What roof flashing is and where it goes

Flashing is a thin sheet of metal — usually aluminum, galvanized steel, or copper — that directs water away from joints where the roof meets a wall, chimney, vent pipe, or valley. Water pools at these joints naturally. Without flashing, it runs under the shingles and into the structure below. Flashing is the barrier that stops that.

The most common places you'll install flashing are where a roof meets a vertical wall (called step flashing), around chimneys (cricket flashing and counter flashing), at roof valleys where two slopes meet, and around penetrations like plumbing vents and exhaust pipes. Each type has a slightly different shape and installation order, but the principle is the same: the flashing overlaps the shingles above it so water sheds downward and outward.

Key Takeaways

  • Flashing is metal that channels water away from roof joints; it must overlap shingles above it so water runs down and off, never under.
  • Step flashing (for walls) installs one piece per shingle course, with each piece tucked under the shingle above and over the shingle below.
  • Chimney flashing requires four pieces: base flashing under the shingles, counter flashing embedded in the chimney mortar, and a cricket (saddle) on the uphill side to split water flow.
  • Valley flashing can be open (metal visible) or closed (shingles over metal); open valleys shed water faster and are easier to inspect for leaks.
  • All flashing seams and edges need roofing cement or sealant, and fasteners should be placed where they won't trap water or puncture the metal.

Tools and materials you'll need

Gather these before you start: a tape measure, a utility knife, tin snips or a circular saw with a metal-cutting blade, a hammer and roofing nails (or a pneumatic nailer), roofing cement or sealant rated for metal, a caulking gun, a pry bar, and safety equipment including work gloves, eye protection, and a roof harness if you're working on a steep pitch.

For flashing material itself, buy pre-formed flashing kits when they're available — they're cheaper and faster than fabricating from sheet metal. Step flashing comes in standard sizes (usually 10 inches by 10 inches); chimney flashing kits include base, counter, and cricket pieces. If you need custom shapes, sheet metal suppliers can bend aluminum or galvanized steel to order, or you can bend it yourself with a metal brake (a tool you can rent).

Installing step flashing on a wall

Step flashing is the most common type. Start by removing the shingles and nails from the area where the roof meets the wall — you'll be working from the bottom of the roof upward. Slide the first piece of step flashing under the shingles above and over the shingles below, with the vertical leg of the flashing tight against the wall. Nail it through the roof portion only (not the wall leg), using two nails placed so they'll be covered by the next shingle course.

Lay the next shingle course over the top of the first flashing piece, then install the next flashing piece the same way — under the shingles above, over the shingles below. Each piece overlaps the one below it by at least 2 inches. Continue this pattern all the way up the wall. Once all flashing is in place, seal the top edge of each piece where it meets the wall with roofing cement, and seal any gaps between flashing pieces. Do not seal the bottom edges; water needs to drain out.

Flashing a chimney

Chimney flashing has four parts: base flashing (under the shingles), counter flashing (embedded in the mortar), a cricket or saddle (a peaked structure on the uphill side), and cap flashing (at the top). Start by installing the base flashing the same way you would step flashing — pieces overlap, nails are covered by shingles above, and the flashing sits under shingles above and over shingles below.

The counter flashing is the critical piece. It's a metal channel that sits in a groove cut into the chimney mortar, overlapping the base flashing below. You'll need to cut a horizontal slot about 1.5 inches deep into the mortar joint (usually 8 to 10 inches up from the roof), then slide the counter flashing in and seal it with mortar or polyurethane sealant. The counter flashing must overlap the base flashing by at least 4 inches. On the uphill side of the chimney, install a cricket — a peaked metal box that splits water flow to either side. The cricket sits under the shingles above and over the shingles below, like step flashing.

Creating and installing valley flashing

A roof valley is where two roof slopes meet. Water naturally channels here, so flashing must be wide enough to handle the flow. Open valley flashing leaves the metal visible; closed valley flashing is covered by shingles. Open valleys are easier to inspect and less likely to clog, but closed valleys look neater.

For an open valley, measure the valley length and width, then order or fabricate a metal channel (usually 24 inches wide, bent down the center). Slide it into the valley with equal overhang on both sides, nail it every 12 inches along both edges, and seal the nail heads with roofing cement. Trim shingles on both sides so they don't cover the metal, and seal the shingle edges with cement.

For a closed valley, install the flashing the same way, but then lay shingles over it. Trim shingles so they meet in the center of the valley without overlapping, and seal the seams with roofing cement. Some roofers use a self-adhesive valley flashing under closed valleys for extra protection.

Sealing and finishing flashing

Every seam, nail head, and edge of flashing that could trap water needs to be sealed. Use roofing cement (a thick, sticky sealant) or polyurethane sealant, depending on what the flashing manufacturer recommends. explore it with a caulking gun, filling gaps and covering nail heads completely. Do not over-seal — water still needs to drain from under the flashing, and too much sealant can trap moisture.

Check that all flashing overlaps are correct: upper pieces should overlap lower pieces by at least 2 inches, and flashing should overlap shingles below by at least 4 inches. Nail placement matters — nails should be placed where they'll be covered by the next shingle course, and they should never puncture the flashing itself (they should go through the roof deck or the shingle, not through the metal). If you're using a pneumatic nailer, set the pressure so nails sit flush without denting the metal.

Common mistakes and how to avoid them

The most common error is installing flashing upside down or backward — the overlap must always direct water downward and outward. Check the manufacturer's diagram before you start. Another frequent mistake is nailing through the flashing itself; nails should go through the roof deck or shingle, not the metal, or they create a path for water to enter. Sealing the bottom edge of flashing is also wrong — water needs to drain out from under it, not get trapped.

Undersizing flashing is another problem. Flashing that's too narrow or too short won't handle heavy rain or snow melt. Use the widest flashing your roof pitch allows, and overlap pieces generously. Finally, don't skip the cricket on the uphill side of a chimney — it's not decorative, it's essential for splitting water flow and preventing pooling.

Frequently Asked Questions

Can I install flashing over existing shingles, or do I have to remove them?

You must remove shingles in the area where flashing goes. Flashing needs to sit under shingles above and over shingles below so water sheds correctly. If you nail flashing over existing shingles, water will run under the flashing and into the structure. Plan to remove at least one shingle course above and below the flashing area.

What's the difference between roofing cement and sealant?

Roofing cement is a thick, tar-based product that hardens over time; it's good for filling gaps and covering nail heads. Sealant (usually polyurethane or silicone) stays flexible and is better for joints that move slightly with temperature changes. Check your flashing manufacturer's instructions — some specify one or the other. Many roofers use both: cement for nail heads and sealant for seams.

How long does flashing last?

Metal flashing lasts 20 to 50 years depending on the material and climate. Aluminum is lighter and cheaper but less durable; galvanized steel lasts longer; copper lasts the longest but costs more. The sealant around flashing typically needs resealing every 5 to 10 years, especially in freeze-thaw climates. Inspect flashing annually for gaps, rust, or separation.

Do I need flashing around every vent pipe on my roof?

Yes. Every penetration — plumbing vents, exhaust pipes, skylights — needs flashing. Pre-formed flashing boots are available for standard pipe sizes (usually 2 to 4 inches). Slide the boot under shingles above and over shingles below, nail it, and seal the edges. Skipping flashing on even one vent is a common source of leaks.

What should I do if I find a gap between old flashing and new shingles?

Clean out any debris, then fill the gap with roofing cement or sealant. If the gap is large (more than 1/4 inch), the flashing may have shifted or settled — check that it's still properly overlapped and hasn't pulled away from the wall or chimney. If flashing is damaged or severely corroded, replacement is safer than patching.