Chapter 5 of 8 · 10 min read
Pitched roof lights, gables, lean-tos and valleys
Compare RL-24 as a gable-ended pitched light and a lean-to, work out a valley bar for an L-shaped version, and see why the valley's water and flashings decided against it.
The hipped roof light of Chapter 4 is one member of a family. The same opening can be covered by a double-pitched light with vertical gable ends, by a single-pitched lean-to against a wall, or, where two roof lights meet at a corner, by an L-shaped light with a valley where its faces meet in a trough. Each changes the glass, the bars and, above all, where water goes.
This chapter works out RL-24’s opening as a gable-ended pitched light and compares it with the hipped design, sets out a lean-to for a wall-side opening, and develops the valley bar of an L-shaped version that the owner asked about early in the design. The failure case is an L-shaped roof light whose valley discharged into a re-entrant kerb corner, where it leaked.
The gable-ended pitched light
Keep RL-24’s 2,400 × 1,200 mm eaves and 25° pitch on the two long faces, but stop the faces with vertical triangular gables at each end instead of hipping them. The ridge now runs the full 2,400 mm, the rise is still 279.8 mm, and every common is still 662.0 mm. There are no hips.
The long faces become simple rectangles, 2,400 × 662.0 mm on the slope, and each can be divided by commons at 600 mm into four identical rectangular panes. Each gable is a vertical triangle with a 1,200 mm base and a 279.8 mm height, an area of about 0.168 m².
| Glass | Bars and junctions | Wind and water | |
|---|---|---|---|
| Hipped, as built | 12 panes: 4 rectangles, 8 triangles in two hands; 3.178 m² | 8 commons, 4 hips, 1,200 mm ridge, two six-way nodes | lowest wind area; water runs off all four faces |
| Gable-ended | 8 equal rectangles plus 4 vertical gable triangles, two per gable; 3.514 m² | 8 commons, 2,400 mm ridge, gable frames | gables catch wind side-on; condensation on the gables runs down onto the kerb top |
The gable-ended light is simpler to glaze: eight identical rectangular panes, no hands, no backing angles. Its costs appear elsewhere. The ridge is twice as long and carries more commons, so it must be deeper. The gable glass is vertical, a different glazing case from the sloping glass, and stands broadside to wind along the roof. Condensation forming on the gable glass runs straight down onto the kerb top, so the gable frame needs its own condensation channel at its base. And from the street, a gabled light reads as a small glass house on the roof rather than as part of it.
The gable frame
A gable is not just a triangle of glass. Its frame must carry the end of the 2,400 mm ridge, which on the hipped light was carried by two hips, and it must resist wind blowing along the roof onto its vertical face. The gable frame is therefore a small triangulated truss: a bottom member on the kerb, two raking members under the end commons, and usually a vertical post at the middle carrying the ridge end down to the kerb. That post divides the gable glass into two right triangles, each 600 mm wide at the base and 279.8 mm high, and it must be fixed through the kerb into the framing below, because it delivers the ridge’s load straight down. On RL-24’s hipped design, the same load is shared by the two hips at each end and reaches the kerb at the corners, where the frame is stiffest. A gabled light with a long ridge and slender gable posts often shows its weakness as a ridge that dips at the middle and gable posts that rack sideways in strong wind, so the gable frame is designed, not improvised from leftover bar sections.
The ridge capping
Both versions finish the ridge with a folded capping over the ridge bar, clamping the top edges of the glass through gaskets. RL-24’s ridge capping is a simple inverted V with two 45 mm legs, each hemmed 15 mm for stiffness and a safe edge, a girth of 45 + 45 + 15 + 15 = 120 mm. On the gabled version it runs the full 2,400 mm and ends in a folded stop-end over each gable; on the hipped version it is 1,200 mm long and laps over the hip cappings at the nodes. Every capping lap faces downhill, and the hip cappings are fitted first so that the ridge capping laps over them.
