How steep can a roof be for a rooflight?
Steeper than any roof you are likely to own. A manufactured roof window from VELUX, Fakro, Keylite or Roto is typically rated to 90 degrees, which is vertical, so on the steep side the roof effectively never runs out of range. The interesting limits at the top of the scale are not about whether the unit is weathertight. They are about which sash type is still rated at that angle, how a person is meant to reach the handle, what a blind does when gravity is pulling it away from the glass, and how the covering behaves on a slope you cannot stand on. The low end of the range is a different question and it sits on the minimum pitch for a rooflight page. This one is about the steep end.
Ninety degrees is the top of the quoted range, and why it is quoted at all
A roof window rated 15 to 90 degrees sounds like an odd specification until you meet the buildings it exists for. Vertical and near-vertical installations are real: the front face of a dormer cheek, a steep mansard lower slope, the vertical element of a stepped roof, and the combination units where a roof window in the slope runs down into a fixed vertical pane below it to give a view out at floor level.
At those angles the unit is behaving more like a window than a rooflight. Water sheds instantly, nothing lies on the glass, and the drainage channels in the frame have gravity entirely on their side. The weathering is the easy part. Everything difficult about a steep installation happens on the roof surface and inside the room.
Where the sash type narrows the band before the roof does
The headline pitch range belongs to the centre pivot sash, which is the version rated across the widest band. Other sash types are narrower, and the numbers are worth knowing before a preference is settled.
| Sash type | Typical rated pitch band | What decides the top of the band |
|---|---|---|
| Centre pivot | 15 to 90 degrees | Nothing structural; the sash balances on its pivots at any angle |
| Top hung, spring assisted | 15 to 55 degrees | The springs have to hold the sash open against its own weight |
| Top hung and pivot combined | 15 to 55 degrees for the top hung action | Same spring limit; the pivot action continues above it |
| Fixed, non-opening | Effectively the full range | No moving parts to hold at an angle |
| Electric opening | 15 to 55 degrees on most chain actuators | Actuator stroke and the load it can hold |
So the steep-roof answer is often a centre pivot, and on a slope above 55 degrees it is usually the only opening choice. The differences between the two actions are set out on the roof window installation page.
Where the handle ends up on a steep slope
On a 30 degree roof, a window with its cill at 900mm above the floor has its head at around two metres, and a top-mounted control bar is a comfortable reach. Raise the pitch to 50 degrees and the same length of frame climbs much faster up the wall of the room, because the sash is closer to vertical relative to the floor. At 60 degrees the head of a 1400mm sash sits well above two and a half metres.
This is why the steepest roofs in the county produce the most rod-operated and electrically opened units. A centre pivot with a control bar at the top of the sash is the standard answer, because the bar stays reachable when a bottom handle would not be, and it lets the sash be rotated for cleaning. Where even that is out of range, an electric opener with a rain sensor is the practical specification.
Coleford and Cinderford at forty five degrees, with rain arriving behind the wind
Forest of Dean terraces in Coleford, Cinderford, Mitcheldean and Lydney are commonly at 40 to 45 degrees in small-format Welsh slate, often over sarking boards. The Forest is wetter and windier than the vale, and the prevailing driving rain comes off the south west with real force.
Steep pitch helps: water leaves the slope quickly and nothing lies on the covering. What steepness does not help with is wind. Rain arriving nearly horizontally against a steep slope is driven across the covering rather than down it, so side flashings are taken wider than the kit minimum and head laps are increased over the manufacturer’s figure. Sarking boards change the detail again, because the underlay sits over solid timber and the cut edge at the opening needs its own support.
Cotswold stone slate at fifty degrees, and the weight overhead
The wolds roofs through Painswick, Bibury and Chipping Campden run at 45 to 55 degrees, and the pitch is not decorative. Stone slate is porous and laid with a modest lap, so it needs a steep slope to shed water before it can soak through. Take that roof shallower and it stops working.
Working on one is a different proposition from a concrete tile semi. The slates are 20 to 30mm thick, the loading runs well over 100 kilograms per square metre, and every slate taken off a 50 degree slope has to be handed down rather than stacked where it lies. Roof ladders, proper edge protection and a boarded lift at the working level are all part of the price, and the honest position is that a steep stone roof takes longer than the same unit on a shallow tiled one.
Mansard lower slopes at seventy degrees
Central Cheltenham’s Regency and Victorian terraces around Lansdown, Montpellier and Pittville are full of mansards, and the lower slope of a mansard is genuinely steep: 65 to 75 degrees is normal, and some are nearer vertical. It is the face you see from the pavement.
Technically a centre pivot roof window is rated for it and will weather it without difficulty. The constraint is consent rather than construction. That steep face is the visible one on a listed building or in a conservation area, and a unit in it is a very different proposition from one on the hidden upper slope behind the parapet. The sensible order is a pre-application conversation with the conservation officer, with photographs, before any unit is chosen.
Blinds, gravity and a pane that is nearly vertical
A pleated or roller blind on a shallow roof window is held against the glass by side channels and behaves itself. On a steep pane the geometry is easier in one way and harder in another: the blind hangs more naturally, but on a centre pivot sash that rotates, anything fitted to the sash rotates with it, and the fixing has to cope with being upside down while the window is open for cleaning.
External awning blinds are worth a mention here because they earn their keep on steep west-facing slopes. They sit outside the glass and stop the heat before it arrives, which matters more on a steep pane than people expect, because a steep west-facing slope takes the low afternoon sun almost square on.
What a steep pitch does to the reveal and the daylight
Inside, a steep pitch turns the reveal into a tall box rather than a shallow one. The lining above the head of the window gets longer, the lining below the cill gets shorter, and the depth of the plastered return grows with the rafter depth and the insulation in it.
That is an opportunity rather than a problem. A tall vertical lining at the head throws light much deeper into the room, and splaying the two sides widens the beam noticeably. On a steep slope the window also sits closer to the occupant’s eye line, so the view out is better than the same unit on a 25 degree roof, where you mostly see sky. Getting the reveal geometry right on a steep pitch is what separates a bright loft room from a lit one.
Working out your own pitch at the steep end
Two figures are enough to answer the question for a specific roof: the pitch, and the covering. A digital level held on a rafter in the loft gives the pitch directly, and a phone level app against the underside of the rafters gets close enough for a first answer. If the loft is boarded and inaccessible, a square-on photograph of the gable end from the garden will let a pitch be scaled off.
Send that with a photograph of the slope itself and a note of the covering. On a steep roof the answer is almost always yes, and the useful part of the reply is which sash type stays in range, how the unit will be opened, and what the access adds to the job.
Next on this subject.
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