Best Chimney Sealers of 2026 — Brick, Crown and Flashing

The best chimney sealer for the brick itself is a breathable, water-based siloxane water repellent — ChimneyRx Masonry Chimney Water Repellent or Drylok Siloxane 7 Brick & Masonry Penetrating Sealer for most stacks. But the brick is only one of three places a chimney lets water in. The crown on top needs an elastomeric crown coating, and the joint where the chimney meets the roof needs a flashing sealant. Sealing the brick while the crown is cracked or the flashing is open is money spent on the part that was not leaking.

The other rule matters more than any brand: never put a film-forming sealer, clear acrylic or masonry paint on chimney brick. A chimney gets wet from the inside and from the top no matter what you put on its faces, and a film stops that water drying out. The result is spalling — brick faces popping off after a few freeze-thaw winters — and it is the most common self-inflicted chimney repair there is.

Why Chimneys Leak

A chimney takes weather on four faces and from above, passes straight through the roof, and carries warm, moist flue gas up its middle. Water gets in at four predictable points, and the product you need depends on which one it is.

1. A Cracked Crown

The crown is the cement slab capping the brick around the flue tile. On many houses it is only a thin "mortar wash" troweled on at a slope, with no overhang or drip edge, and it cracks within a few years. Every rain then sends water down into the core of the chimney, soaking the brick from behind. No face sealer can stop water that enters from the top.

2. Failed Flashing

Flashing turns the roof's waterproofing up against the chimney: step flashing woven into the shingles, with counter-flashing let into a mortar joint and lapped over it. The usual failure is the sealant where the counter-flashing enters the brick, or a gap someone filled with roofing tar instead of metal. It is the most common cause of "chimney leaks" inside the house.

3. Porous Brick and Mortar

Older, softer brick and weathered mortar joints absorb a surprising amount of water in wind-driven rain. That water freezes, carries salts to the surface as white efflorescence, and works its way into the attic. This is the problem a siloxane water repellent solves — and only this one.

4. A Missing Cap

A standard 9 × 13-inch flue with no cap admits roughly half a gallon of water for every inch of rain, rusting the damper and soaking the smoke chamber. A stainless steel cap with mesh sides, typically $50–$200, fixes it outright and keeps animals out. It is the cheapest chimney waterproofing there is.

Find the Leak Before You Buy Anything

• Ceiling or rafter stains beside the chimney — almost always flashing.
• Water in the firebox or a rusted damper — a missing cap or a cracked crown.
• Efflorescence, damp patches or flaking brick faces outside — water moving through porous brick and mortar, often fed by the crown.
• Brown, smelly staining on the chimney breast indoors, even in dry weather — often condensation from an appliance venting into an unlined flue. That is a liner problem, and no exterior sealer will touch it.

If the source is not obvious, do a hose test. With a helper watching the attic, run a hose gently on one zone at a time for 10–15 minutes: the roof below the chimney first, then the flashing on each side, then the brick faces, then the crown. Work bottom-up — starting at the top sends water over everything and tells you nothing.

Why the Brick Sealer Must Breathe

A chimney cannot be made fully waterproof, and trying to is what destroys it. Water vapour reaches the brick from the flue, from the house, and from any rain that gets past a crown or a joint, and it has to dry outward through the faces. A film-forming coating — clear acrylic "masonry sealer", waterproofing paint, elastomeric wall coating, anything that leaves a sheen — slows or stops that drying. The trapped water freezes behind the surface and shears the brick faces off. Once they go, the fix is replacing brick, which costs far more than any sealer.

Penetrating repellents soak into the pores and line them with a water-repellent layer without filling them. Liquid water beads off; vapour still passes out; the surface looks unchanged. Siloxane or a silane/siloxane blend suits the porous clay brick most chimneys are built from — see silane vs siloxane sealer for the chemistry, and our brick sealer guide for the rest of the house.

The Best Chimney Sealers

Six products, covering the three jobs: three breathable water repellents for the brick, one crown sealer, and two flashing sealants for different sizes of problem.

Side-by-Side Comparison

The "Membrane" entries are deliberately waterproof films — right on a crown or roof joint, wrong on the brick faces, which must dry.

Testing Absorption

Before sealing, and every couple of years afterwards, check whether the brick actually needs it. Mist a few bricks and a mortar joint with a spray bottle and watch for a minute. Water that beads and runs off means the brick is dense or still protected. Water that darkens the brick and vanishes within seconds means it is absorbing freely. Check every face — the side facing prevailing weather usually goes first.

