Question answered
Do chimney waterproofing sealants work?
The short answer
Chimney waterproofing sealants can work, but only in the right place. A breathable water repellent on sound brick can reduce how much rain the face soaks up. A film-forming or non-breathable coating on damp or damaged brick can trap moisture and make the stack worse. No sealant mends open joints, a cracked flaunching or a lifted flashing, which is where most stack leaks start.
Sealants promise a dry chimney from a tin and a brush. Some help a little on the right brick. Others trap the water they were meant to keep out.
Two kinds of sealant, two outcomes
Sealants for brickwork fall into two broad groups. A breathable water repellent soaks into the brick and lines the tiny pores, so rain beads and runs off while water vapour can still pass out. A film-forming coating sits on the surface like a thin skin, and it blocks the pores.
The difference matters because brick is always taking in a little moisture and drying out again. That cycle is how a stack stays healthy. A breathable treatment leaves it intact. A non-breathable one stops the drying half, so any water that gets behind the film, through a crack or from inside the wall, stays there.
When trapped water freezes, it pushes the face off the brick. That is the flaking called spalling. A coating bought to protect the stack can end up causing the damage, and the failure often shows only after a couple of winters.
What a sealant cannot do
A water repellent affects the face of the brick. It does nothing for a gap in the mortar, a cracked flaunching, a pot that has loosened, or a flashing that has lifted at the roof junction. Those are the usual routes for water into a stack, and a coating passes straight over them.
It also cannot reverse damage that has been done. Bricks that have already crumbled stay crumbled. Wet brickwork sealed in that state keeps its moisture. Treating a damp stack without fixing the source is like painting over a stain on a ceiling.
Repellents wear off with weather and sun, so even a sound application needs renewing at some point. Product claims vary, and a manufacturer's data sheet will say how long a treatment should last and on which surfaces it is meant to be used. Read it before you buy anything.
When a sealant might be worth considering
There are cases where a breathable treatment has a place. A stack in good condition, with sound pointing and flashings, built from a porous brick that soaks up driven rain, may benefit from a repellent after the repairs are done. The treatment goes on last, over clean, dry, repaired brickwork, as an extra layer rather than a rescue.
For a stack that leaks into the loft or shows damp on the breast below, the order is different. Trace the entry point, fix it, let the brickwork dry, and only then think about a coating. Our chimney inspection page describes how we trace water from the damp patch back to its source.
On older stacks built in soft lime mortar, we are cautious about adding anything that changes how the wall breathes. Let the surveyor look at the brick and joints before you decide.
Fixing the real routes for water
Where water is getting in, the repair is usually mechanical. Open joints are raked and repointed using a mortar matched to the brick. The flaunching is cut out and renewed with a sloped bed that sheds water away from the pots. Lead is dressed back into the brick joint, or replaced where it has split. Our page on chimney breasts removed covers a different cause of damp in the room below.
A cowl or a cap can stop rain going down an unused flue, and a bird guard keeps nests from blocking it. Fitting a bird guard is a simple job that also helps keep the flue ventilated.
Where bricks have failed, they can be replaced, and where the stack is too far gone, it is taken down and rebuilt. We aim for the smallest job that properly fixes the fault: a repair where a repair is enough, and the survey pictures show you why.
Questions to put to anyone selling a coating
If a roofer or salesperson proposes a sealant, ask a few plain questions. Is it breathable, and how do they know? What did they find wrong with the stack, and have they shown you? Will the joints, flaunching and flashings be dealt with first? What does the manufacturer say about how long it lasts?
Be wary of a quote that offers waterproofing alone for a stack that is visibly cracked or leaking. A coat of liquid on top of a defect is a cheap-looking answer that rarely holds. Also be wary of anyone who cannot say what the product is.
Insist on a written quote, and ask whether the figure includes VAT and scaffolding. Ours follows a survey, arrives with photographs and gives a single total. If you would like the stack looked at before deciding, the full range of work is on our chimney repairs page, and our guidance on taking a stack down covers the other end of the scale.
Sealants: two more questions
Can I apply a waterproofer to my chimney myself?
Applying it is easy; reaching the stack safely is not, and the brick needs to be clean, dry and repaired first. A coating applied over damp or damaged brick can do harm. Have the stack surveyed before you buy a product.
Is a clear sealant better than a coloured one?
Colour is a separate question from breathability. A clear repellent can still block the brick if it forms a film, and a coloured paint is almost always a film. Check the data sheet for whether the product lets vapour through before you look at the finish.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.