Roof window cill height and the view out

Two different heights, both called the cill

Confusion starts here, and it costs money. A roof window has a cill height measured vertically from the finished floor of the room to the bottom of the frame, and it has a position on the slope measured along the rafter from the eaves. They are not the same number and they do not move together.

The vertical figure is what the room experiences: whether you can see out sitting down, whether a desk fits underneath, whether the escape rules are satisfied. The along-slope figure is what the roof experiences: whether there is enough covering below the apron, whether the frame clears a purlin, whether the head has a whole course to tuck under. A survey records both, because a change to one always changes the other by a factor set by the pitch.

Where the measurement is actually taken from

Take the vertical height from the finished floor level, not from the joists and not from the loose boards that are down while the work happens. On a loft conversion the finished floor is often 70 to 100mm above the existing ceiling joists once a deck, insulation and a floor covering are added, and a window set out against the joists ends up that much higher in the room than the drawing said.

The other end of the measurement is the bottom of the frame, not the bottom of the visible glass. On a manufactured roof window the frame and sash together take somewhere between 70 and 100mm off the bottom of the aperture before the glass starts. So a frame cill at 900mm gives glass starting nearer 990mm, and the sightline is worked from the glass rather than the timber.

Sitting and standing are two different rooms

A standing adult’s eye is around 1,550 to 1,650mm above the floor. Sitting in an armchair it is around 1,150mm, at a desk around 1,200mm, and lying in a bed around 700mm. Those four numbers are the whole design problem, because a roof window in a sloping plane only shows you what lies along a line drawn from your eye through the glass and out.

Raise the cill above eye height for a seated person and the seated person sees nothing but the ceiling of the reveal. That is a perfectly reasonable outcome in a bathroom or a landing, where nobody sits still. It is a poor outcome in a study, a snug or a kitchen, where the room is used from a chair for hours at a time.

Pitch does more to the view than height does

On a vertical window the view is straightforward: the horizon is level with your eye, and anything below the cill is hidden. In a sloping plane the glass leans away from you, so the lower edge of the frame cuts off the view at an angle that changes with pitch.

Work it the simple way. Stand where you will normally stand, sight along the underside of the frame at the cill, and continue that line outward. On a steep 45 degree slope that line drops away quickly and lands on the ground twenty or thirty metres out, so you see garden, fence and neighbouring rooftops. On a shallow 22 degree slope the same line stays high and lands on the sky a long way off, so you see cloud and the tops of trees and nothing else. Same window, same cill height, completely different room.

Three slopes, worked through with real numbers

Take a cill at 900mm above the floor and an eye at 1,600mm, so the eye sits 700mm above the cill.

On a 45 degree slope, standing about a metre back from the glass, the downward sightline over the cill is steep enough to take in the garden fence at fifteen metres and the ground closer in than that. This is why loft rooms in Forest of Dean terraces and Cotswold cottages feel connected to the outside in a way a shallow roof never does.

On a 30 degree slope, the interwar standard across Longlevens and Charlton Kings, the same setup gives a middle-distance view: rooftops, treetops, the upper half of a neighbouring house. Perfectly pleasant, and it is the reason 30 degrees is the pitch people describe as normal.

On a 22 degree slope, typical of a vale rear addition lean-to, the sightline over the cill is almost flat. You see sky. To get anything else you drop the cill, and the cill on a lean-to that is only 1.8 metres of run from gutter to abutment has very little room to drop into.

The head of the window does the daylight

Daylight into a room in the roof comes overwhelmingly from the upper part of the glass, because that is the part with the widest area of sky in front of it and the part that throws light deepest into the room. Push the head of the window up the slope and the far corner of the room gets brighter. Push the cill down and the near floor gets brighter but the back of the room barely changes.

Which means the two ends of the window are doing separate jobs and can be specified separately. A taller unit does both. Where the run of slope will not take a taller unit, the honest question is which job matters more in that particular room: light at the back of a long attic, or a view from a chair. Sizing arithmetic sits on the roof window sizes page.

A cill set too low, and the covering you end up looking at

Dropping the cill has a limit that has nothing to do with the room. Below a certain point the sightline out passes through the plane of the roof itself, and what you see is not the garden but the tiles or slates running away below the window.

On a 30 degree slope with a cill at 500mm, a seated person is looking at roof covering for the first several metres before the ground appears. On a hipped interwar rear slope the hip line comes into the view as well, running diagonally across the bottom of the glass. It is not a defect, but it surprises people, and it is easy to check at survey by holding a straight edge at the proposed cill line and sighting along it.

Knee walls, and the height a conversion forces on you

Most loft conversions have a knee wall: a short vertical stud wall carrying the sloping ceiling, usually somewhere between 900mm and 1,500mm high, built to give a usable floor area and to hide the shallow eaves space behind it.

The knee wall sets a floor on the cill height, because the window has to start above it. A 1,200mm knee wall means the visible glass begins at roughly 1,250mm, which on a 30 degree roof rules out any seated view at all. Where the view matters, the knee wall gets built lower and the sloping ceiling brought further down, which costs floor area at the edges of the room. That is a real trade and it belongs in the conversation before the studwork goes up rather than after.

