Does chimney waterproofing spray work?

Chimney sprays fall into two groups. Breathable water repellents, usually silane or siloxane based, soak a short way into the surface and line the pores of brick and stone so that rain beads and runs off. They do not form a skin, and water vapour can still pass outwards. Film-forming sealers behave differently. They sit on the surface like a coat of varnish, shut the pores and hold water in the wall behind.

Both come with promises of years of protection. Treat those claims with caution. Manufacturers test products on clean, sound masonry in controlled conditions, and a chimney stack at roof height, with open joints and a cracked cap, is a different thing. Be wary of any lifespan quoted by a seller who has not looked at how your stack was built.

The question is not whether a repellent works in a laboratory, but whether it works on your stack, in its present state.

On a stack with sound brick or stone and sound joints, a breathable repellent can reduce the amount of rain absorbed through the face. A drier stack is less exposed to frost damage and to the salts that drive spalling, and may leave less damp to show on the chimney breast indoors. That is a modest but real benefit, provided everything else is in order.

It works best on porous bricks and open stone that take up water readily, and where the stack is tall and exposed on several sides. It is applied after cleaning, by brush or low-pressure spray, to a dry surface in dry weather. Re-treatment may be needed as the product weathers.

For an old stack, the aim is not to make it waterproof. Historic masonry is designed to take in water and let it out again. The more sensible aim is to keep it from staying wet for weeks, and a repellent is only one way to do that.

Two tests tell you something. A few drops of water on a dry face should soak in slowly on bare brick and bead on a treated one. If water darkens the brick quickly and stays dark, the face is highly absorbent, or the joints are letting it in.

A repellent cannot bridge a crack, refill a joint or seal the junction at a pot. Water that enters through a failed flaunching, a cracked pot or a lifted piece of flashing runs inside the masonry whatever the surface coating says. It is the same for a back gutter that has split, where water finds its way down regardless.

Our guide to why Stroud chimneys leak traces each leak path and the repair for it. In most cases one of those paths, not porous brick, is the reason a stack is wet. Spraying over them hides the symptom for a season and lets the real fault carry on.

A repellent also does nothing for a flue that is damp inside, or for condensation in an unused chimney. Those need ventilation or a cap, not a coating. Nor will it restore bricks that have already lost their faces; they have to be replaced.

The cap on top matters more than any spray. A cracked flaunching that lets water pour down between the pot and the stack soaks the upper courses faster than any rain on the face, and a repellent will not touch that route.

The risks come from sealing a wet wall. If a film-forming paint, a masonry sealer or a cement render is put over damp brick, moisture cannot leave. It builds up behind the coating, freezes in cold weather and pushes the brick face off. Salts crystallise under the skin and add to the damage. The result is a stack that looks sound from the road and is crumbling behind its coat. Our answer on why chimney bricks crumble explains the process in detail.

Listed houses, and homes in one of Stroud District's conservation areas, should ask the council before any treatment goes on a historic stack, because altering the look of the masonry can need consent. A film that changes the colour of stone is a change to the building.

If a coating has already been applied, a roofer can usually tell from the sheen, the dark patches where it has failed, and how water behaves on the surface.

The survey comes first. We examine the pointing, flaunching, pots, flashing and back gutter, photograph each, and list what needs repair. Those repairs come first, and a repellent is discussed only if the stack is otherwise sound. A tall stack that is being shortened to reduce its exposure is another route; see reducing a chimney's height for when that is an option.

After the survey the written quote spells out the work, and any coating worth having is yours to take or leave. The work is covered by a written 10-year workmanship guarantee. Everything else we do on stacks is described on our chimney repairs page, and send photographs if you want an opinion before booking a visit.

If you have already had a stack treated and still have damp, tell us when it was done and which product was used. That helps us separate a fault in the coating from a fault in the stack.