Architectural Sheet Metal by the Numbers
Chapter 6 of 8 · 10 min read
Wall junction flashings
Flash a roof where it meets a wall with separate under- and over-flashings, step the over-flashing into brick courses with calculated runs, turn water away at the bottom and allow for movement.
Wherever a roof meets a wall, water running down the wall and across the roof meets at a joint between two different structures. The roof moves with its frame, the wall with its foundations, and the two rarely move together. A flashing at that junction must shed water from both, cover the joint between them and still let them move. Trying to do all three with one piece of metal, screwed to both, is the classic mistake. The reliable answer is two pieces: one belonging to the roof and one belonging to the wall, overlapping so that water cannot get between them but movement can.
This chapter flashes the junction where W-40’s entrance canopy meets the brick facade. It explains the two-part principle, sizes the under-flashing that belongs to the roof, steps the over-flashing into the brick courses with the run of each step calculated from the pitch, turns the water away at the bottom of the junction, and closes the top where the two slopes meet the wall. The failure case is a one-piece flashing screwed to both canopy and wall, which tore as the canopy moved.
The junction
The entrance canopy’s ridge runs straight out from the facade, so each of its two 15° slopes meets the wall along a sloping line, climbing from the eaves at each side to the apex in the middle. The facade around the entrance is face brick, laid in courses 86 mm high. Each slope is 3 m wide in plan, so the junction line on each side rises about 3,000 × tan 15° ≈ 804 mm from the eaves to the apex.
The roof sheets on each slope run downhill, parallel to the wall, so the sheet next to the wall lies alongside it. The junction is therefore a raking side junction: water runs along it, down the slope, rather than across it.
Two parts, two owners
| One-piece flashing | Two-part flashing | |
|---|---|---|
| Fixed to | both roof and wall | under-flashing to the roof, over-flashing to the wall |
| Movement between roof and wall | resisted; the metal or its fixings tear | absorbed by the overlap sliding |
| Replacing the roof sheets | disturbs the wall fixing | over-flashing stays, under-flashing comes off with the roof |
| Weatherproofing | depends on sealant at the wall | depends on laps and the cover of the over-flashing |
| Choice for W-40's canopy | no | yes |
The under-flashing, sometimes called the apron or upstand flashing, belongs to the roof. It lies over the last roof sheet next to the wall, dressed over the sheet’s outer rib, and turns up against the wall, but is not fixed to it.
The over-flashing, or counter-flashing, belongs to the wall. It is let into the wall’s mortar joints, or a cut chase, and hangs down over the under-flashing’s upstand, covering it without touching the roof.
Water running down the wall runs onto the over-flashing, off its lower edge onto the under-flashing, and down the roof. Water on the roof runs along the under-flashing and down the slope. The joint between the two flashings is covered from above and open to movement.
The under-flashing
W-40’s under-flashing on each slope is a single raking length, about 3.1 m along the slope, with an upstand of 150 mm against the brickwork, a flat across the last sheet’s pan and a turn down over the sheet’s outer rib. The 150 mm upstand is a teaching figure; the roofing maker’s instructions and the industry installation code, SA HB 39, set the requirements for real work. Its upper end runs into the closing piece at the apex, described below, and its lower end into the diverter at the eaves.
The upstand’s top edge is parallel to the roof, 150 mm above it all the way up. That line matters, because the over-flashing’s steps must cover it everywhere.
Step the over-flashing into the courses
Brick courses are horizontal; the roof climbs at 15°. A counter-flashing let into the mortar joints cannot follow the slope in one piece, so it is made as a series of steps, each let into one horizontal joint and hanging down over the upstand, with each step lapping the one below it.
The horizontal length of each step is set by how far along the wall the roof climbs one course.
- Course height
- 86 mm
- Roof pitch
- 15°, tan 15° ≈ 0.268
- Rise of the junction on each side
- about 804 mm
- run per step = 86 ÷ 0.268 ≈ 321 mm
- number of steps = 804 ÷ 86 ≈ 9.35, so 10 steps on each side, the last one shortened
Each step piece is about 321 mm of run plus a 50 mm lap over the next step down, about 371 mm long. It has a short leg, about 25 mm, turned into the mortar joint, and a face hanging down the wall. Because the joint is horizontal and the roof climbs along the piece, the face is deeper at its downhill end than at its uphill end, by one course: if it hangs 100 mm below the joint at the uphill end, it hangs 186 mm at the downhill end. The bottom edge of each face, parallel to the roof, sits about 75 mm below the top of the under-flashing’s upstand, so the upstand is covered everywhere by that much, as a teaching figure.
Every step is the same shape on a given slope, which suits the brake: one set of dimensions, cut ten times for each side, handed for the two sides. The bricklayer, or a cutter on a finished wall, raking out each joint about 25 mm deep, takes the leg, which is wedged and sealed in the joint with a sealant suited to masonry.
Lap the steps the right way
The lapping rule from Chapter 5 applies to the steps too. Each step’s downhill end laps over the uphill end of the step below it, so water running down the face of one step runs onto the outside of the next. Fixing the steps from the bottom of the slope upwards makes that order natural: each new step is let in above and slipped over the end of the one already in place. Once all ten are in, the roof plumber runs a hose down the wall above the junction and watches the water step from face to face and onto the under-flashing, with none appearing behind the upstand. Any step whose lap runs the wrong way shows immediately, and is far easier to reverse before the joints are sealed and pointed.
