Chimney flashing is typically corrosion-resistant metal—aluminum, steel, copper, or lead—backed by sealant and a self-adhered membrane.
Why Chimney Flashing Matters
Where a chimney intersects a roof, water wants to run down the brick and under the shingles. Flashing forms a metal “gutter” at that seam so runoff drops back onto the roof surface and exits with the rain.
Modern codes keep the materials consistent. Metal flashing must be corrosion-resistant and at least 0.019 inch thick, which aligns with 26-gauge galvanized sheet. You can verify that in the International Residential Code.
Chimney Flashing Materials And What They’re Made Of
The table below compares the common options and where each one fits best around a chimney.
| Material | What It Is | Where It’s Used / Notes |
|---|---|---|
| Galvanized Steel | Steel sheet with a zinc coating | Widely used for base, step, and back-pan pieces; protect cut edges; keep away from copper runoff. |
| Stainless Steel | 304/316 alloy steel | Stiff, clean, and salt-resistant; great for long pans and crickets. |
| Aluminum | Light, formable non-ferrous sheet | Easy bending; good with asphalt shingles; isolate from copper and fresh cement. |
| Copper | Cold-rolled sheet, often 16–20 oz | High-end look; solderable; long service on slate, tile, or high-end asphalt. |
| Lead | Soft sheet, often 2.5 lb/ft² | Conforms to rough brick; popular for counterflashing; handle with care. |
| Lead-Coated Copper | Copper with factory lead layer | Copper strength with a gray surface that stains less. |
| Self-Adhered Membrane | Rubberized asphalt or butyl | Backup layer beneath metal at aprons, back pans, and crickets; not a stand-alone fix. |
| Sealants | Polyurethane or hybrid mastics | Used in reglets and sealed laps; look for ASTM C920 on the label. |
What Is Chimney Flashing Made Of At Each Layer
A chimney detail is a stack of simple parts that work together. Each piece handles water in a specific spot and overlaps the one below it.
Apron At The Downslope Face
This L-shaped pan sits under the shingle course in front of the chimney and turns up the brick several inches. Galvanized steel, aluminum, stainless, copper, and lead all see duty here.
Step Flashing Along The Sides
Sidewalls get individual L-shaped “steps,” one per shingle course, lapped like shingles. That stair pattern kicks wind-driven water back onto the roof at every course. Trade guides such as the GAF steep-slope field guide call for legs of about 5 inches on the roof and 5 inches up the wall, plus a matching counterflashing over the step tops to stop blow-back.
Back Pan And Cricket
The pan on the high side spans the width of the chimney. Wider stacks get a small ridge, called a cricket, to split water and snow so nothing pools behind the brick. Stainless or heavier copper hold shape on long runs.
Counterflashing In The Mortar Joint
Counterflashing is the skirt that keeps water from curling behind the base pieces. The top edge tucks into a reglet or a raked mortar joint; the bottom leg laps the base flashing. Copper.org details show base flashing rising at least 4 inches and the counterflashing overlapping it by a few inches. Brick Industry Association notes the same concept and stresses a clean reglet and tight laps.
Reglet, Sealant, And Fasteners
A straight kerf lets the counterflashing sit flat. Drive soft lead wedges every foot, then tool a polyurethane sealant in the joint. Match fasteners to the flashing metal. When metals must mix, slip in a separator—membrane strip, paint, or plastic shim—so galvanic action doesn’t chew up the joint.
Metal Choices: Pros, Cons, And Fit
Pick based on roof type, exposure, and build method.
Galvanized Steel
Affordable and predictable. It folds cleanly and pairs well with asphalt shingles. Keep cut edges treated and avoid direct contact with copper or copper runoff.
Stainless Steel
Strong and stain-resistant. It shines on long pans and near salt spray. It’s stiffer to form.
Aluminum
Lightweight and easy to work. Isolate it from wet treated lumber and from copper above.
Copper
Elegant and durable. Common weights are 16 oz for steps and aprons, 20 oz for wide pans. Soldered seams seal cleanly, reliably. Keep copper away from galvanized or aluminum below.
Lead And Lead-Coated Copper
Lead hugs irregular stone and wavy brick with minimal cutting. 2.5 lb/ft² is a frequent spec for counterflashing. Lead-coated copper offers similar workability with added stiffness and a low-stain gray finish.
Coatings And Finishes
Factory-painted steel and aluminum resist chalking and stains. Clear coats on copper slow early color change. Seal bare edges so rust can’t start. Skip asphalt over copper; it smears in heat.
Code Rules And Field Standards
The code sets minimum thickness and requires corrosion-resistant metal. Copper.org shows base flashing climbing at least 4 inches with counterflashing overlapping it; see the chimney flashing page for clear laps and reglet work.
