Shingles & materials
Drip edge, step flashing and valleys: the pieces that stop leaks
Roofs rarely leak through shingles; they leak at edges, walls and valleys. Here is each flashing, what it does and how much of it a roof needs.
Where water gets in
The field of a shingle roof, the open expanse between edges, is the easy part: each course laps the one below and water runs off. Leaks start where the pattern breaks: at the edges, where the roof meets a wall or a chimney, in valleys where two planes pour together, and around anything that pokes through. Flashing is the set of metal and membrane pieces that keep the pattern going across those breaks. Almost every flashing works on one rule: each piece laps over the piece below it, so water always runs over a joint, never into it.
Drip edge at the eaves and rakes
Drip edge is an L- or T-shaped metal angle along every edge of the roof. It holds the shingles’ edge, protects the fascia, and throws water clear of the wall. The International Residential Code (R905.2.8.5) requires it on shingle roofs at both eaves and rakes, lapped at least 2 inches at joints and nailed at 12-inch centres.
The order of layers is the detail that matters:
- At the eaves, drip edge goes on the bare deck, and the ice barrier or underlayment laps over it. Water running down the underlayment lands on the metal.
- At the rakes, the underlayment goes on first and the drip edge over it, so wind-driven water on the metal cannot get under the felt.
Drip edge for a gable house
80 feet of eaves and 72 feet of rakes, 10-foot sticks lapped 2 inches, 5% waste.
- Edge: 152 ft; with waste 159.6 ft
- Effective stick: 9 ft 10 in; pieces: 159.6 ÷ 9.83 = 16.2, so 17 pieces
The drip edge calculator splits them between eaves and rakes; the same edge length sets the starter strip order.
Step flashing along walls
Where a roof plane runs alongside a wall, at a dormer cheek or a second-storey wall, each course of shingles gets its own L-shaped piece of step flashing, typically 4 by 4 by 8 inches: 4 inches on the roof under the shingle, 4 inches up the wall behind the siding or housewrap. Each piece laps the one below by the difference between its length and the shingle exposure. The count is therefore one per course:
Twenty-four feet of sidewall at a 5⅝-inch exposure takes 52 pieces plus starters. Step flashing is nailed to the roof deck only, at the top corner, never to the wall, so the wall can move without tearing it.
Kick-outs: the piece most often missing
At the bottom of every sidewall, where the roof edge ends against the wall, the first step flashing piece is a kick-out diverter, bent to throw water out into the gutter. Without it, water running down the step flashing pours behind the siding at exactly the corner where it does the most damage. Missing kick-outs are one of the most common causes of rotted sheathing and framing that inspectors find.
Chimneys and crickets
A chimney is flashed in four parts, installed with the shingles:
- an apron across the front (downhill) face, over the shingles;
- step flashing up both sides, one piece per course;
- a back pan or a cricket behind;
- counter flashing let into the mortar joints on all sides, lapping down over the step flashing and apron.
The code (IRC R1003.20) requires a cricket, a small gable roof behind the chimney, when the chimney is more than 30 inches wide across the slope, so water and snow split around it instead of ponding. Narrower chimneys take a single bent back pan. Counter flashing on masonry is cut into a raked mortar joint; surface-mounted metal held on with caulk fails within a few seasons. The step and wall flashing calculator counts every part and applies the 30-inch rule.
| Flashing | Where | Counted by | Rule |
|---|---|---|---|
| Drip edge | Eaves and rakes | Edge length ÷ (stick − lap) | Under underlayment at eaves, over at rakes |
| Step flashing | Roof against a wall | One per shingle course | Nailed to the deck, not the wall |
| Kick-out | Bottom of each sidewall | One per wall run | Throws water into the gutter |
| Apron and back pan | Chimney front and back | One each | Cricket if over 30 in wide |
| Counter flashing | Masonry walls and chimneys | Perimeter + 10% | Let into the mortar |
| Valley lining or metal | Valleys | Valley length + run-out | 36 in lining; 24 in metal if open |
Valleys: open, closed-cut and woven
Every valley gets a 36-inch lining of self-adhering membrane or mineral-surfaced roll roofing (IRC R905.2.8.2). On top of it, three treatments:
- Open metal valley. A 24-inch-wide strip of metal, often W-shaped with a centre rib, with shingles cut back to leave a channel. Fastest to shed water and debris; preferred under trees and in snow country. The metal is fastened only at its edges.
- Closed-cut valley. One plane’s shingles run across the valley; the other plane’s are cut in a straight line 2 inches from the centre. No metal shows; the commonest choice on architectural shingle roofs.
- Woven valley. Shingles from both planes interlace course by course. Only possible with flexible three-tab shingles.
The valley flashing calculator counts lining rolls and metal pieces, with a foot of run-out at the eave for each valley.
Pipe boots and vents
Plumbing vents and exhaust pipes pass through rubber-collared pipe boots woven into the shingles: the boot’s upper flange under the shingles above, its lower flange over the shingles below. The rubber collar cracks in sunlight after 10 to 20 years, and a failed boot is among the most common leaks on a roof that is otherwise sound, which the repair or replace guide discusses.
Reuse or replace?
On a reroof, replace the drip edge, the step flashing and the pipe boots: they are cheap, and old step flashing has nail holes in the wrong places for the new shingle exposure. Good counter flashing on a chimney can stay if it is sound and properly let into the mortar. The membrane under all of it is covered in ice and water shield, where the code wants it, and the field underlayment in felt vs synthetic.