\n\n

Chimney Flashing on a Metal Roof: Standing Seam Details

Chimney Flashing on a Metal Roof: Standing Seam Details

How to flash a chimney on a standing seam metal roof: base, side, back pan and counterflashing, cricket sizing, clip and sealant choices, and common leak points.

Chimney flashing on a standing seam roof is a different job from flashing on shingles. The panels run continuously from eave to ridge and move with temperature, so the flashing has to seal the chimney without pinning the panels in place. Get that one idea right and most of the leaks, oil-canning and callbacks around chimneys go away. This guide is written for roofing contractors and builders who install or order the metal: what each flashing piece does, when a cricket is needed, which metals and sealants to use, and what to hand your supplier so the pieces arrive made to fit.

MidAtlanticMetalPanels
midatlanticmetalpanels.com
Plan Chimney Flashing Around Standing Seams
Mid-Atlantic Metal Panels supplies standing seam panels, trim packages, and flashing packages for contractors across five Mid-Atlantic states.
Explore contractor supply

TL;DR

  • Standing seam panels expand and contract, so chimney flashing must tie into the seams or a curb without screwing through the panel flats and locking the roof in place.
  • Use a base (apron) flashing, step or side flashing, a back pan or cricket on the uphill side, and counterflashing set into the masonry or over the chimney’s cladding.
  • A cricket is generally required when the chimney is more than 30 inches wide parallel to the ridge. Many pros add one on narrower chimneys too.
  • Match the flashing metal to the panel metal, and use sealants only as a backup to the mechanical laps, never as the main defense.
  • Shop-fabricated, one-piece pans and measured, custom-length trim cut field labor and leak points.

Table of Contents

Why a metal roof changes the flashing

On a shingle roof, step flashing is woven into each course. The roof surface is static, and every piece is small and overlapped. A standing seam roof has no courses to weave into. Each panel is a long, rigid sheet with raised ribs, and it slides on its clips as the sun heats it and the night cools it. A chimney, on the other hand, does not move at all.

That difference is the whole problem. The Roofing Brief makes the point that a metal-roof chimney detail succeeds or fails on sealant chemistry, panel attachment and keeping dissimilar metals apart, none of which matter much on shingles. A flashing that is rigidly fastened to both the chimney and the panel flat will either tear its sealant, buckle the panel or open a gap. The fix is to let the flashing bridge the movement, a point also made by Rescreening Masters, which notes that flashing should not depend on rigid sealant beads to hold everything together.

If you want the numbers behind the movement, our piece on sizing standing seam clips for thermal movement walks through the calculation. For clip selection by profile, run and substrate, see matching clips to the panel.

The four flashing components, in order

The U.S. Department of Energy’s Building America Solution Center describes the roof-to-chimney water connection for brick chimneys as step flashing, base flashing and counterflashing working together. On a standing seam roof the same logic holds, with each piece adapted to the panel.

Base flashing (downslope apron)

The apron sits on the low side of the chimney. It runs up the chimney face, then laps out over the panel flats and ribs below. On standing seam, the apron is cut and formed to the panel profile so the ribs are closed off rather than left as open channels. The lower edge should lap over the panels, not under them, and should not be face-screwed through the pan into the panel without a slotted or floating detail.

Formed chimney apron lapping over roof ribs

The apron’s lower hem is where the profile matters most. A cut-and-hem in the field at this location is a common leak point, which is why many contractors order the apron shop-formed to the profile.

Side flashing

At the sides, the flashing has to cross raised seams. A common field approach is to run the flashing out past the nearest seam, fold it over the rib and fasten only through the flashing hem, so the panel stays free. Where the chimney sits close to a seam, shift the detail so a seam does not run directly into the chimney face and make a water dam.

Metal Construction News warns that penetrations placed over or beside standing seams can form a dam that impedes both water flow and panel movement. Coordinate the detail so water can run freely around the flashing.

We cover the sidewall version of this detail in 26-gauge sidewall flashing for standing seam, and the principles carry over to chimney sides.

Back pan or cricket (upslope)

The uphill side is where water and debris collect. A wide chimney acts like a dam, so the back needs either a flat back pan that drains to both sides, or a full cricket, which is a small framed ridge that splits the flow around the chimney. More on that below.