Choosing the pitch
Whatever the form, the pitch must be chosen, and it changes every number in Chapter 4. RL-24’s 25° was a balance, and the alternatives show what was traded.
| Rise | Common | Hip true length | Backing | Sloping glass | |
|---|---|---|---|---|---|
| 15° | 160.8 mm | 621.2 mm | 863.6 mm | 10.55° | 2.982 m² |
| 25°, as built | 279.8 mm | 662.0 mm | 893.5 mm | 17.39° | 3.178 m² |
| 35° | 420.1 mm | 732.5 mm | 946.8 mm | 23.93° | 3.516 m² |
At 15°, the glass and rise are least, but water moves slowly over the glass and lingers against the capping edges, leaving dirt lines, and condensation on the underside is more likely to drip from the glass than run to the gutters, which Chapter 6 explains. At 35°, everything sheds briskly, but the rise of 420 mm makes the roof light a feature on the skyline, the glass area grows by about 11% over the 25° design, and its summer gain and winter loss grow with it. Roof light makers commonly publish a minimum pitch for their systems, and the glazier may have one for the gaskets and cappings used; the designer works within them. RL-24’s 25° gave positive drainage with a modest rise.
Water across the slope
On a pitched or hipped light, water should run straight down the glass to an eave without crossing anything. Cappings that run down the slope, over commons, hips and valleys, are harmless, because water runs beside them. Anything that runs across the slope, a transom between two panes in the length of the slope or the head of a lean-to, is a dam: water stops against it, dirt collects, and a gasket that is perfectly sound on a vertical window lets water through when it stands in a puddle. That is why the old trade practice for long glass runs lapped the upper pane over the lower one at a cross bar, like slates, with a gutter under the lap, rather than butting both panes against a flat transom. RL-24’s panes run in one length from eave to ridge, 641 mm, and need no transoms at all.
The lean-to
Some openings sit against a wall: a roof light at the junction of a low extension roof and the house wall, for example. Here a single-pitched lean-to roof light is natural. Its bars run from a wall plate at the top to the eaves frame at the bottom, its top edge is weathered into the wall with a head flashing and its two sides are triangles or sloping frames.
For an opening 1,200 mm deep from the wall at the same 25° pitch, the lean-to’s bars run the full depth, 1,200 ÷ cos 25° ≈ 1,324.1 mm, and its top stands 1,200 × tan 25° ≈ 559.6 mm above its eaves, twice the hipped light’s rise. The bars are twice as long as RL-24’s commons and, by Chapter 3’s rule, deflect sixteen times as much at the same spacing and load, so lean-to bars are deeper or more closely spaced. The head flashing is a wall junction flashing, detailed as in Architectural Sheet Metal by the Numbers in Wall junction flashings, with the roof light’s head frame sitting under a stepped or raked over-flashing let into the wall.
The L-shaped option and its valley
Early in the design, the owner asked whether the roof light could turn the corner over the kitchen in an L: the 2,400 × 1,200 mm wing as drawn, and a second wing, also 1,200 mm wide, running at right angles from one end. Both wings would be hipped at their free ends at 25°. Where the two wings meet on the inside of the L, their faces meet in a valley.
Valley geometry
A valley over a re-entrant corner, between two faces at the same pitch, is geometrically a hip turned upside down. It runs at 45° in plan, from the inside corner to the point where the two ridges meet. It has the same plan length, 848.5 mm, the same rise, 279.8 mm, the same true length, 893.5 mm, and the same pitch, 18.25°, as RL-24’s hips. Its jacks, called valley jacks, run up the faces from the valley to the ridges, with the same 42.19° face cut against the valley.
The difference is the backing. At a hip, the two glazing faces fall away from the bar’s centre-line; at a valley, they rise away from it. The valley bar’s rebates are folded upwards at 17.39° from its centre-plane, and the angle between the faces, measured on the top, is 145.22°, a trough rather than a ridge.
