Flashings at roof penetrations

Every hole in a roof is a leak waiting for a reason. Flues, vent pipes, roof hatches, skylights and plant kerbs all interrupt the sheeting, and water running down the roof meets them from above. A penetration flashing has two jobs: to seal the gap between the roof and the thing passing through it, and to send the water arriving from upslope around the obstruction rather than letting it pond and find its way in. The second job is the one most often neglected, because a penetration looks watertight on a dry day.

This chapter flashes W-40’s two kinds of penetration: a 150 mm flue pipe and a 900 mm square roof hatch kerb. It sets out the lapping rule that governs every flashing, flashes the round pipe with a base tray, upstand and storm collar, flashes the square kerb with an upstand and a cricket on its high side, develops the cricket’s true shape from its edge lengths, and allows for the movement of a hot flue and a long roof. The failure case is a hatch on another roof with a flat back flashing and no cricket, which dammed leaves and leaked.

The lapping rule

Water runs downhill, over laps and never up into them, provided every upslope piece lies on top of the piece below it. Roof sheets are lapped that way, and every flashing must join them the same way. At a penetration, that means:

• on the high side, the flashing goes under the roof sheets above it, so water from the sheets runs onto the flashing;
• on the low side, the flashing goes over the roof sheets below it, so water from the flashing runs onto the sheets;
• at the sides, the flashing is turned up or dressed into the sheet’s ribs so water running past cannot get under its edge.

A flashing that breaks this rule anywhere will leak there in heavy rain, however much sealant it has. Sealant ages and cracks; laps in the right direction work for the life of the roof.

Figure 1Rules for every penetration flashing on W-40's roof

• Do: High-side flashing under the sheets above, low-side flashing over the sheets belowthe lapping rule
• Do: Upstand at least 150 mm above the roof on the high side, as a teaching figuremaker's and code requirements govern
• Do: Upslope water diverted around wide penetrations, not dammed behind themcricket or back gutter
• Do: Sheet pans beside the penetration can carry the diverted watercheck the sheet maker's data
• Do: Movement allowed between the flashing and anything hot or movingslip joint, not a rigid seal
• Do: Materials compatible with the roof and with everything upstreamno copper or lead draining onto coated steel
• Avoid: Relying on sealant to keep water out of a lap that runs the wrong wayit will fail

The flue pipe

W-40’s 150 mm diameter flue from a gas heater rises vertically through the 5° front slope. The hole in the roof sheeting is an ellipse, because the pipe cuts the sloping sheet at an angle; at 5°, its long axis is only 150 ÷ cos 5° ≈ 150.6 mm, so the hole is very nearly round. The geometry of a pipe cut by an inclined plane, and the pattern of the hole, are worked through for a steeper roof in Sheet Metal Patterns by the Numbers; at W-40’s pitch, a round hole with clearance is all that is needed.

The flashing has three parts:

1. A base tray: a flat sheet, wider than the sheet pans it sits across, with a raised collar around the hole. Its upslope edge goes under the roof sheet above, or is cut into the sheet’s ribs and sealed, and its downslope edge is dressed over the sheet’s ribs onto the sheet below, with its sides turned up beside the ribs.
2. An upstand: a short cylinder, part of the tray’s collar or riveted and sealed to it, rising at least 150 mm above the roof surface around the pipe, with a small clearance to the pipe.
3. A storm collar: a cone clamped to the pipe above the upstand, overlapping it like an umbrella, so rain running down the pipe is thrown clear of the gap between pipe and upstand.

The gap between the pipe and the upstand is never sealed solid. The flue expands as it heats, and the roof sheeting moves with temperature too; the storm collar keeps the water out while letting the pipe slide. Many roofs use proprietary flexible pipe flashings instead, a moulded boot that is dressed to the sheet profile and clamped to the pipe. They work well within their temperature rating, which for a gas flue must be checked, and in their maker’s instructions.