Counterflashing

Counterflashing is the piece that covers the top edge of the base flashing and keeps water from getting behind it. On brick, it is set into the mortar joints. On a framed and sided chimney, it is hung over the cladding or tucked under the siding. In masonry work, cut kerfs are typically about an inch deep. Professional Builder’s JLC article on cricket retrofits describes cutting kerfs 1 inch deep with a diamond-blade angle grinder, then setting the flashing hook and packing it tight.

The counterflashing is the part that gets to be rigid, because it hangs from the chimney and is not fastened to the moving panels. The base flashing below it floats; the counterflashing above it stays put. That split is the cleanest way to think about the detail.

When a metal roof needs a chimney cricket

The commonly cited threshold is 30 inches. The Roofing Brief’s cricket guide reports that the IRC calls for a cricket when the chimney’s dimension parallel to the ridge exceeds 30 inches and the chimney does not intersect the ridgeline, citing Section R1003.20 of the 2021 code. A municipal handout from West St. Paul, Minnesota says the same for a chimney or penetration more than 30 inches wide, and adds that the cricket covering must be sheet metal or the same material as the roof covering.

Two cautions on that. First, the 2024 International Residential Code governs chimney construction and flashing in general, but the specific way standing seam panels tie into a cricket is usually set by the panel manufacturer’s details and standard sheet-metal practice, not by code text. Second, your jurisdiction adopts its own code edition with its own amendments, so confirm locally.

When a metal roof needs a chimney cricket — overview diagram

There is also a practical view. CapOut’s component guide recommends a diverter at any chimney width, to keep debris from piling up. On a snow-shedding metal roof, a narrow chimney can still collect leaves, ice and slush on its uphill face. A small cricket costs little compared with a return trip.

Quick reference by chimney width

Chimney width (parallel to ridge) What to plan Notes
Under 30 in. Back pan or small diverter Code often does not require a cricket, but a diverter keeps debris moving
30 in. or more Full cricket Typically required unless the chimney intersects the ridge
Chimney at the ridge Flash both sides, no cricket Water has no uphill face to collect on

Building the cricket on a standing seam roof

The general sequence, as New England Metal Roof describes it, is to frame the cricket with blocking or plywood to create the slope and ridge, install waterproof underlayment, form the metal cricket panels, then tie the cricket’s edges and the chimney into the base and counterflashing.

A few points specific to standing seam:

  1. Frame to the right slope. The cricket ridge should shed water to both sides. Do not make the slope so shallow that standing water remains on the pan.
  2. Underlayment first. Use a high-temperature, self-adhered underlayment under metal, and lap it up the chimney face.
  3. Prefer one piece. Roofing pros quoted by Metal Construction News favor shop-fabricated, one-piece cricket pans. Controlled shop fabrication, welded or soldered where the metal allows, cuts field workmanship risk, and a single pan has fewer joints, fasteners and sealant-dependent failure points.
  4. Lock to the panels without screws. The same source describes using a return hem so panels lock to the pan without cuts, breaks or screws through the pan.
  5. Finish with counterflashing. Tie the cricket up under the counterflashing so the entire chimney face sheds into the pan.

If the cricket sits at a slope change, the transition details matter too. See our write-up on slope transitions on standing seam roofs.

Attachment: float the flashing, do not pin the panels

The single most expensive mistake is pinning a panel with a fastener near the chimney. Once a panel cannot move on its clips, it will oil-can, tear a sealant bead or both.

The Roofing Brief recommends non-penetrating methods: seam-clamp flashing, or a floating curb that ties into the standing seams, to avoid distorting the panels as they move. For a larger chimney, or one with a wide pan, a curb-style approach lets the panels slide past a fixed frame. We describe a shop-built version in curb flashing for standing seam.

Some rules of thumb:

  • Fasten flashing only to the chimney or curb side, and to the upper, fixed part of the panel run where the manufacturer permits.
  • Use slotted holes or hemmed edges on the downslope side.
  • Keep sealant in a supporting role. It belongs under hems and behind laps as a backup, not as a bead that carries movement.
  • Keep fasteners out of the pan where water collects.

Metals and sealants

Flashing metal should match the panels. The Roofing Brief notes that matching the flashing to the panel metal avoids galvanic corrosion, and recommends butyl tape or neutral-cure silicone that tolerates movement. On a Galvalume roof, that means Galvalume or compatible flashing, not bare copper or an uncoated dissimilar metal. A lead strip is common in masonry kerfs but should not touch exposed aluminum or zinc-coated steel without a barrier. Our explanation of the 10:1 area ratio for dissimilar metals covers when contact becomes a problem.

