to dry back out through the face.

Now the water going in has a harder time coming out. It sits in the wall through the winter, freezes, and the brick starts spalling on a wall that had only needed pointing. We have seen handsome sealing jobs on walls that were being destroyed underneath them.

So: point it first, replace whatever brick has already gone, and then seal. In that order it is genuinely worth doing.

When it is worth it

• Immediately after a full repointing, to protect the work you just paid for
• On elevations that take the prevailing weather and stay wet longest
• On soft, porous older brick that drinks heavily during long rain
• On chimneys and parapets, where exposure is worst

And when it is not worth it: on a wall with open joints, on a wall with active spalling, or as an answer to an interior damp problem whose source has not been identified yet.

Coatings that seal the wall shut

Be careful with heavy film-forming paints and waterproof coatings sold for masonry. Unlike a breathable repellent, these can seal the face of the wall completely — and a wall that cannot breathe out is a wall holding its water in.

On old Chicago brick that usually ends badly. If a previous owner has already coated your brick, tell us, because it changes what the wall needs and it changes what we can sensibly do to it.

Common questions

Should I seal my brick?

Only once the wall is sound. If the joints are open or the brick faces are popping off, sealing first makes it worse — the repair comes first and the sealer protects it afterwards. On a freshly pointed wall, or on soft old brick that drinks heavily, it is worth doing.

How long does a masonry sealer last?

Several years, depending on the product and how much weather the wall takes. It is a maintenance coat that gets reapplied, not a one-time fix, and the most exposed walls — chimneys and parapets — wear through it first.

Will sealing change the look of my brick?

A breathable water repellent is meant to be close to invisible: no shine, no colour change. Film-forming paints and coatings are a different thing — they change the look, and they can trap water in the wall, which is why we warn against them on older brick.

Will sealer fix a damp wall inside the house?

Not on its own, and not until the source is known. Damp on an inside wall can come from open mortar joints, a failed lintel, a leaking parapet or the roof. Sealing the outside without finding which one just hides the evidence while the water keeps coming.

Related work

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