Chimney Waterproofing

What waterproofing actually does

Brick, stone and mortar are porous. Every rain, some water soaks in, and in winter that water can freeze and slowly break the masonry apart. A water repellent applied to the outside of the chimney reduces how much water the surface absorbs.

The word "waterproofing" oversells it a little. The treatment doesn't seal cracks, close open mortar joints, or stop water coming through the crown, flashing or an uncapped flue. It reduces absorption through sound masonry. That's useful, but it's one part of keeping a chimney dry.

Think of it as the last layer of protection rather than the first. Water that gets into a chimney usually finds the easiest path: the top of the flue, a cracked crown, a gap in the flashing, an eroded joint. Those paths need to be closed by repair. A repellent then reduces the slower soaking that happens across the whole surface of the brick in every rain.

Breathable vs. film-forming products

This is the choice that matters most.

A chimney gets moisture from inside too: from combustion exhaust and from humid indoor air. That moisture needs a way out. That's why a vapor-permeable product made for masonry is the right type for a chimney. Our guide Do chimney sealers work? goes further into product types.

When it helps and when it doesn't

It tends to help when

• The brick and mortar are sound, or have just been repaired
• The chimney is exposed to wind-driven rain
• The brick is a type that absorbs water readily
• You see mild efflorescence on otherwise sound masonry

It won't fix

• Open or eroded mortar joints
• Cracked or missing crowns
• Loose, missing or poorly installed flashing
• An uncapped flue
• Condensation inside a flue that vents a gas or oil appliance

Applying a repellent over these problems can hide them for a season without stopping the leak.

How we assess it

Before recommending a water repellent, we look at whether the chimney actually needs one and whether it's ready for one. From the roof, when it's safe, we check the brick and mortar for open joints, cracks and spalling, and look at the crown, cap and flashing that are the more common water entry points. Inside, the damper, smoke chamber and flue show whether water is getting in from the top; a chimney camera helps where the flue allows. If there's an active leak and the source isn't clear, a controlled water test can narrow it down.

You get photos and our findings. If repairs are needed first, we'll say so before any treatment is scheduled.

We also look at what the masonry is made of and whether it has been coated or painted before. Some old brick and mortar, and masonry already treated with another product, need a different approach or a test area first.

Preparing the masonry

The product label and manufacturer's instructions govern how a repellent is applied. Typically that means:

• Repairing joints, brick and crown before treatment
• Cleaning the surface so the product can penetrate
• Letting new mortar cure for the period the manufacturer specifies
• Applying to dry masonry in suitable weather
• Protecting the roof, siding and windows nearby

Efflorescence is a sign of water moving through the masonry. It should be cleaned and its source understood first. Our guide to white staining on brick explains why.

Timing and upkeep

Water repellents work best on masonry that's dry, clean and fully repaired, in weather the product label allows. In practice that means:

• Schedule it after repairs. If repointing or crown work is planned, the repellent comes last, after new mortar has cured for the time the manufacturer specifies.
• Avoid wet spells. Masonry that's still damp from rain may not absorb the product properly.
• Watch the temperature. Most products have a minimum application temperature, which limits winter work.
• Check it over time. A simple sign the treatment is working is water beading on the brick after rain. When the brick starts darkening and soaking up water again, it may be time to reapply.

Keep an eye on the parts the repellent doesn't protect. A new crack in the crown or a lifted piece of flashing will let water in no matter how well the brick is treated. Checking these at your yearly inspection keeps small problems from undoing the work.

Limits and what affects the scope

• Repellents wear over time; how long they last depends on the product and exposure
• The size and height of the chimney, and how many faces are exposed
• How much repair is needed before treatment
• Stucco-clad chimneys have their own issues. See stucco chimney repair
• Painted or previously sealed masonry may not accept a penetrating product until the old coating is dealt with

Next step

If your chimney has been repaired recently, or you're seeing early signs of water absorption, request an assessment and ask about waterproofing. A team member will contact you to confirm a time.

Request a waterproofing assessment

Questions homeowners ask