How to Flash a Headwall Where a Roof Dies Into a Wall
One continuous apron, lapped over the top shingle course and tucked behind the cladding. The detail that stops wall water reaching the roof deck.
- Advanced
- 1 to 3 hours
- Updated
The short answer
A headwall is where the top of a roof slope ends against a wall that rises above it. It is flashed with apron flashing: a continuous L-shaped piece with a roof leg at least 4 inches wide lapped over the shingles, and a wall leg at least 4 inches tall behind the cladding. Install ice and water shield first, fasten only the roof leg, and lap the wall barrier over the top of the apron.
Where a roof slope runs up into a wall, everything on that wall drains down onto the top of the roof. All the water on the face of the wall, plus the water blown against it, collects in a line right at the joint. Headwall flashing, also called apron flashing, catches it. It is a wide, continuous piece of metal that sits on top of the shingles, turns up behind the cladding and carries water out over the shingle surface.
Unlike step flashing, apron flashing is not stepped. Water arrives from above across the entire width of the wall, so a continuous piece is required. It works together with step flashing at the ends, where the headwall meets the sidewalls.
Headwalls are common on dormer fronts, house additions, and any place a lower roof butts against a taller wall. They also collect moss, needles and debris in the Fraser Valley, which keeps the joint wet. See the wider overview in roof flashing types. Where the roof is metal panels, the headwall is a formed trim instead, see metal roof wall flashing.
Tools
- Tin snips (straight cut)
- Hand seamer or sheet metal brake
- Flat bar for lifting siding
- Hook blade utility knife
- Caulk gun
- Tape measure
- Roofing hammer or coil nailer
Materials
- Apron flashing, 26 gauge galvanized or prefinished steel, in lengths with a roof leg of 4 to 6 inches and a wall leg of 4 to 6 inches
- Ice and water shield
- Housewrap or building paper and flashing tape
- Roofing nails, 1-1/4 inch
- Sealant rated for metal and cladding
- Step flashing for where the headwall meets sidewalls
Safety first
- Headwalls are often above a long roof plane or over a second storey. Tie off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- You will be working at the top of the slope with your back to the roof. Set anchors above you and keep the rope short.
- Sheet metal edges and cut siding are sharp. Wear cut resistant gloves.
Step by Step
Step 1: Open the wall and inspect the sheathing
Headwalls leak quietly. Water follows the wall paper down and into the top of the roof deck. Probe the sheathing and the top edge of the roof deck with a screwdriver. Dark or soft wood means water has already been getting in and needs to be replaced before new flashing goes on.
Remove old apron flashing. Reusing it is rarely worth the risk, since the nail holes and bends will not seat properly under new shingles.
Watch out: Do not flash over soft sheathing. The new flashing will hold the moisture in and the rot will keep going.
Step 2: Run ice and water shield up the wall
Fold a strip lengthwise and press it tightly into the corner. On the wall, it goes under the wall's building paper or housewrap, so water coming down the wall lands on top of it. On the roof, it laps over the field underlayment or is covered by it, depending on the sequence. Follow the lapping order in the lapping rule. See also how to install ice and water shield.
Step 3: Shingle up to the headwall
Trim the last course so it reaches within about 1/2 inch of the wall. A gap lets water drain and stops the shingle tab wicking and buckling. On the last course, use full-height shingle pieces rather than narrow cut strips, which can curl and lift. See cutting shingles.
Step 4: Form and cut the apron flashing
Standard roof leg width is 4 to 6 inches, and wall leg height is 4 to 6 inches. Longer is better on low slopes or exposed walls. Cut pieces so each laps the next by at least 4 inches in the direction of water flow (usually toward the lower end of the wall). Bend a slight kick, or hem, in the bottom edge of the roof leg to reduce capillary draw under the metal.
Preformed apron flashing is cheap and sized for standard walls. On long walls, bending on site with a sheet metal brake produces consistent, straight lengths.
Step 5: Set the apron over the shingles and nail the roof leg
Fasten the roof leg with roofing nails about 12 inches apart, into the roof deck, following the flashing and shingle manufacturer's detail for exactly where they go. Because the roof leg is exposed, nail heads are too: cover them with a small dab of sealant. Never nail the wall leg, because the roof and wall move independently and a fixed wall leg tears.
A headwall has no course above it to hide the nails, which is why the apron is sized generously and the heads are sealed. Keep fasteners out of the bend, where the metal carries the most water.