Masons quantify this with a Karsten (RILEM) tube: a small graduated tube stuck to the brick with putty, read for how many millilitres soak in over set intervals, before and after treatment. Asking a contractor whether they tube-test is a fair way to judge how seriously they take the job. Either way, try the sealer on a few bricks on the least visible face first and check the colour the next day.

Applying It Safely From a Roof

Most chimney sealing is a roof job, and the roof is the dangerous part, not the chemistry.

Know When Not to Go Up

• Steep pitch. Above roughly a 6/12 pitch — six inches of rise per foot — walking unaided is genuinely unsafe. Use a roof anchor and harness, roof brackets with a plank, or hire it out.
• Two storeys or more. A fall from a second-storey roof is life-changing. Many chimney companies will waterproof a stack for a few hundred dollars, which is cheap against that risk.
• Wet, frosty, windy or very hot days. Wet shingles are slippery, wind carries spray across the roof, and hot asphalt shingles scuff and soften underfoot.
• Tile, slate or metal roofs. Brittle or slippery — leave them to a roofer.

Ladder and Footing

Set the extension ladder at a 4:1 angle — one foot out for every four feet of height — on firm, level ground, extended at least three feet above the eave, and tie it off if you can. Wear soft-soled shoes, haul tools up in a bucket on a rope, and have someone on the ground. Never step onto shingles you have just sprayed: they are slick.

Prepare the Chimney

Brush off loose debris, moss and efflorescence with a stiff nylon brush. Repoint failed joints and let new mortar cure — 28 days is the conservative standard — before sealing. Sheet the shingles around the base and keep overspray off windows and skylights.

Wait for Dry Brick and Dry Weather

Penetrating sealers need empty pores to soak into. Wait for at least two or three dry days after rain — longer after a wet spell — and pick a day with no rain forecast for the product's stated window (Drylok specifies six hours). Most water-based siloxanes want temperatures above about 40°F during application and drying; check the label.

Apply Bottom to Top, Flood Coat

Use a pump sprayer at low pressure — under about 50 psi, so the product flows rather than mists — and work upward from the bottom of each face, keeping a run of about six inches of liquid below the spray line; that run is what saturates the brick. Treat mortar joints thoroughly, and apply any second coat wet-on-wet within the label's window.

Crown and Flashing Before the Faces

Do the crown coating and flashing repair first, on a separate day if necessary. Stop the crown coating at the edge rather than letting it run down the brick, where it would form exactly the film this guide warns against.

How Often to Reapply

A siloxane water repellent is not a permanent coating, but it does not wear off like paint either. It degrades slowly under UV and weather, and the sign is simple: the brick stops beading water. On a typical above-roof stack, expect five to ten years from a good water-based siloxane, with weather-facing sides and harsh freeze-thaw climates at the shorter end. Manufacturer figures — Drylok estimates more than ten years for Siloxane 7 — assume ordinary wall exposure, and a chimney is not ordinary exposure.

Do the spray test every two to three years, ideally at the annual sweep inspection. When water darkens the brick instead of beading, clean the faces and reapply — no stripping needed, because there is no film.

Crown coatings and flashing sealants need a look every year: peeling, splits at corners, or coating pulled away from the flue tile are easy to touch up if caught early.

DIY vs Professional Costs

Materials are cheap; the roof is the cost. On a single-storey house with a low-pitch roof, doing it yourself saves several hundred dollars and is well within reach. On anything steeper or taller, the professional price buys the safety equipment and a trained eye for crown, liner and flashing problems, and bundling it with an annual sweep often brings it down.

The Short Version

Put a cap on the flue if it has none. Coat a sound crown with MasonryDefender Chimney Crown Sealer, or have a failing one rebuilt. Reseal leaking flashing with ChimneySaver FlashSeal and fabric, or Sashco Through The Roof! for small gaps. Then treat the brick with a breathable siloxane: ChimneyRx Masonry Chimney Water Repellent as the default, Drylok Siloxane 7 for the best coverage and value, or MasonryDefender Chimney Brick Sealer for a small stack ordered online.

And keep anything that leaves a film — acrylic sealers, masonry paint, elastomeric wall coatings — off the brick faces, as on any exterior brick wall. More chimneys are damaged by the wrong waterproofer than by the weather it was meant to stop.

Frequently Asked Questions

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