Kerbs, upstands and flashings

A roof light does not sit on the roof sheeting. It sits on a kerb, a short wall built up around the opening, and the kerb is flashed to the roof so that water running down the roof passes around it. The glass and bars get the attention, but most roof light leaks start at the kerb: a back flashing too low, an apron too short, a side flashing that lets water run behind it, or a kerb that was never level to begin with.

This chapter sets RL-24’s kerb heights on its 5° roof, deals with the rain that arrives from the roof above, and works out the girths of the flashings that join the kerb to the roof sheeting. The failure case is a roof light whose high side had no back tray, which leaked in the first heavy storm.

Why a kerb

A kerb does three things. It lifts the bottom edge of the glazing frame above the water running on the roof, so that a gust or a blocked rib does not drive water into the frame. It gives the frame a level, true seat to sit on, whatever the slope of the roof. And it separates two trades: the roof plumber flashes the kerb to the roof, and the roof light, made in the shop, sits over the kerb and its flashings and can be lifted off later without disturbing the roof.

RL-24’s kerb is built from treated timber, 50 mm thick, around the 2,400 × 1,200 mm opening, so its outside dimensions in plan are 2,500 × 1,300 mm. Its top is level, because the roof light’s frame and bars are set out from a level base. The roof falls at 5°, so the kerb is lower on its high side than on its low side.

Kerb heights on a sloping roof

The shop’s own rule for RL-24 is that the kerb stands at least 150 mm above the roof surface at its lowest point, which on a sloping roof is the high side, where the roof surface is highest. This is the shop’s minimum for this job, not a code value; a roof light maker’s instructions or the roofing maker’s details may ask for more, particularly in high-rainfall or cyclonic areas.

The roof drops across the kerb’s 1,300 mm plan depth by 1,300 × tan 5° ≈ 113.7 mm. With the top level, the low side of the kerb stands 150 + 113.7 = 263.7 mm above the roof. The two side kerbs taper from 150 mm at the top to 263.7 mm at the bottom.

Figure 1Section through RL-24 down the slope, not to scale

Setting the kerb out on site

A level kerb on a sloping roof is easy to draw and easy to get wrong. The carpenter builds the kerb from the roof framing, which follows the roof slope, so every height is measured from a sloping surface. RL-24’s kerb was set out from a laser level: the top of the high kerb was fixed at 150 mm above the roof surface at its highest point, and every other point on the kerb top was set to the same laser level. The low kerb’s height was then measured, not assumed, and checked against the calculated 263.7 mm. The two agreed within 3 mm. A kerb whose top is out of level by 10 mm over 2.5 m makes every bar in the roof light a little wrong, and the error appears as a twisted frame or a ridge that is not straight.

Water from the roof above

RL-24’s long side runs across the slope, 2,500 mm wide outside the kerb. All the rain falling on the roof directly above it runs down to the high kerb and has to be turned aside. The roof above the roof light runs back about 4.0 m in plan to the extension’s ridge, so the catchment that arrives at the kerb is about 4.0 × 2.5 = 10 m².

Figure 2Flow arriving at RL-24's high kerb in a heavy storm

flow (L/s) = catchment area (m²) × rainfall intensity (mm/h) ÷ 3,600

Catchment

4.0 m × 2.5 m = 10 m²

Intensity, teaching value

200 mm/h

Flow

10 × 200 ÷ 3,600 ≈ 0.556 L/s, about 33 L/min

1. About two buckets of water a minute arrive at the back of the kerb and must be turned to the sides

That water cannot be allowed to pond against the kerb. On a metal deck roof it runs in the pans between the ribs, so the sheets above the roof light are cut off short of the kerb and their pans are turned up, or stop-ended, so that water does not run back under the sheet end. A back tray runs under the cut sheet ends and across the full width of the kerb, turned up the kerb face and beyond each side, with a fall from its middle to each end. Water from the cut pans drops into the tray and runs sideways to the pans either side of the kerb, which continue uninterrupted down the roof.

The tray’s fall is set by the shop at about 1 in 50 from the middle. Over half its length, 1,450 mm from the middle to each end, the fall is about 1,450 ÷ 50 ≈ 29 mm. That is easy to form with a raised centre, or cricket, in the tray. The tray must be deep enough to carry 33 L/min without overtopping; at 300 mm wide with a 40 mm upturn at its back edge, it has plenty of capacity, but its ends must discharge cleanly into a pan, not onto a rib.

The flashings and their girths

Four flashings join the kerb to the roof: the back tray on the high side, an apron on the low side, and a side flashing on each side. Each is a folded profile, and each girth is the sum of its flats, as Architectural Sheet Metal by the Numbers explains in Profiles, girth and stretch-out.

Figure 3Girths of RL-24's kerb flashings

	Flats	Girth
Back tray, high side	150 up kerb, 50 over top, 15 down inside, 300 tray, 40 upturn	555 mm
Apron, low side	12 hem, 150 on roof, 263.7 up kerb, 50 over top, 15 down inside	490.7 mm
Side flashing, top end	12 hem, 150 on roof, 150 up kerb, 50 over, 15 down	377 mm
Side flashing, bottom end	12 hem, 150 on roof, 263.7 up kerb, 50 over, 15 down	490.7 mm

The side flashing’s blank is therefore a long trapezium: 377 mm wide at its top end and 490.7 mm at its bottom end, cut from one blank with the taper taken out of the kerb face. Its roof leg sits over the nearest rib and its hem is turned down into the pan beside it. The side flashings lap under the back tray at the top and over the apron at the bottom, so that every lap faces downhill.