- Plan length
- 600 × √2 ≈ 848.5 mm
- True length and pitch
- √(848.5² + 279.8²) ≈ 893.5 mm at 18.25°
- Backing, rebates folded upwards
- 17.39° each side
- Plan area draining to the valley
- two triangles of 600 × 600 ÷ 2 = 0.36 m²
- Flow at the teaching 200 mm/h
- 0.36 × 200 ÷ 3,600 ≈ 0.020 L/s, about 1.2 L/min
- About a cup of water every ten seconds, delivered to one corner, in every heavy storm, for the life of the roof light
Why the valley decided the design
The valley’s geometry is no harder than the hip’s. Its trouble is water. On a hipped light, water on every face runs straight down to an eave and off over the kerb flashing onto the roof. On the L-shaped light, water on the two triangular areas beside the valley runs into the valley’s capping, down to the inside corner and off there. It all lands at the one place on the kerb that is hardest to flash: the inside corner of an L-shaped kerb.
That corner is a re-entrant corner on a sloping roof. Depending on how the L sits on the slope, either it is on the high side, where it collects water from the roof above as well, and needs its own back tray turning two ways, or it is on a side, where the side flashings of the two wings meet in a folded internal corner that must stay watertight with water running into it from above. Condensation is also concentrated there: the valley capping’s internal condensation channel brings all the condensation from the two triangular faces to the same corner.
The owner chose RL-24 as a single rectangular hipped light, with a second, separate hipped light possible later over the other part of the kitchen. The decision was made on paper, in an afternoon, with the numbers above. That is far cheaper than making it after the first wet winter.
A failure case: water at the inside corner
This is an illustration. On an earlier house, the same shop made an L-shaped hipped roof light on an L-shaped kerb, with the inside corner of the L on the high side of the roof. The valley capping discharged onto the kerb top at the inside corner, and the back trays of the two wings met there in a folded internal corner sealed with sealant.
Within a year, the owner reported a stain on the ceiling under the inside corner after heavy rain. The detection, as so often, came from the stain; the diagnosis came from a hose test that released water first on the roof above the corner, which produced no leak, and then on the glass near the valley, which did. The valley water, running off the capping at speed, was jumping the back tray’s upturn at the corner and running down the inside of the kerb flashing.
The correction was to extend the valley capping out past the kerb with a spout, discharging into a small sump formed in the back tray at the inside corner, with its own outlet onto the roof pans below. The shop’s rule now is to avoid valleys in roof lights where a straight-kerbed alternative exists, and, where a valley is essential, to detail its discharge before anything else.
How this applies to a small Australian business
Before settling on a form, sketch the opening as a hipped light, a gabled light and, against a wall, a lean-to, and compare the glass, the bars and where the water goes. Treat a valley as a hip turned over: same lengths and pitch, backing folded upwards. Then follow the valley’s water to the corner it lands on and design that corner before anything else. Where an opening turns a corner, consider two straight-kerbed roof lights before one with a valley.
Exercise: the gable glass
On the gable-ended version of RL-24, what is the area of the two gable triangles, and what fraction of the total glass do they make? If the gables are glazed in the same 6.38 mm laminate at 15 kg/m², what does each gable’s glass weigh? A designer suggests replacing the gables with insulated sheet metal panels; what would that do to the glass area and to the condensation at the gable base?
Worked answer. Each gable is 1,200 × 279.8 ÷ 2 ≈ 0.168 m², so the two make 0.336 m². With the sloping glass of 3.178 m², the total is 3.514 m², and the gables are about 9.6% of it. Each gable’s glass weighs about 0.168 × 15 ≈ 2.5 kg, split by the central post into two panes of about 1.26 kg. Insulated metal gables would cut the glass to 3.178 m², the same as the hipped light, and remove the cold vertical glass whose condensation runs onto the kerb top, though the panels’ own inner surfaces and their junctions with the frame would still need insulating and sealing.