One more field rule saves a lot of callbacks: separate dissimilar metals. When copper, aluminum, and steel touch in the presence of water, the less noble metal corrodes first. An isolator such as a membrane strip or paint breaks that reaction and keeps seams clean.
Sizes, Thickness, And Joining
Step flashing legs of about 5×5 inches are common; each piece should extend past the shingle exposure by at least 2 inches. On big stacks, size up the back pan so its top edge hides under a full shingle course. Hemmed drips stiffen edges and shed water. Keep nails out of the vertical leg; fasten through the shingle only.
Thickness picks start at the code minimum. Typical residential choices include 26-gauge galvanized or stainless, 0.019-inch aluminum, 16 oz copper for steps and aprons, and 20 oz copper for crickets and back pans. Lead for counterflashing is often 2.5 lb/ft². On long seams, solder or rivet-and-seal laps.
Workmanship Details That Make Or Break A Chimney
Layout and overlaps do most of the waterproofing.
Lap And Overlap
Counterflashing should lap the base pieces by a few inches. Each step must shield the shingle course and the step below. Aim seams away from the water path.
Cut The Reglet Clean
Keep the kerf straight and consistent. Drive soft lead wedges every foot, then gun polyurethane into the joint. Leave a hairline gap for movement.
Back-Pan Drain Path
Slide the pan far enough up-slope to sit under a full shingle course. Where snow or leaves pile up, a cricket keeps water moving. Kickout flashing at the lower sidewall steers runoff clear of the chimney shoulder and siding below.
Mind Dissimilar Metals
Separate copper from aluminum or galvanized steel and match fasteners to the flashing metal. Small choices here prevent stains and pinholes.
Durability And Care
Galvanized steel eventually thins at raw edges once the zinc sacrifices itself, so sealed cuts pay off. Aluminum doesn’t rust, but alkaline wash from fresh masonry can attack bare surfaces. Stainless shrugs off most grime. Copper darkens to brown and green while staying watertight. Lead holds a flat gray and seats into rough mortar. Once installed well, only a quick look each spring and fall is needed to catch loose wedges, split sealant, or branch damage. A tight chimney cap and crown also limit wash down the stack so flashing sees less runoff.
Local exposure steers the pick. Near salt or constant wet, stainless or copper stays stable. In shaded yards where debris builds, size a cricket so the pocket can flush during heavy rain.
Cost And Value Calls
On a typical reroof, labor outweighs sheet cost. Crews often upgrade the back pan and counterflashing on big stacks even when the rest of the roof gets galvanized or aluminum. A smart mix keeps the bill in check: galvanized or aluminum for steps and apron, stainless or copper for the back pan, and a matching counterflashing set in a clean reglet.
Second Table: Quick Spec Guide By Metal
Use this cheat sheet when choosing a chimney detail. Values are common residential picks, not rigid rules.
| Metal | Common Residential Spec | Service Tendencies |
|---|---|---|
| Galvanized Steel | 26 ga; 5″×5″ step pieces; coated back-pan | Performs well when edges are sealed and kept clear of copper runoff. |
| Stainless Steel | 26 ga or heavier on long pans | Holds shape on wide spans; solid near salt and constant wet. |
| Aluminum | ~0.019″ coil stock; painted steps | Bends easily; isolate from copper and fresh cement; avoid direct contact with wet treated lumber. |
| Copper | 16 oz steps/apron; 20 oz back-pan | Accepts solder; strong pick for slate, tile, and asphalt. |
| Lead | 2.5 lb/ft² counterflashing | Fits rough masonry; handle safely; follow local rules on use and disposal. |
Two Quick Myths, Debunked
“A Bead Of Caulk Is Enough”
Sealant isn’t the flashing. It’s a gasket for the reglet and a helper at laps. Metal base pieces and counterflashing do the waterproofing.
“One Long Side Panel Beats Steps”
Continuous side strips trap water. Step flashing sheds at every course. That’s why shingle makers and trade manuals call for individual steps on sidewalls.
Picking The Right Setup For Your Roof
Asphalt shingles with a brick stack: galvanized or aluminum steps with a stainless back pan and stainless or copper counterflashing is a balanced setup. Slate or tile: copper throughout, or stainless back-pan with copper steps and counterflashing, keeps lines crisp. Wood shakes move more than shingles, so larger steps and generous laps keep the joint tight.
Bottom Line On Chimney Flashing Materials
What is chimney flashing made of? In most homes, it’s corrosion-resistant sheet metal—galvanized steel, stainless steel, aluminum, copper, or lead—formed into base pieces and finished with true counterflashing. Add a peel-and-stick membrane where it makes sense, use a quality polyurethane in the reglet, and separate dissimilar metals. Follow the code for thickness and the trade guides for sizes and overlaps. Do that, and the stack stays dry through hard rain, drifting snow, and gusty crosswinds.