Laps and corners

At each corner of the kerb two flashings meet. The rule is that water must always run from an upper piece onto a lower piece, never into a lap. At the high corners, the back tray extends past the kerb and over the top of the side flashings by at least 150 mm. At the low corners, the side flashings run past the kerb and over the apron’s ends. Each corner is folded as a closed, soldered or riveted-and-sealed box corner on the kerb face, so that there is no open seam where the upstands meet. Architectural Sheet Metal by the Numbers develops corner mitres for profiles; for a simple kerb upstand, a folded and sealed corner gusset is quicker and drier.

The roof light’s skirt: the second line

The kerb flashings are the first line of defence. The roof light’s own frame carries a skirt, a folded apron hanging down over the outside of the kerb flashing upstand, which is the second. The skirt laps over the upstand by at least 50 mm and has a drip at its bottom edge, standing clear of the flashing so that water cannot be drawn up between them by capillary action. This two-part arrangement, upstand and skirt, means that the roof light can be lifted and refitted for maintenance without disturbing the flashings, and that a small movement between frame and kerb opens no path for water.

Figure 4The order of work at RL-24's kerb

1. 1Frame and level the kerblaser level; heights checked against 150 and 263.7 mm
2. 2Fit the apronlow side first, so later pieces lap over it
3. 3Fit the side flashingsover the apron, under where the tray will sit
4. 4Stop-end the sheets above and fit the back traytray over the sides, under the sheet ends
5. 5Hose test the kerb before the roof light goes onwater from above for 15 minutes
6. 6Fit the roof lightframe skirt over every upstand, fixed through the kerb top

The hose test of the kerb alone is worth its quarter of an hour. Before the roof light is fitted, the kerb is an open box and any leak in its flashings can be seen from inside. Once the roof light is on, the same leak appears as a drip somewhere under the glass, and is easily blamed on the glazing.

Fixings, materials and the cold kerb

Every flashing must be fixed, and every fixing is a hole. The rule is to fix where water does not run. RL-24’s upstands are fixed through the kerb top, under the roof light frame, where the frame and its skirt cover the screw heads. The roof legs are fixed through the crests of the ribs, never through the pans, with sealing washers, at the spacing the roofing maker gives. No fixing goes through the face of an upstand below the skirt’s lap.

The flashing material must also be compatible with everything it touches and everything that drains onto it. RL-24’s roof is prepainted metallic-coated steel, and its flashings are in the same material and colour, so there is no galvanic pairing between them. The roof light’s frame and bars are aluminium; aluminium is compatible with metallic-coated steel in normal exposure, but copper, lead and some treated timbers are not, and water that has run over copper must never drain onto zinc or aluminium coatings. Where the treated timber of the kerb touches the flashing, a barrier strip separates them, following the flashing maker’s advice for the timber treatment used.

The kerb is also a thermal bridge. Its timber walls run from the warm ceiling space up into the cold roof light, and the inside face of the kerb is where condensation often first appears. RL-24’s kerb is lined on the inside with rigid insulation behind the light-well lining, so that its inner surface stays closer to room temperature. Chapter 6 shows why a few degrees on that surface matter.

Kerbs that are not timber

Many roof lights sit on steel or aluminium upstands made in the shop, either as a folded box riveted and sealed at its corners or as a factory-made upstand from a roof light maker. A metal upstand can be made accurately, with its level top built in, and delivered ready to fix to the roof framing. It is a better conductor of heat than timber, so it needs insulating on its inside face more carefully. The flashing rules are the same: stop-ended sheets and a back tray above, an apron below, tapered sides lapped downhill, and a frame skirt over every upstand.

A failure case: no back tray

This is an illustration. On an earlier job with the same 2,400 mm width across the slope, the roof plumber ran the metal deck sheets up to the high kerb, cut them and turned the upstand flashing over the sheet ends, sealing the joint with a bead of sealant. There was no back tray and the pans were not stop-ended.

The first heavy storm ran water down the pans to the kerb, where it ponded against the sealant bead. Wind-driven rain pushed it under the upstand and along the top of the sheets, and it appeared as a stain on the ceiling at the roof light’s upper corner. The owner reported the leak; the roof light maker was called first and found nothing wrong with the glazing.

The detection came from a hose test run with water released on the roof above the kerb, which reproduced the leak within minutes, and from lifting the upstand flashing, which showed that the sealant had pulled away from the sheet ribs. The correction was to stop-end the sheet pans, fit a proper back tray running out to the pans either side, and refit the upstand over it. The shop’s procedure now shows a back tray on every kerb wider than one sheet pan across the slope.

How this applies to a small Australian business

Build the kerb with a level top and measure, rather than assume, the heights on each side. Work out where the water from the roof above goes, and if the kerb is wider than a pan or two across the slope, fit a back tray and stop-end the sheets above. Calculate the girth of every flashing, and make the side flashings tapered to suit the kerb. Lap every piece downhill. Hose test the kerb before the roof light covers it. Use sealant as a backup, never as the thing that keeps water out.

Exercise: a steeper roof

The same 2,500 × 1,300 mm kerb is to go on a 10° roof, with the long side across the slope and the same 150 mm minimum at the high side. What is the low kerb height? What is the girth of the apron, using the same flats as RL-24’s apron? If the catchment above the kerb is 6 m deep in plan, what flow reaches the kerb at the teaching intensity?

Worked answer. The roof drops 1,300 × tan 10° ≈ 229.2 mm across the kerb, so the low kerb is 150 + 229.2 = 379.2 mm high. The apron’s girth is 12 + 150 + 379.2 + 50 + 15 = 606.2 mm. The catchment is 6.0 × 2.5 = 15 m², so the flow is 15 × 200 ÷ 3,600 ≈ 0.83 L/s, about 50 L/min, half as much again as RL-24’s, and a back tray is essential.