Should You Waterproof a Chimney With Sealant?

Brick and lime mortar are porous on purpose. They take in some water in wet weather and let it out again when wind and sun return. A stack that dries between storms stays sound for a long time.

What does a sealant do to that cycle?

A film-forming sealant, such as a clear coat or a silicone-type liquid, closes the surface pores. Water already inside the wall has no way out, and more can keep arriving from the top of the stack and from the flue. That trapped water freezes in winter and pushes the face off the brick, the damage described in our answer on crumbling chimney bricks.

The coating can also hide the first warning signs. A coated stack may look dry from the garden while the brick behind the skin is soaked. The first you notice is a flaking face, or a stain on the wall indoors, by which time the damage is well advanced and costs more to put right. Brick that has been soaked for a season takes a long time to dry once the cause is fixed.

A chimney is wet for a reason, and the reason is usually a defect, not the brick itself.

Where does the water usually get in?

At the top, the flaunching, the sloping mortar crown that holds the pot, can crack and let rain run down. Joints on the shaft erode and open. The lead flashing, the metal strip where the stack meets the roof, can lift or split. A pot without a cap lets rain fall down the flue. Any one of these can feed a damp breast, the thick wall around the fireplace, and several together often do.

Each fix is specific. The crown is renewed in fresh mortar. Repointing restores the joints. New lead replaces failed flashing. A cap closes the flue.

That costs more than a tin of sealant, but it deals with the water instead of covering it. Afterwards the brick can dry out, and the stack settles back into balance.

Spend the money on the cause and the coating question often answers itself. A dry, sound stack seldom needs a coat of anything.

A few products are made to be vapour-open, meaning they let moisture pass outward while shedding rain. Sellers call them breathable water repellents, and they are a different thing from a sealing film. Even so, no coating mends a crack or a missing joint.

Is a breathable repellent a good idea?

Sometimes, on sound brickwork with good pointing, and only after the faults are mended. Even then the product must suit the brick, because a poor choice can still stain or trap moisture. Follow the maker's data sheet, and ask a roofer or builder who knows the stack before you buy.

On older, softer brick pointed in lime, the safer route is to repair and repoint in the right mortar and leave the surface alone. Our answer on lime mortar on chimneys explains the match between mortar and brick.

Never brush any sealant onto a stack that is cracked or loose, and never put it over a joint that is still wet. The first job there is to make it safe, and a coating adds nothing to that. Once the stack is stable, the repair can be planned in the right order.

If your chimney looks wet or stained, the sensible order is survey first, then repair, then a decision on whether anything extra is needed at all.

How do we find the cause?

We inspect the stack from a safe position, look closely at the crown, joints, flashing and pot, and photograph each so you can see them. Indoors, the pattern of the damp on the breast often points to where the water is coming in. Damp that shows low in the room, high on the wall or only after wind-driven rain each point to a different route in.

If the survey turns up several faults, the repair is planned as one job and not a string of small ones, with the order of work set out so nothing is done twice. Where a stack is badly weathered or leaning, repair may not be enough, and a rebuild is the better route.

A bead of sealant squeezed along a flashing is only a stopgap, because it hardens and splits as the metal expands and shrinks. The survey is followed by one written price. The work is described in plain words, so you know what each part of the repair is meant to cure. All the stack work we offer sits under chimney repairs.