The escape band, which fixes the cill for you

Where the room in the roof is a habitable room reached by a new staircase, one window normally has to serve as the escape opening, and that requirement sets the cill within a narrow band: the bottom of the clear opening between 800 and 1,100mm above the floor. It is a small window of choice, and it exists so somebody can climb out without a ladder inside the room.

The practical effect is that in a loft bedroom the cill height is often already decided before anybody thinks about the view. The full set of figures, including the clear openable area and the hinge arrangements that satisfy it, is on the escape openings page.

Reaching the handle from where you actually stand

Cill height and handle height move together, and on a top hung sash the handle is at the bottom of the sash rather than the top. A cill at 1,100mm on a 45 degree slope puts a bottom-mounted handle within easy reach. The same cill on a 22 degree slope puts the handle further away horizontally than it is vertically, and a shorter person is leaning across the reveal to get at it.

On a centre pivot the handle runs across the top of the sash, so it climbs with the cill and with the length of the unit. A 1,400mm long unit with its cill at 1,100mm has its handle above two and a half metres, which is a rod or an electric opener rather than a hand. The two hinge arrangements are compared on the centre pivot and top hung page, and powered operation on electric rooflight installation.

Low cills, the drop below, and when guarding comes into it

A roof window with a low cill in a room above ground level raises the same question a low vertical window does: what stops somebody falling against it. The glass in a roof window is toughened outside and normally laminated inside, so it is not going to give way, and the sash is held by hinges rated for the load.

Where the question becomes real is a bottom-hung or side-hung opening arrangement with a low cill and a long drop outside, or a large fixed unit set almost to floor level in a converted attic on a Stroud hillside where the ground falls away sharply behind the house. Building Control will look at guarding in those situations, and a restrictor or a low guard rail inside the reveal is the usual answer. It gets raised at survey rather than at inspection.

The horizontal cill lining, and what it costs the view

The reveal below a roof window is normally built horizontal rather than following the slope, so warm air from a radiator underneath rises straight up the face of the glass and keeps condensation off it. That is the right detail and it is worth keeping.

It does have a consequence for the view: the horizontal shelf projects into the room and cuts the downward sightline a little further than the frame alone would. On a shallow pitch, where the sightline is already marginal, that extra few degrees can be the difference between seeing the garden and seeing the shelf. The fix is a deeper radiator recess or a slimmer cill board rather than abandoning the horizontal detail. Reveal geometry generally is on the reveals and splaying page.

Furniture under the window, and the height it wants

What sits under the glass often settles the cill height on its own. A desk needs the cill at around 750 to 800mm so the writing surface tucks under and a seated eye clears the frame. A bed head against the wall below the window wants the cill at 1,000mm or more so the sash and its handle are not directly over a pillow. A kitchen worktop in a single storey rear addition needs 900mm plus an upstand.

A radiator is the one that gets forgotten. A standard panel radiator is 600mm high and needs 150mm above it, so a cill below 800mm and a radiator on the same wall are in conflict, and one of them has to move. Deciding this before the studwork is set out costs nothing. Deciding it afterwards costs a plasterer.

Gloucestershire slopes, and where the cill lands on each

The county’s roof stock produces four recurring situations. On an interwar hipped rear slope at Longlevens, Hucclecote or Charlton Kings, 30 to 35 degrees with a purlin at mid height, the usable band sits between the purlin and the ridge and the cill often ends up around 1,100mm. That is a standing view of rooftops, and it is why two shorter units set lower, below the purlin, sometimes read better in the room.

On a Severn Vale rear addition in Gloucester or Tewkesbury at 22 to 26 degrees, the run of slope is short and the cill has to sit high enough to leave covering below the apron, so the view is sky. In that situation the daylight is the point and the view is a bonus that may not arrive.

On Forest of Dean and Cotswold slopes at 40 to 50 degrees, the geometry is generous. A cill at 1,000mm still gives a seated view of the valley, which is why steep-roofed cottages at Chalford and Coleford get more out of a single window than a shallow roof gets out of two. On a Cheltenham mansard, the steep lower slope behaves almost like a wall and the cill is set to the room exactly as a dormer would be, while the shallow slope behind the parapet is a kerb-mounted unit with no view at all by design.

Setting the height out before the covering comes off

The cill height is marked on the inside of the slope in tape at full size, with the frame outline, the glass line inside it and the reveal linings drawn on. Then somebody stands where the desk will be, sits where the chair will be, and looks. That takes ten minutes and it settles an argument that is otherwise settled by a drawing nobody can read.

What gets recorded at that point: finished floor level against the existing joists, the knee wall height if there is one, the pitch off a digital level, the clear run of slope with the purlin position marked, the escape band if the room is habitable, and the furniture layout. From those six figures the cill height and the size code fall out together. The roof window installation page covers what happens next, and costs explains how the two figures work.