Chimney Flashing vs Step Flashing: Which One Should You Buy?

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Chimney flashing vs step flashing: the quick answer

Chimney flashing is the umbrella term for every piece of metal or sealant that waterproofs the chimney-to-roof joint: base flashing along the bottom, step flashing up the sides, counter flashing embedded in the chimney, and a cricket or saddle behind it. Step flashing is the specific part made of small L-shaped pieces, each one overlapping the shingle below it, that sheds water down the sides of the chimney. When people say their chimney flashing failed, the step flashing is the usual suspect, because each piece has to sit right and stay sealed.

What is chimney flashing

Chimney flashing is the weatherproofing system where a chimney passes through the roof. It has several parts: base (or apron) flashing at the downhill side, step flashing up both sides, counter flashing tucked into the chimney to cover the step flashing tops, and sometimes a cricket to divert water on the uphill side. Materials are usually galvanized steel, aluminum or copper, though sealants play a role too. Its only job is to move water off the chimney and onto the shingles without letting any under them.

What is step flashing

Step flashing is the set of small L-shaped metal pieces installed in steps up the sides of the chimney, one piece per shingle course. Each piece overlaps the shingle below and sits under the shingle above, so water runs down the steps and off the roof. It is the part most exposed to movement: the chimney and the roof expand and shift differently, which is why step flashing loosens and its sealant cracks over time. When you see water stains on the ceiling near the fireplace, failed step flashing is the first place to look.

Which one do you need

If the leak is at the chimney sides and the metal looks intact, you need step flashing work: either new pieces or a sealant over the old ones. If the leak is at the base or the whole joint looks tired, the full chimney flashing needs attention, including the counter flashing and the cricket. For a pipe or stove vent rather than a masonry chimney, neither applies: that takes a pipe boot flashing, which is a different product. The gear section below ranks four listings that match these jobs.

Gear for the job, ranked

Ranked by stated features and warranty, strongest first. The ChimneySaver FlashSeal leads: a 1 gallon elastomeric sealant in synthetic rubber that creates a flexible waterproof barrier, applied with a brush at a 1/8 in coating, and backed by a 7-year warranty when used with FlashSeal Fabric. It bonds to asphalt shingles, metal flashing and brick, and comes in white, black and brown. Second is the Aothotop #7 silicone pipe flashing boot: a high-temp silicone boot for 6 to 11 in OD pipes, stated to withstand +500°F heat and -100°F cold, at 14.2 by 14.2 by 7.1 in and 2.1 lb, with roofing screws and a hex socket included. One caution: the listing says no tools are needed, but it also includes screws and a hex socket, so basic tools are part of the job. Third is the Therwen 120-piece aluminum flashing sheet set: 5 by 7 in silver plates, 120 pieces, for step-flashing repairs around chimneys and skylights, cuttable with tin snips and attachable with nails, screws or adhesive tape. Fourth is the UNITED STATES HDW 8 in rainproof flashing: a thin listing with little stated beyond plastic construction and a 1 lb weight, so buy it only if the 8 in size matches your need.

How to fix flashing around a chimney

Start by finding the failed part: look for lifted step flashing, cracked sealant or rusted base flashing. Small cracks and holes take a sealant like the FlashSeal, applied at the stated 1/8 in coating over the cleaned joint. Loose or missing step pieces need new metal: the Therwen 5 by 7 in sheets cut to size and slide under the shingles. Reseal the tops with the elastomeric sealant. For a full replacement, the old flashing comes off, new step pieces go in course by course, and counter flashing covers the tops.

Frequently Asked Questions

What is a chimney flashing?

It is the weatherproofing system where the chimney meets the roof: base flashing, step flashing up the sides, counter flashing and sometimes a cricket. Its job is to shed water onto the shingles without letting any under them.

How does chimney flashing work?

Each piece overlaps the one below it so water runs downhill over the metal and onto the shingles. Step flashing does this in steps up the chimney sides, one L-shaped piece per shingle course, with counter flashing covering the tops.

How do I fix flashing around a chimney?

Find the failed part first: cracked sealant, lifted step pieces or rusted base flashing. Seal small cracks with an elastomeric sealant, replace loose step pieces with new metal cut to size, and reseal the joint tops.

What size flashing do I need for a chimney?

Step flashing pieces are small: the Therwen set here is 5 by 7 in, a typical step size. For pipe vents, measure the pipe OD: the Aothotop boot fits 6 to 11 in pipes. Match the product to your chimney or pipe, not the other way around.

Can I seal chimney flashing instead of replacing it?

For cracks and small holes, yes: the FlashSeal listing states a 1/8 in brush coating that seals flashing, vents and stacks, with a 7-year warranty when used with its fabric. Rusted-through or missing metal needs replacement, not sealant.

Does step flashing go under or over the shingles?

Each step piece goes under the shingle above it and over the shingle below it, so water steps down the roof. Counter flashing then covers the tops of the step pieces where they meet the chimney.

Related guides: our best chimney flashing for a flat roof picks, the best chimney flashing for a slate roof, the double wall vs triple wall chimney pipe comparison, the stainless vs galvanized chimney cap comparison and the best chimney brush for a metal chimney.