Step 6: Join the apron to the sidewall step flashing
Step flashing on a sidewall ends at the headwall. The apron runs over the top of that last piece, so water coming down the headwall drops onto the step flashing and continues down the roof. Wrapping the corner with a formed end cap or a folded corner piece keeps the joint tight. See step flashing and flashing a dormer for the corner detail.
Step 7: Lap the wall barrier and cladding over the top
The wall's water resistive barrier must lap over the top edge of the apron, like shingles. On vinyl siding, leave the clearance the manufacturer specifies, commonly about 1/2 to 2 inches. On stucco, use a receiver or a weep screed detail so the stucco does not bridge to the roof. See flashing a roof to a wall by cladding type.
Step 8: Seal only where the detail calls for it
Sealant is a backup. A continuous bead along the top edge of the wall leg can trap water behind it. See roofing sealants.
Headwall Flashing by Cladding
- Vinyl siding: wall leg goes behind the J-channel or the siding courses, with housewrap lapped over. Keep cladding off the roof.
- Fibre cement: lowest board sits off the roof at the manufacturer's clearance. Seal cut edges with paint.
- Stucco: use a receiver or weep screed detail so the stucco does not bridge onto the flashing or the shingles.
- Brick and stone: use counter flashing set into a reglet. See counter flashing.
Fraser Valley Notes
Moss and fir needles collect at a headwall because it is a flat line at the top of a slope where debris stops. Wet debris against metal and shingles accelerates wear, so cleaning the joint each year makes a real difference. Wind-driven rain also hits headwalls hard, so use the longer end of the leg sizes.
Common Mistakes
- Using step flashing on a headwall. Water arriving across the whole wall finds the gaps between pieces.
- Nailing the wall leg so the flashing tears as the roof and wall move.
- A roof leg too narrow. With less than 4 inches, wind-driven rain can go over the top edge of the metal.
- Wall barrier lapped behind the apron. Water running down the wall slips behind the metal and into the deck.
- Siding touching the shingles. It wicks water, holds debris and rots the siding's lower edge.
- A continuous bead of sealant along the top of the wall leg, which traps water behind it instead of letting it drain.
When to Call a Professional
If the wall is stucco, brick or stone, if the sheathing is soft, or if the headwall is tall and sits above a steep roof, have a roofer handle it. Headwall leaks often travel inside the wall and show up far from the source.
A roofer can also check that the sidewall step flashing, kickout and headwall apron work together as one system, since leaks at the corners are the most common failure.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Flash a Headwall (Apron Flashing): Questions
What is headwall flashing?
Headwall flashing, also called apron flashing, is a continuous L-shaped piece of metal installed where the top of a roof slope ends against a wall. Its roof leg lies over the top course of shingles and its wall leg runs up behind the siding or stucco. It catches all the water running down the wall and sends it out over the shingles.
What is the difference between step flashing and apron flashing?
Step flashing is a row of small pieces, one per shingle course, used where a slope runs alongside a wall. Apron flashing is one continuous piece used where the top of a slope meets a wall. Water runs along step flashing but arrives across the full width of an apron, so the apron needs to be unbroken and lapped at joints.
How wide should headwall flashing be?
Commonly at least 4 inches on the roof leg and at least 4 inches up the wall, with 6 inches being better on low slopes or exposed walls. Pieces lap each other by about 4 inches. Sizes vary by shingle and flashing manufacturer, so confirm the detail in the product's installation instructions.
Should apron flashing go over or under shingles?
The roof leg of apron flashing goes over the top course of shingles, so water coming off the wall lands on the metal and runs out over the shingles. The wall leg goes behind the siding and is covered by the wall's water resistive barrier. The lower edge is never tucked under shingles at a headwall.
Can you caulk a headwall instead of flashing it?
Caulking alone is not reliable. A bead along the wall joint cracks and lets go within a few seasons and gives water a direct path into the deck. A correctly lapped apron flashing sheds water by gravity. Sealant is a backup at laps and fasteners, not a replacement for continuous metal under the cladding.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
Want someone to look at your roof?
Kory & Johnny have roofed homes across the Fraser Valley since 2016. Free inspections, honest estimates, no pressure.
604-728-0146Text UsEmailServing Hope, Agassiz, Chilliwack, Rosedale, Abbotsford & the entire Fraser Valley