Flashing: Valleys, Step Flashing, and Pipe Boots

What Flashing Does and Why It Matters

Flashing is sheet metal or formed rubber placed at every point where a roof plane meets a wall, a valley, or a penetration. Water always runs downhill and sideways under wind pressure, so any joint that is not protected by properly lapped flashing will eventually leak. No amount of sealant makes up for a flashing piece that is installed backwards or missing entirely. The goal is to create overlapping layers that force water to stay on the surface of the roof system and drain off the eave.

Valley Flashing

A valley is the internal angle where two sloping roof planes meet. Valleys concentrate large volumes of water, so they require one of two methods.

Open valley uses W-metal (a pre-formed metal strip with a raised center rib) or smooth valley metal laid over the underlayment before shingles are applied. Shingles on each side are cut back several inches from the centerline, leaving the metal exposed. The raised rib prevents water from one plane from shooting across into the other. Metal pieces must be lapped at least 6 inches in the direction of water flow, with the upper piece over the lower piece — never the reverse.

Closed or woven valley uses shingles lapped across the valley; metal or modified-bitumen strip is still installed beneath. Even in a closed valley, the underlayment flashing layer must be correct.

When cutting valley metal, use aviation snips. Wear cut-resistant gloves and safety glasses — fresh-cut sheet metal edges are razor-sharp. Keep fingers curled away from the blade at all times.

Step Flashing at a Wall-to-Roof Intersection

Where a sloping roof meets a vertical wall — such as at a dormer or a second-story wall — step flashing is used. Each piece is an L-shaped rectangle of galvanized steel or aluminum bent to 90 degrees. One leg lies flat on the roof deck; the other turns up against the wall sheathing.

The installation sequence is critical:

1. Apply one shingle course.

2. Slide one step flashing piece on top of that course so its bottom edge aligns with the bottom edge of the shingle, and its uphill edge is hidden under the next course.

3. Nail the flashing to the roof deck only — never nail through both legs, and do not nail it to the wall, because thermal movement and building settling will crack or loosen a rigidly nailed flashing.

4. Apply the next shingle course over the flashing.

5. Repeat, so every shingle has its own flashing piece.

Each piece must overlap the one below it by at least 2 inches so no gap is exposed between pieces. The wall cladding (siding, stucco, or brick) goes on afterward and overlaps the vertical leg, forming a second line of defense. Never substitute a continuous strip of bent metal for individual step flashing pieces — it cannot move with the building and will crack.

Bending step flashing on a hand brake: mark the bend line with a scribe, align the line with the brake's jaw edge, clamp, and push the leaf up firmly to 90 degrees. Check the angle with a small square before releasing. If bending by hand with a seamer, grip the stock tightly, work in short movements, and verify the angle each time.

Pipe Boot Installation

A pipe boot (also called a pipe flashing or pipe collar) seals the gap around a pipe or vent stack that penetrates the roof deck. Most modern boots are pre-formed with a rubber cone that compresses around the pipe and a metal or plastic flange that integrates with the shingles.

Installation steps:

1. Slide the boot over the pipe and press the flange flat onto the roof deck.

2. The downhill flange goes over the shingles below it so water runs onto the shingle — not under it.

3. The uphill and side flanges go under the shingles above and beside it.

4. Cut the field shingles carefully around the pipe with a utility knife or snips. A tight fit is better than a wide gap.

5. Apply roofing sealant only where the manufacturer's instructions specify — typically along the top edge of the uphill flange where it tucks under shingles, and around the pipe collar itself. Do not caulk the entire perimeter of the flange as a substitute for proper lapping; caulk eventually fails and hides an improperly installed boot.

Pipe boots have a maximum pipe diameter rating printed on them. Match the boot size to the pipe — forcing an undersized boot creates gaps, and a boot too large will not seal.

Safety Rules for This Lab

• Safety glasses are required at all times when cutting or bending metal.
• Cut-resistant gloves when handling freshly cut sheet metal edges.
• No one works on an elevated roof surface in this lab — all work is on bench mockups, so fall-protection requirements do not apply here, but maintain good posture and footing around the bench.
• Roofing sealants and adhesives may contain solvents; work in a ventilated area and follow the SDS (Safety Data Sheet) for the specific product.
• Keep snips and seamers pointed away from yourself and others when not in use.

The Spray Test

After installation, your instructor will spray water at every joint with a spray bottle held 6 to 12 inches away. Watch the back side of the mockup for any drips. A passing installation shows zero water intrusion at any flashing joint. If water appears, identify which lap is incorrect before re-doing the affected section.