Can a rooflight be fitted to a hipped roof?
Yes. A hipped roof takes a rooflight perfectly well, and across Gloucester’s interwar suburbs it is one of the most common jobs there is. What the hip changes is the shape of the space available rather than whether the job can be done at all. On a gable-ended roof the slope is a rectangle and you can put a unit almost anywhere in it. On a hip, the slope is a trapezium that narrows towards each corner, and the usable rectangle inside it is smaller than the roof looks from the garden. Almost every disappointment on a hipped roof comes from measuring the roof and not measuring the rectangle. The general job sequence sits on the roof window installation page; this is what the geometry adds to it.
Trapezium and triangle: the two shapes a hip gives you
A hipped roof has no gable walls. The two long slopes are trapeziums, wide at the eaves and narrower at the ridge because the hip lines cut in at each end. The two short ends are triangles, rising from the eaves to a point at the end of the ridge.
That gives three usable planes on a typical semi and four on a detached house, but they behave differently. The long trapeziums have plenty of width low down and lose it as you go up. The triangular hip ends have width only at the bottom and almost none at the top, which is why a unit in a hip end is nearly always low in the slope and modest in size.
Jack rafters, and why every one is a different length
Inside the roof, a hip is built from jack rafters: common rafters cut short at an angle where they meet the hip rafter, each one shorter than the last as it approaches the corner. Their spacing stays the same, usually 400mm or 450mm on an interwar cut roof, but their length changes continuously.
This matters for two reasons. The bay you choose has to be long enough top to bottom to take the frame plus its trimmers, and towards the corner the bays run out of length before they run out of width. And a jack rafter is supported at its top end by the hip rafter alone, so shortening one transfers its load onto the hip rather than onto the rafters either side, which is not how a common rafter behaves.
The hip rafter stays whole
The hip rafter runs from the corner of the wall plate to the end of the ridge and carries the head of every jack rafter on both sides of it. It is the single most heavily loaded timber in that part of the roof, usually a deeper section than the commons, and on older roofs it is often doubled or has a purlin framing into it.
It does not get cut. An opening either sits clear of it in the common rafters, or it sits clear of it in the jacks, and there is no version where a frame crosses the hip line. That is a structural statement about the building, and it is the same answer whether the roof is a 1930s cut roof or a modern hipped truss arrangement. The broader question of what trimming does to a roof is covered on the roof window installation hub and in more depth on the page about whether fitting a rooflight weakens the roof.
How far the frame has to sit back from the hip line
Above ground, the constraint is the hip covering. A hip is finished with bonnet tiles, ridge tiles laid over the angle, or on a stone slate roof with a swept or mitred detail. Under them sit the raking cut tiles of both slopes, mechanically fixed because their nibs have been cut away, and usually a lead soaker or a dry-fix system.
The side flashing of a rooflight needs a full run of undisturbed covering between itself and that hip detail. A working minimum is 300mm from the centre of the hip line to the edge of the frame on plain tile and slate, and 400mm on a profiled tile or a bonnet hip where the covering is deeper. Less than that and the cut tiles at the side of the opening become the same tiles that are already cut for the hip, which leaves nothing whole to fix into.
Charlton Kings and Warden Hill: working out the rectangle a hip leaves
The interwar and post-war semis across Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings are hipped, pitched at 30 to 35 degrees, and covered in concrete interlocking tile. Structurally they are straightforward, which is exactly why the shape is the thing that catches people out.
Take a typical rear slope: 6 metres along the eaves, narrowing to 3.5 metres at the ridge, with a rafter length of about 4 metres up the slope. Subtract 300mm at each hip, 300mm at the ridge for the head flashing to lap under a full course, and 400mm at the eaves so the cill flashing has tiles below it. What is left is a rectangle perhaps 2.8 metres wide and 3.3 metres up the slope, and the purlin crossing at mid-height splits it into two zones. That still takes a good-sized unit. It does not take the enormous single window people picture.
The purlin, the binder and the strut that arrive with the hip
A hipped cut roof needs support in two directions, so it carries more internal timber than a simple gable roof. There is usually a purlin running horizontally across each slope at about mid-height, strutted down onto a load-bearing wall, and the purlins are mitred together where they meet at the hip. In the ceiling below there is a binder tying the joists, and often a hanger dropping from the purlin to the binder.
All of this lands in the middle of the space where a window would go. A purlin at mid-height gives two zones, one above and one below, and the practical answer on most Gloucestershire hips is a unit in the lower zone where the head height in the room is comfortable and the run of clear rafter is longest.
Hipped trussed roofs at Quedgeley and Bishops Cleeve
Modern estates are hipped too, but the construction is different. The hip end is formed with a girder truss set back from the corner, carrying a set of progressively shorter monopitch trusses on hangers, with the corner filled by a jack rafter set or a prefabricated hip system. The girder truss is often two or three trusses bolted together and it is the most heavily loaded member in the roof.
Nothing in that assembly gets cut, and the girder truss is the last thing anyone would touch. What is left is the standard 600mm module between the ordinary trusses on the long slopes, which suits narrow units 550mm wide and 780, 980 or 1180mm long. Two of those in adjacent bays give more glass and more even light than one wide unit would, and neither needs a designed member.
A unit in the hip end itself
The triangular hip end is a genuine option and it gets overlooked. On a semi it usually faces the side boundary or the front, so it is worth checking what can be seen from the pavement and whether an Article 4 direction applies in that street. Technically it works: the jack rafters run into the hip rafter from both sides, and a small unit low in the triangle can often sit in one bay with head and cill trimmers only.
The constraint is height. Because the plane narrows to a point, a unit set high in a hip end has almost no covering either side of it, so the size drops away quickly as you move up the slope. A short unit low down, lighting a landing or a bathroom, is where this position earns its keep.
Getting the answer before an order is placed
The survey does three things on a hipped roof. It measures the eaves length, the ridge length and the rafter length so the trapezium is known rather than estimated. It finds the purlin, the binder and any strut from inside the loft and marks where they cross. And it identifies the covering properly, because a bonnet hip on a clay tile roof and a dry-fix ridge on a concrete tile roof need different clearances.
From that, the largest unit the slope will actually take falls out as a number, and it is usually worth comparing it against a pair of narrower units in adjacent bays. Photographs of the rear elevation from the garden, taken square on rather than at an angle, will get you an indicative answer by email before anyone visits.
Next on this subject.
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