How to Waterproof a Brick Chimney? | Fix Mortar First, Then Seal

The exterior masonry face of a brick chimney is porous by design. That porosity is fine until the joints fail, the crown cracks, or the cap stops shedding water, at which point water absorption, efflorescence, and freeze-thaw spalling follow.

The order of operations is the whole job: fix the structure, then treat the surface. Get that backward and you trap moisture inside a chimney that has no way to dry out.

Fix The Chimney Before You Seal Anything

Penetrating water repellent makes a sound chimney more water-resistant; it does not fix a chimney that is already failing. Repointing or repairing degraded mortar comes first, because sealing over failed joints locks water into the wall instead of keeping it out.

Work through the structure in this order:

• Trace the water to its actual source before touching the brick face. Roof flashing, step flashing, and the flashing-to-chimney joint are common offenders, and no sealer on the market stops water that is arriving from the roof.
• Repair or repoint deteriorated mortar joints. Crumbling or recessed joints are a leak path no coating can close.
• Inspect the crown — the sloped concrete cap at the top. If it is cracked, spalled, or missing its drip edge, repair or rebuild it.
• Confirm the chimney cap fully covers the flue tile and sits properly fitted, so rain falls past the flue instead of into it.

Skip any of these and the sealer is money spent on a wall that still leaks.

Does Waterproofing Actually Work On Brick?

Yes — a clear, breathable water repellent reduces water absorption on brick and mortar that is structurally sound, and it is the treatment most often recommended for fired brick chimneys. The product types to look for are siloxanes, silanes, and blends of the two.

One example from the market is A-T Brick Chimney Sealer, which Applied Technologies describes as a penetrating silane/siloxane blend.

What to avoid: film-forming coatings. Paint and non-breathable surface sealers hold up fine for a few seasons, then trap moisture behind the film. Trapped moisture drives efflorescence, staining, and — in freeze-thaw climates — spalling brick. For the specific products that get this right, this roundup of tested chimney waterproofing products covers what is worth buying.

Applying And Caring For A Breathable Masonry Sealer

Once the chimney is structurally sound, the application itself is straightforward — the results hinge on surface condition and cure time.

1. Clean the brick and mortar so the sealer can penetrate bare masonry, not dirt or biological growth.
2. Let the wall dry completely. Applying to damp masonry traps water under the treatment and stops the sealer from soaking in.
3. Apply the vapor-permeable sealer with the method the manufacturer specifies for brick and mortar.
4. Follow the cure-time instructions exactly. The sealer needs that window to set before rain hits it.

Two limits are worth knowing before you spend the afternoon. First, weather exposure drives the value here: in regions with heavy rainfall or wind-driven rain, a water repellent earns its cost quickly; in a dry climate it matters less. Second, historic or soft brick needs extra caution, because softer brick absorbs water differently from modern fired brick.

Compatibility is another quiet trap. Clear water-repellent treatment is the common choice for brick chimneys; opaque coatings get used more often on concrete brick or lightweight concrete masonry, not on typical fired brick chimneys. Match the product to the material in front of you.

FAQs

Can I seal a chimney that has crumbling mortar? No. Repair or repoint the mortar first, then treat the brick. Sealing over degraded joints hides the problem while trapping water inside the wall, which accelerates the damage you were trying to stop. Structure first, surface second.

Will waterproofing stop an active leak in my chimney? It depends on where the water is actually coming from. A sealer reduces wind-driven rain penetration through the masonry face, but it will not solve flashing failures or leaks traced to adjacent roof components. Diagnose the source before you treat the brick.

Does a clear sealer or a painted finish last better? Clear, penetrating, breathable sealers are the safer choice for fired brick. Painted or film-forming coatings hold water against the masonry and are tied to spalling and staining over time. Opaque coatings generally suit concrete brick or lightweight concrete masonry instead.