Can a rooflight be fitted to a hipped roof?

The usable rectangle on a slope that narrows

Start by drawing the slope as it actually is. A hip end is a triangle with its base at the eaves and its apex at the ridge or, on a hip with a short ridge, a trapezium. A main slope on a hipped roof is a trapezium too, long at the eaves and shorter at the top, cut back at one or both ends by the hips.

Now take the exclusion zones off it. Somewhere between 300mm and 450mm of undisturbed covering along each hip, a course or more of covering below the ridge, and a clear run at the eaves. What remains is the rectangle a rooflight can occupy, and on a hip end of a modest semi that rectangle can be surprisingly small, sometimes no more than a metre wide at the height where you wanted the glass.

This is why hipped roofs so often end up with the unit lower down the slope than the owner first imagined. Lower is wider.

How far the unit stays off the hip line

The hip is a ridge running diagonally, and like any ridge it is a junction covered by hip tiles bedded in mortar or held on a dry hip system. Water from both adjoining slopes runs away from it, so it is not a drainage channel in the way a valley is, and the clearances are less severe.

They are not nothing, though. The unit’s side flashing has to land on whole tiles or slates, and the covering along a hip is already cut on the rake, so the courses running into it finish in tapered pieces. Putting a flashing skirt onto a run of raking cuts gives you a line of small, lightly fixed pieces holding down a lead edge, which is exactly what a gale finds. We set out for full-width covering between the flashing and the first cut piece, which in practice means around 300mm minimum and more on a large-format tile.

The hip rafter carries load and is not touched

Structurally the hip rafter is the important member. It runs from the corner of the wall plate to the ridge, it is deeper than a common rafter, and the jack rafters from both slopes are cut against it and bear into it. On a cut roof it is often doubled or a heavier section.

It is not trimmed for a rooflight, ever, and neither are the two or three jack rafters immediately adjacent to it, because they are short members getting their support from the hip. That pushes the nearest possible opening one or two full bays in from the corner, and on a hip end with a shallow triangle that can be most of the width you thought you had.

Where the roof is trussed the position is fixed harder again, and what may and may not be cut is set out on rooflights in a trussed roof.

The interwar semi, and where the glass usually lands

The typical 1930s semi around Headington, Cowley and Botley has a hipped main roof, concrete interlocking tiles or plain clay, a 40 to 45 degree pitch and rafters at 400mm centres. The rear main slope is the useful one: it is the longest plane, it usually faces a garden with no obstruction, and the hips are far enough apart that neither is a factor in the middle of it.

On that roof a single unit between rafters, or one trimmed to a wider size, sits comfortably around a third of the way up the slope. Two units spaced across the same plane is the arrangement that lights a converted loft properly.

The hip ends themselves, the short triangular slopes at each gable position, are where the difficulty is. They are usually the smallest planes on the house and they are the ones facing the street or the boundary. If a unit has to go there, it will be a narrow one, and it will sit low.

Head clearance where the slope is running out

On a rectangular slope the top of the roof is a single constraint. On a hipped slope it is two, because the trapezium narrows as it rises and the ridge cuts it off. A unit 1400mm long needs 1400mm of slope plus its clearances at head and foot, and at the height where the head lands the slope may be a good deal narrower than at the foot.

The practical consequence is that on a hip end you often cannot use a long unit at all, and a shorter one placed lower gives you more glass than a longer one placed higher. How close the head can come to the top is a separate question covered on how close to the ridge can a rooflight go?

Two narrow units instead of one wide one

Where the hip clearances have eaten the width, the answer is frequently two units in separate rafter bays rather than one wide unit that would need trimming. Two narrow openings need no structural work at all if each fits between rafters, and they light a room more evenly than one central unit does.

They also read better from outside on a hipped roof. A hip end is a small, strongly shaped plane, and a single large rectangle sitting in it fights the geometry. Two smaller ones spaced symmetrically sit within it. On a plane you can see from the road, that is worth thinking about before the size is fixed.

Spacing and appearance for paired units are covered on two rooflights side by side on one slope.

Hip-to-gable conversions, and what that job actually is

The standard loft conversion on a hipped semi removes the hip at one end, builds the gable wall up in blockwork or timber, and reroofs the new triangle. It converts a cramped triangular loft into a usable room and it creates a full-height wall for a window.

That is structural work by a builder with an engineer’s design behind it, and it is not us. We install and replace rooflights, and nothing else. What we do is come in once the new gable and roof structure are formed and the covering is on, and fit the rooflights into the reworked slope. Coming in at that point is straightforward, because the battens are new, the gauge is regular and the position can be agreed before the covering goes down.

If your conversion is going that way, the useful thing is to tell your builder the rooflight positions early. Setting out a new roof around known openings is far easier than working openings into a finished one.

Setting out on a hipped slope at the survey

We measure the eaves length, the ridge length, the slope length, the hip positions at both ends and the rafter or truss spacing, then draw the plane and mark the exclusions on it. What is left is what you can have, and it is usually clear within ten minutes on the roof.

The covering and its condition come next, because on a hipped roof the cut pieces along the hips are the vulnerable ones and we want them undisturbed. Then the internal arrangement: ceiling type, where the light needs to fall, whether the reveal will be splayed.

Fifteen years of hipped roofs across this county says the answer is nearly always yes, with the size and position adjusted rather than the idea abandoned. You get a range by email once we know the roof, a fixed price after the survey, the roof is never left open overnight, and the workmanship carries a ten-year guarantee.