Roof Flashing Types and When to Use Each: Step, Counter, Valley, Pipe Boot, Chimney Apron

A roof leaks at transitions, not in the middle of the field. That means flashing is the real waterproofing. Here''s the field reference for every flashing type my crews install, with material trade-offs and install sequence.

Step Flashing

Use it at: any vertical wall meeting a sloped roof. Dormer sidewalls, chimney sides (not fronts), skylight sides.

What it is: a bent L-shaped piece of metal, usually 5" tall x 7" wide x 90 degree bend. One flange lies on the roof under a shingle course, the other goes up the wall.

Install rule: one piece per shingle course. Each piece laps the one below by at least 2 inches. The wall flange sits behind the siding or counter flashing. Never caulk step flashing as the primary seal, caulk is backup only.

Common failures: one long piece of bent metal instead of stepped (leaks because water gets under the top edge), or step flashing face-nailed into the wall without counter flashing (water runs behind in 2 years).

Counter Flashing (Reglet Flashing)

Use it at: brick chimneys, masonry walls, anywhere step flashing terminates against a hard vertical surface that can''t have siding laid over it.

What it is: a Z-shaped piece of metal that tucks into a mortar joint or kerf cut above the step flashing and bends down to cover it.

Install rule: grind a 1-inch-deep reglet kerf into the mortar joint (not the brick face), tuck the top leg of the counter flashing in, fill the kerf with polyurethane sealant (Sika Sikaflex-1a, NP1). The bottom leg covers the step flashing by at least 3 inches.

Material: copper or lead for chimneys that will last 30+ years, aluminum for quick work.

Valley Flashing

Use it at: any roof plane intersection forming a V.

Three styles:

• Open metal valley: W-shaped painted steel, 18 to 24 inches wide, shingles cut back to leave 3 to 6 inches of metal exposed each side of centerline.
• Closed-cut valley: shingles from one side run fully through, shingles from other side cut 2 inches short of centerline. No metal visible.
• Woven valley: shingles from both sides interwoven. Not allowed with laminated / architectural shingles per most manufacturer warranties.

Install rule: 36-inch peel-and-stick first, then metal (if open) or nothing (if closed), then shingles. Closed-cut is the most common in 2027 because it looks clean and performs well if the peel-and-stick is right.

Pipe Boot Flashing

Use it at: plumbing vent pipes penetrating the roof.

Types:

• Rubber (EPDM) boot with aluminum base: $12 to $18. Seal fails at year 8 to 12 from UV.
• Silicone boot (Perma-Boot, Lifetime Tool): $28 to $45. 20-year life.
• Lead boot: $35 to $70. 30+ year life, turn down into pipe for total seal.

Standard rubber boots are what''s on 80 percent of roofs and they''re why so many attics leak at year 10. Upgrade to silicone or lead on any job where the homeowner is staying. Pair with a 12-inch peel-and-stick patch under the boot.

Chimney Apron Flashing

Use it at: the downhill (front) side of a chimney or other rectangular penetration.

What it is: a single piece of bent metal, usually 18" wide across the front, that goes under the shingles above the chimney and bends down to cover the top of the chimney face brick and the first 4 inches of roof deck downhill.

Install rule: apron first at the down-slope face, then step flashing up both sides, then saddle (cricket) flashing at the back if the chimney is over 30 inches wide.

Cricket / Saddle

Use it at: the uphill side of any chimney or wide penetration over 30 inches wide per IRC R1003.20.

What it is: a small framed slope built behind the chimney that diverts water around it instead of damming against the back.

Install rule: frame with 2x lumber, sheathe with 1/2" OSB or plywood, peel-and-stick the entire cricket, then flash with step and counter, then shingle. Skipping the cricket on a wide chimney is a classic builder shortcut that causes a leak at year 5.

Skylight Flashing Kits

Buy the manufacturer kit (Velux EDL, Fakro, Sun-Tek). Don''t piece one together. The kit includes the apron, sill, head flashing, and step flashing sized for the specific skylight frame. Deck mount gets one kit, curb mount gets a different kit.

Lead vs Aluminum vs Galvanized

MaterialBest forLifespanAvoid LeadChimney aprons, pipe boots on high-end work50+ yearsPulling off unused lead, EPA waste handling AluminumStep, counter, valley metal on asphalt30 to 40 yearsContact with copper or pressure-treated lumber (corrodes) Galvanized steelRough work, sheds, barns20 to 30 yearsRusts at cut edges, not for coastal Painted steel (Kynar)Matching metal roof35+ yearsScratching during install CopperSlate, tile, historic75+ yearsGalvanic contact with aluminum

Material I Won''t Mix

Copper and aluminum together is galvanic corrosion waiting to happen. Copper wins, aluminum dissolves. Don''t run aluminum valley metal up to a copper chimney cap. Don''t use aluminum step flashing next to copper gutters. Match materials or isolate with rubber gasket.

RoofKnockers Flashing Specs

On each job in RoofKnockers the flashing list lets you spec material, linear feet, and upgraded pipe boots so the estimate doesn''t undershoot. See features. Also check the drip edge guide for eave and rake metal.

FAQ

Can I reuse existing flashing on a reroof?

Only if it''s less than 10 years old and in perfect shape. Most step flashing on a 20-year-old roof has torn nail holes and rust at the bottom fold. Tear it off and replace. It''s an hour of labor and $60 in material to save a 10-year callback.

Is caulk a flashing?

No. Caulk is a backup seal, never a primary seal. A roof waterproofed with caulk is a roof that''s going to leak in 18 months when the caulk cracks.

What flashing do I use at a valley on a metal roof?

Metal roofs use either a W-shaped matching valley panel from the panel manufacturer or a flat valley pan with hemmed edges. Never use asphalt shingle valley metal under metal panels.