For sealants, avoid acetic-cure silicones, which are known to attack some metal coatings. Use what the panel manufacturer’s instructions and the sealant’s data sheet both approve.

Leak points we see most often

The American Society of Home Inspectors lists common chimney flashing defects: missing components, counterflashing not embedded properly in mortar joints, incompatible materials, rust, loose or opened flashing, and deteriorated mortar or fasteners. It also names differential movement between the roof and the chimney as a cause. Add the metal-specific ones:

  • Pinned panels beside the chimney from screws through the flats.
  • Seams dumping into the chimney face, forming a dam.
  • Open rib ends at the apron where the profile was not closed off.
  • Field-cut hems that were never sealed or formed properly.
  • Sealant-only joints that fail within a few seasons.
  • No cricket on a wide chimney, so debris and ice load the upslope joint.

Perspective: build the pieces before you are on the roof

We supply standing seam panels, trim and flashing to contractors across Virginia, Maryland, Washington DC, Pennsylvania and West Virginia, and the pattern is consistent: the chimney is rarely hard because of the chimney. It is hard because the pieces were bent on the roof, in the wind, to a profile nobody measured twice.

Our view is simple. Measure the chimney and the seam layout before you order. Decide where each seam lands relative to the chimney faces. Then have the apron, side pieces, back pan or cricket pan and counterflashing made to that layout, in the same gauge and finish as the panels. Cut-and-drop delivery of custom-length panels means the panels arrive sized for the run, so you are not trimming and re-hemming at the chimney to make up for a length that did not fit. Fewer cuts at the penetration means fewer places to leak.

We would also rather you read the panel manufacturer’s installation details for your profile than rely on any generic article, ours included. Details differ between a 1-inch mechanical lock and a 1.5-inch snap lock, and the chimney is where those differences show.

— Matt Catino

Order the chimney package with the panels

If you are ordering a standing seam job with a chimney on it, put the flashing on the same takeoff as the panels. We make trim and flashing packages in 24-gauge and 26-gauge steel, and we can coordinate custom pieces through our custom trim and flashing page, so the apron, pan and counterflashing match the panels’ metal and finish.

MidAtlanticMetalPanels

Send your chimney dimensions, roof pitch and panel profile to our team through the contractor supply page, and we will help with the takeoff. Panels come as custom-cut lengths delivered to the job site, and for hard-to-reach jobs we also offer on-site roll forming.

How to check your detail against the manufacturer and local code

This is the step that gets skipped. A short routine:

  1. Get the manufacturer’s installation manual for your exact profile and gauge. Look for its chimney, curb or penetration details and for its rules about fastening near seams.
  2. Find your jurisdiction’s adopted code edition and amendments through the building department. The IRC is a model code; local adoption varies.
  3. Compare cricket requirements. Check the width threshold, slope and covering material your inspector will apply.
  4. Check the flashing metal against the panel metal and the sealant against both.
  5. Ask the building department about permits or inspections that apply to reroofing with a chimney.
  6. Photograph the detail before covering it, so you can answer any question later.

Where the manual and a generic guide disagree, the manual wins for warranty purposes, and the local code wins for the inspection.

FAQ

Can I screw flashing directly through a standing seam panel near a chimney?

Avoid it. Fasteners through the flats pin the panel and stop it from moving on its clips, which leads to oil-canning, sealant tearing and leaks. Use seam clamps, a floating curb or hemmed edges that let the panel slide, and follow the manufacturer’s details.

Do I need a cricket on a narrow chimney?

Code generally ties the requirement to chimneys more than 30 inches wide parallel to the ridge, though your local adoption controls. On narrower chimneys a cricket is optional, but a small diverter or back pan helps keep debris and ice from collecting on the uphill face.

Can I use silicone to seal chimney flashing on a metal roof?

Only as a backup. Use a neutral-cure product or butyl tape that the manufacturer approves, and keep the main water-shedding job on mechanical laps and hems. A rigid sealant bead cannot carry the roof’s movement for long.

What metal should the flashing be made of?

Match the panels. Galvalume or painted steel panels should get compatible flashing in the same finish, which avoids galvanic corrosion. Be careful where lead, copper or aluminum touch steel, and check the manufacturer’s guidance.

Should chimney flashing be shop-made or bent on site?

Shop-made is usually better for crickets and pans because a one-piece formed pan has fewer joints and fasteners. Simple counterflashing can still be cut and set in the field.

Sources