Figure 2Two ways to flash W-40's 150 mm flue

	Fabricated tray, upstand and storm collar	Proprietary flexible pipe flashing
Material	prepainted steel, same as the roof	moulded polymer with a metal-reinforced base
Temperature	suits a hot flue with clearance	limited by its rating, which must suit the flue
Fit to the sheet profile	base tray folded or dressed to the ribs	base dressed by hand onto the ribs
Movement	upstand and pipe slide freely under the storm collar	boot flexes with the pipe
Choice for the W-40 flue	yes	no

The roof hatch and its kerb

The roof hatch from the roof drainage design sits in the front slope, 4 m below the ridge, on a steel kerb 900 mm square. The kerb’s sides rise at least 150 mm above the roof, and the roof sheets are flashed to them on all four sides by the lapping rule: under-flashed on the high side, over-flashed on the low side, and turned up the kerb’s sides between the sheet ribs.

The difficulty is the high side. The kerb is 900 mm wide across the slope, and all the water from the strip of roof above it, 0.9 m wide and 4 m long, arrives at its back. At the 1% AEP intensity of 250 mm/h used for W-40, that is only about 0.25 L/s, a modest flow. But it arrives against a vertical face across the full width, where it slows, drops leaves and grit, and ponds. A flat back flashing, a horizontal strip turned up the kerb, has no fall across the slope at all, so the water must find its way sideways while the debris builds a dam.

The cricket

A cricket, or saddle, is a small ridged roof built against the high side of the kerb. Its ridge runs straight up the slope from the centre of the kerb, and its two faces fall sideways from the ridge towards the kerb’s corners. Water arriving from above meets the cricket, runs down its faces to the two diagonal valleys where it meets the roof, and flows along those valleys and round the kerb’s corners into the sheet pans beside the kerb.

W-40’s cricket has its ridge 450 mm long, running up the slope from the kerb’s centre, and rises 100 mm above the roof surface at the kerb. Each face is a triangle with three corners: the kerb’s corner on the roof surface, the top of the ridge at the kerb’s centre, 100 mm up the kerb, and the ridge’s upper end on the roof surface, 450 mm upslope.

The cricket’s true shape

The pattern for each face is the triangle’s true shape, found from its three edge lengths. Measured in the roof’s own plane, with the 100 mm rise at right angles to the roof:

Figure 3True edge lengths of one cricket face

each edge = √(difference across² + difference up the slope² + difference in height²)

Kerb corner to ridge top at the kerb

across 450, up 0, height 100

Ridge top to ridge end

across 0, up 450, height 100

1. corner to ridge top = √(450² + 100²) ≈ 461.0 mm
2. ridge top to ridge end = √(450² + 100²) ≈ 461.0 mm
3. kerb corner to ridge end = √(450² + 450²) ≈ 636.4 mm, the valley edge on the roof

The face is an isosceles triangle, two sides of 461.0 mm and a base of 636.4 mm, with an angle of about 87.3° at the ridge top. The pattern adds a flange along the kerb edge to turn up the kerb, a flange along the valley edge to dress over the roof sheets’ ribs, and an extension at the ridge end to lap under the roof sheet above. The two faces can be folded from one piece along the ridge line, which avoids a joint at the ridge.

The diagonal valleys lie in the roof plane. Running at 45° to the slope, each falls at about 0.71 × tan 5° ≈ 0.062, about 1 in 16, which carries the water round to the kerb’s corners readily. The faces themselves fall from the ridge to the valleys more steeply still.

Set out the cricket on the bench

On the flat sheet, the fabricator draws the valley edge first, a straight line 636.4 mm long. With a tape or trammel set to 461.0 mm, arcs are swung from both ends; where they cross is the ridge top at the kerb. Joining the three points gives one face. The second face is drawn on the other side of the ridge line, sharing it, to make a single folded piece. As a check, the distance from the ridge top to the midpoint of the valley edge should be about √(461.0² − 318.2²) ≈ 333.6 mm; if it is not, one of the arcs was set wrongly. The flanges are then added: about 50 mm along each kerb edge to turn up the kerb, about 40 mm along each valley edge to dress over the sheet ribs, and an extension at the ridge end to lap under the sheet above. The piece is folded along the ridge to the angle that brings both kerb edges flat against the kerb, checked against the kerb on site, and fixed.