Turn the water away at the bottom
At the eaves, the junction ends, and all the water that has run down it, from the wall and from the strip of roof alongside, arrives at the bottom corner, where the roof edge meets the wall. If nothing turns it, it runs straight down the wall face beside the canopy’s gutter, saturating the brickwork and staining it.
A diverter, a short folded piece at the foot of the under-flashing, kicks the water outwards into the canopy’s gutter. It is formed as part of the under-flashing’s lower end, turned out away from the wall at about 45° and dressed into the gutter. It is a small piece with a large effect: most wall staining under roof junctions on buildings comes from its absence, and it costs a few minutes of folding to include.
Close the top
At the apex, the two slopes’ under-flashings meet at the wall, and the two flights of steps meet the highest brick course the junction reaches. A small closing piece, a saddle, sits over the tops of both under-flashings where they meet the ridge, lapping under the ridge capping and covering the joint between the two upstands. The highest step on each side laps over the saddle’s upstand. It is developed like the cricket in Chapter 5, from its edge lengths, and mocked up on the wall before the ridge capping goes on.
- Roof sheetslaid on each slope, the last sheet beside the wall
- Under-flashingsdressed over the last sheet's rib, upstand against the brick, not fixed to it
- Divertersformed at the foot of each under-flashing and dressed into the gutter
- Saddleover the tops of both under-flashings at the apex, under the ridge capping
- Joints raked out25 mm deep in each course the steps will use
- Over-flashing stepslet in from the bottom up, each lapping over the one below, wedged and sealed in the joint
Walls without courses
Not every wall has mortar joints to step into. W-40’s main facade, away from the entrance, is precast concrete panels, and a rendered or clad wall offers no courses either. Against such walls, the over-flashing can follow the slope in one raking piece, because there is no horizontal joint to force steps. It is let into a chase sawn along the wall at the slope, or fixed under a proprietary surface-mounted channel that holds a sealant bead, with its face hanging over the under-flashing’s upstand exactly as the steps do. Against a clad wall, the cladding itself, lapped over the under-flashing’s upstand with a gap above the roof, acts as the over-flashing. In every case the principle is the same: a piece belonging to the wall covers a piece belonging to the roof, and nothing fixes them together.
Horizontal junctions: the apron
Where the top of a roof slope runs up to a wall, rather than alongside it, the junction is horizontal, and the under-flashing is an apron: a flashing that lies over the top ends of the roof sheets and turns up the wall. Water on the roof now runs away from the wall, down the sheets, so the apron’s job is to cover the sheets’ top ends and stop wind-driven rain being blown up under them. The sheets’ pans are turned up at their top ends, called stop-ending, so water cannot run back under the apron, and a profiled closure strip fills the gaps between the ribs under the apron. The over-flashing above a horizontal apron is a single straight piece let into one joint, with no steps at all. W-40’s office annex, if it is built, will have a skillion roof rising to the facade with exactly this junction.
Movement at the junction
W-40’s canopy is a steel frame bolted to the building’s structure, and its roof moves with the frame: with temperature, with wind and with the slight settlement of new footings. The brickwork moves with the wall. Across the junction, those movements may be only a few millimetres, but repeated through every day and every season, they are enough to work loose any fixing that ties the roof to the wall. With the two-part flashing, the under-flashing’s upstand slides up and down behind the over-flashing’s face. Nothing ties them together, so nothing tears.
A failure case: one piece, two owners
This is an illustration. An earlier canopy on another building was flashed against its wall with a single folded flashing: a flat over the last roof sheet, screwed through into the roof frame, and an upstand screwed into plugs in the wall, with sealant along its top edge.
Through its first summer, the canopy’s steel frame expanded and contracted daily, and the canopy settled a few millimetres as its new footings took up load. The flashing, fixed to both, was pulled in two directions at once. The screws into the wall loosened in their plugs, the sealant along the top edge cracked, and in one place the metal itself tore beside a screw.
The detection came from water staining on the inside face of the wall below the canopy after an autumn storm. The investigation found the torn flashing and the cracked sealant. The correction was to remove the one-piece flashing, fit a separate under-flashing with an upstand not fixed to the wall, rake out the joints and fit stepped over-flashings into them. The business’s detailing standard now shows every roof-to-wall junction as a two-part flashing.
How this applies to a small Australian business
At every roof-to-wall junction, use two flashings: an under-flashing that belongs to the roof and an over-flashing that belongs to the wall, overlapping but not fixed together. On a sloping junction against brick or block, step the over-flashing into the horizontal joints, with each step’s run calculated as the course height divided by the tangent of the pitch, lapped the right way and installed from the bottom up. Turn the water away from the wall at the foot of the junction with a diverter, close the top with a saddle, and check the details against the roofing maker’s instructions and the installation code.
Exercise: a steeper canopy against blockwork
A second canopy, pitched at 30°, meets a wall of concrete blockwork laid in 200 mm courses. Each slope is 3 m wide in plan. How far does the junction rise on each side? What is the run of each step, and how many steps are needed on each side?
Worked answer. The junction rises 3,000 × tan 30° ≈ 1,732 mm on each side. The run per step is 200 ÷ tan 30° ≈ 200 ÷ 0.577 ≈ 346 mm. The number of steps is 1,732 ÷ 200 ≈ 8.7, so nine steps on each side, the last one shortened. Each step piece is about 346 mm of run plus a 50 mm lap, about 396 mm long, and its face is one course, 200 mm, deeper at its downhill end than at its uphill end.