Best Chimney Flashing for a Slate Roof (2026)

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Specifications are manufacturer rated, as published on each listing and collected 22 October 2026. Where a listing states a figure two ways, the table follows the title and bullets.

How we scored them

Each product is scored on performance, build, value, safety and warranty, with the weights published on our How We Pick page. A spec the listing does not publish scores nothing; only the Sobalai boot states a warranty term.

Scores run from 9.2 to 9.8: each product’s point total is placed on that range, so more points always means a higher score. Safety counts only for powered gear or where the listing states safety features; where it does not apply, the score is taken over the remaining 90 points.

Buying guide: best chimney flashing for a slate roof

Step flashing is the real answer

For a slate roof, the correct flashing is step flashing: small plates woven into each slate course where the roof meets the chimney. The Therwen 20-piece set gives 5 by 7 inch aluminum plates you trim to size and fix with nails, screws or adhesive. Nothing else here does this job; tapes and sealants repair flashing, they do not replace it. If your chimney flashing has failed, step flashing is what goes back in. Twenty plates cover a good stretch of chimney base, and because each plate is independent, a future repair means replacing one plate, not the whole run.

Sealant for joints, tape for patches

The ChimneyRx brushable sealant is the joint sealer: a black elastomeric coating that stays flexible where tar-based sealants crack, bonds to brick, shingles and aluminum flashing, and covers about 14 square feet per 1/2 gallon with a 12-hour dry time. The BXI tape is the patch: 4 inches by 32 feet of self-adhesive SBS rubberized asphalt with an aluminum foil backing, sticking to smooth, flat, dry surfaces with no primer or torch. Use sealant for planned repairs, tape for the leak you find during a storm. The tape’s aluminum foil backing is paintable and UV resistant per the listing, so a patch does not have to look like one for the months it sits there, and at 0.06 inches thick it bridges small gaps without a buildup that catches the next slate course.

Know what the other two are for

The Sobalai #6 boot seals round pipes from 5 to 9 inches where they pass through a roof, with a 12 by 12 inch square base, high-temp silicone rated from -100 to +500 degrees Fahrenheit, and 50 roofing screws in the box. It is built for metal roofs and vent pipes, not for a masonry chimney on slate, but it is the best-specified product here with a 2-year warranty. The Selkirk 6-inch adjustable flashing is for Type B gas vent pipe at 0 to 6/12 pitch, in 28 gauge galvanized steel: the right part for a gas vent, the wrong part for a slate chimney.

Aluminum and slate

Aluminum flashing and slate are a standard pairing, and the Therwen plates are aluminum. One honest note from the listing: the surface turns black after long rain exposure and the plates need timely replacement. That is the trade of aluminum step flashing: easy to cut and fit, but not a fit-and-forget material. Inspect it when you inspect the slates.

Counter-flashing and mortar joints

Step flashing is only half the detail: the top edge of each plate tucks under counter-flashing set into the chimney’s mortar joints. Where old counter-flashing has cracked mortar, the ChimneyRx sealant bonds directly to brick and buys time, but repointing the joint and resetting the counter-flashing is the lasting repair. The listing’s 14 square feet of coverage per 1/2 gallon goes a long way on joints, which are narrow by nature.

Temperature and surface prep

Sealants and tapes have application windows. The ChimneyRx applies between 45 and 90 degrees Fahrenheit and dries in 12 hours in dry weather. The BXI tape handles 5 to 158 degrees once applied and sticks best to smooth, flat, dry surfaces. The Sobalai boot tolerates -100 to +500 degrees. Do the work in dry weather on a dry roof, whatever you use. The ChimneyRx needs 12 hours of dry weather to set, so watch the forecast, and the BXI tape wants the surface dry at application time even though it tolerates a wide temperature range afterward.

When to call a roofer

Slate is brittle underfoot and unforgiving of mistakes: cracked slates around the chimney turn a flashing job into a slate repair job. If the chimney flashing has failed across multiple courses, or if counter-flashing needs grinding out of mortar joints, that is roofer work. The products here suit competent DIY repairs: sealing joints, patching with tape, and replacing accessible step flashing plates one course at a time.

Frequently Asked Questions

What flashing goes on a slate roof chimney?

Step flashing woven into the slate courses, plus counter-flashing in the chimney mortar joints. The Therwen 20-piece aluminum set is the step flashing pick here: 5 by 7 inch plates you trim and fix around the chimney and skylights.

Can I seal chimney flashing with a brush-on product?

For repairs, yes. The ChimneyRx sealant brushes onto flashing joints, stays flexible so it will not crack like tar, and covers about 14 square feet per 1/2 gallon. It bonds to brick, asphalt shingles and aluminum flashing.

How do I match new step flashing to old slate?

Work one course at a time from the bottom up, sliding each plate under the slate above and over the slate below, exactly as the original was laid. The Therwen plates trim with tin snips, so you can match odd widths around the chimney corners. Keep the old plates as templates for the new cuts.

Is flashing tape a permanent fix?

No. The BXI 4 inch by 32 foot tape is a strong patch: self-adhesive, no primer or torch, aluminum foil backing that is UV resistant. Treat it as the repair that holds until proper step flashing goes in. On a slate roof especially, tape buys a dry winter, not a decade.

Will a pipe boot work on a slate roof?

A pipe boot seals round pipes, not chimney bases. The Sobalai #6 fits 5 to 9 inch pipes with a square 12 by 12 inch base and is designed around metal roofing. On slate, use step flashing for the chimney and a boot only for a round vent pipe.

Which of these has a warranty?

Only the Sobalai boot: the listing states 2 years. The other four state no warranty term.

Related: the best chimney flashing for a metal roof, best chimney liner kit and the stove accessories hub.