Where the diverted water goes

The cricket moves the water sideways but does not remove it. Both halves of the 0.25 L/s now arrive in the sheet pans either side of the kerb, a little more than those pans would carry without the kerb. On W-40’s roof, the sheet maker’s data showed the profile could carry far more than that, so no further work was needed. On a long, low-pitched roof with a wide kerb near the top, the extra flow in two pans can exceed what they carry, and water then overtops the ribs and finds the side laps. That check is part of every penetration on a large roof.

Allow for movement

Roof sheeting moves with temperature, mostly along its length, down the slope. A long roof sheet fixed at its top and bottom moves relative to anything rigid passing through it, such as a steel kerb fixed to the structure below. The flashings must allow that relative movement: the upslope flashing slides under the sheet above rather than being screwed through both; the sheets are fixed to the kerb’s flashings in ways the roofing maker permits for expansion; and the cricket is fixed to the kerb and lapped, not screwed, under the sheet above it. On W-40’s 12 m slope, with sheets over 12 m long, the designer checked the roofing maker’s guidance on sheet length and fixing and confirmed no expansion joint was needed in the sheets themselves.

A failure case: the dam behind the hatch

This is an illustration. On another building, a roof hatch the same size sat near the top of a long, low-pitched roof under a large tree. Its high side had a flat back flashing: a strip turned up the kerb and lapped under the sheets above, with no fall across the slope.

After a few seasons, leaves collected against the kerb and decayed into a mat. In a heavy storm, the water arriving behind the hatch could not drain sideways through the mat, rose against the kerb and over the top of the back flashing’s upstand, and ran down inside the kerb into the building. The stain appeared on the ceiling below the hatch, and the leak recurred in every heavy storm, worse each time.

The detection came from the ceiling stain directly under the hatch, which pointed to the penetration rather than the roof. The inspection found the leaf dam and water marks on the kerb above the back flashing’s upstand. The correction was to clean the roof, fit a cricket developed from true lengths as above, and add the hatch to the building’s six-monthly inspection list. The roof never leaked there again.

How this applies to a small Australian business

Flash every penetration by the lapping rule: under the sheets on the high side, over the sheets on the low side, turned up between the ribs at the sides. Give every upstand enough height, to the roofing maker’s and the installation code’s requirements. Flash hot pipes with a slip joint and a storm collar, never a rigid seal. Put a cricket on the high side of any wide kerb, and develop it from its true edge lengths rather than trimming it to fit on the roof. Check that the sheet pans beside a penetration can carry the water diverted into them. And inspect penetrations regularly, because they collect debris.

Exercise: a wider kerb

A plant kerb 1,500 mm wide is planned for W-40’s front slope. Its cricket is to have a ridge 750 mm long, rising 150 mm at the kerb. What are the true lengths of each face’s three edges? About how steeply does each diagonal valley fall on the 5° roof?

Worked answer. From a kerb corner, 750 mm across, to the ridge top at the kerb, 150 mm high: √(750² + 150²) ≈ 764.9 mm. From the ridge top to the ridge end, 750 mm up the slope: also √(750² + 150²) ≈ 764.9 mm. From the kerb corner to the ridge end, on the roof: √(750² + 750²) ≈ 1,060.7 mm. Each valley still runs at 45° to the slope, so it falls at about 0.71 × tan 5° ≈ 0.062, about 1 in 16, the same as before. The larger kerb diverts water from a strip of roof 1.5 m wide, so the flow into the pans beside it should be checked against the sheet maker’s data.