Roof window cill height and the view out

The one dimension to settle before anything is ordered

Measure from the finished floor level, vertically, to the bottom inside edge of the roof window frame. That is the cill height. Every other decision about sightlines follows from it, and it is worth writing it on the drawing rather than describing it in words.

A common working range on a standard domestic pitch puts the cill somewhere between 900mm and 1,200mm above the finished floor. Below about 900mm you start running into guarding questions. Above about 1,200mm a standing adult loses the ground from the view and the window becomes a light source rather than a window.

The reason the figure has to be settled early is that it drives the rafter cutting, the trimmer positions and often the product size, and none of those can be adjusted once the structure is in. A roof window is not something you nudge down 200mm on a Tuesday afternoon.

Why a sloping window behaves differently from a wall window

In a vertical window, your eye and the glass are on parallel planes and the view is essentially horizontal. In a roof window the glass leans away from you, so your sightline through it points upwards and continues upwards. Every millimetre you raise the cill lifts the ground you can see further away from the house, until it disappears over the horizon of the roof edge.

That is why a roof window that looks generous on a section drawing can still deliver no view of the garden. The aperture is large, the daylight is excellent, and the sightline still lands in the sky. The two things are not related in the way people expect.

It also means the same window at the same cill height performs completely differently on a 25 degree roof and a 55 degree roof. Pitch is the other half of the calculation and cannot be ignored.

How roof pitch changes the answer

On a shallow roof the glass is closer to horizontal, so the sightline through it climbs steeply and the ground disappears quickly. To get a downward view on a 25 degree pitch, the cill has to come down a long way, sometimes further than the room comfortably allows.

On a steep roof the glass is closer to vertical and behaves more like a wall window. At 55 degrees a cill at 1,000mm gives a genuine view out at standing height with the ground visible at a reasonable distance from the house. That is why steep Victorian and Edwardian roofs, which are common around Oxford, tend to produce better roof window views than post-war shallow pitches.

Standing eye level, seated eye level and who uses the room

Eye level for a standing adult is roughly 1,500mm to 1,700mm. Seated at a desk it drops to around 1,150mm to 1,250mm, and sitting up in bed it is lower still, nearer 1,000mm. Those three figures produce three different answers, and the room has to pick one to prioritise.

A study where somebody sits at a desk all day wants a lower cill than a landing somebody walks across. A bedroom where the view matters from the bed wants lower again. A bathroom where nobody wants to be seen may deliberately want the cill high, which is the one case where losing the ground view is the objective.

Ask who will be in the room and what they will be doing. That question does more for the specification than any product comparison.

Setting the line out with a laser and a batten

The way to stop this being theoretical is to mark it in the space before the roof is opened. Stand where the person will stand, hold a batten or a laser line at their eye height, and project it up the slope. Where that line crosses the rafter line is the point above which glass gives you sky and below which glass gives you ground.

Do it a second time from the seated position, and a third from wherever the bed or desk will sit. You end up with two or three marks on the rafters, and the cill goes at or below the lowest of them.

This takes ten minutes and it is the most useful ten minutes on the whole job. We do it as part of the survey, with the household in the room, because nobody can judge this from a drawing.

The head height, and why it is not just the cill

The top of the frame matters too, though for a different reason. The head governs how far daylight throws into the depth of the room and how much sky is in the view. A common target puts the head somewhere between 1,900mm and 2,200mm above finished floor, which on most pitches means a unit around 1,180mm tall in a standard range.

Raising the head pushes light deeper into the room and towards the back wall, which matters in a long loft where the far end would otherwise be gloomy. Lowering it brings the light closer to the window and can leave the room reading as bright at one end and dim at the other.

Cill and head together define the unit height, and the available heights in a manufacturer’s range are not continuous. You will be choosing between fixed steps, which is covered in more detail on our page about roof window size codes.

Guarding, and how low the cill can legally go

Approved Document K deals with protection from falling, and it becomes relevant where an opening in a habitable room sits close to the floor with a drop beyond it. On a roof window the question is whether somebody could fall through the aperture when the sash is open.

The practical trigger point is a cill below about 800mm from finished floor level where there is a significant drop outside, though the assessment depends on the pitch, the eaves height and what is below. Where guarding is required, the answer is usually an internal balustrade or a restrictor arrangement rather than abandoning the low cill.

Building Control will form a view on this for a loft conversion, and it is far better to raise it at design stage than to be told about it at inspection. It is one of several places where the view you want and the regulations have to be reconciled rather than argued about.

Where an escape opening pulls the cill downwards

Where a roof window is providing the escape opening from a loft bedroom, the clear openable area and the height of the cill above floor are both constrained, and those constraints are not negotiable in the way a view preference is. The cill height for escape is typically expected between 600mm and 1,100mm above finished floor.

That range overlaps with a good view height, which is convenient, but it removes the option of going high. The full requirements are set out on our page about escape openings and the rules, and where escape applies, that requirement wins and the view arranges itself around it.

It is worth establishing at the first conversation whether the window is carrying escape duty, because it changes the product, the size and the cill in one go.

Low knee walls and the dwarf wall problem

Many loft conversions have a knee wall, sometimes called a dwarf wall, running along the eaves at 900mm to 1,200mm high, with the sloping ceiling starting above it. The rafter is not accessible below that line without opening the wall up, and the storage behind it is often part of why the wall exists.

That puts a hard floor under the cill height. If the knee wall is 1,100mm and the slope starts there, the cill cannot sensibly go lower than that without redesigning the wall and losing the eaves storage. It is a common reason a loft ends up with a sky view rather than a garden view.

The options are to lower the knee wall, accept the higher cill and choose a taller unit to compensate at the head, or bring the window into a position where the knee wall is lower. All three are decisions for the design stage, and all three are cheaper to make on paper.

Tall units, and buying the view back at the bottom

Where the cill cannot come down, the next move is to make the unit taller so the glass extends further down the slope from a fixed head. A 1,398mm tall unit reaches roughly 200mm lower than a 1,180mm unit for the same head position, and on a moderate pitch that 200mm can be the difference between seeing the fence and seeing the sky above it.

The cost of that is structural. Taller units mean a longer trimmed opening, which can mean cutting an additional rafter and a heavier trimmer, and on some roofs it means the opening no longer fits between existing rafter centres without significant work.

There is also the practical effect of glass reaching lower down the slope: it comes closer to the floor and closer to whatever furniture sits under it. That is not always welcome in a bedroom.

Two windows at different heights on one slope

On a wide slope, one high unit and one low unit does not usually look right. Roof windows read as a set from outside, and a mismatch in cill line is visible from the street in a way that a mismatch in width is not.

Where two units sit on the same slope, align the cills and align the heads. If the rooms behind them want different things, that tension usually resolves in favour of the elevation, because the inside can be adjusted with blinds and layout and the outside cannot be adjusted at all.

Where a genuine difference is unavoidable, a clear step of 400mm or more reads as deliberate. A 60mm difference reads as a mistake, and it is the sort of thing that becomes impossible to unsee.

Where the cill lands relative to the rafters

The cill height you want and the rafter layout you have do not automatically agree. A trimmed opening needs a trimmer across the bottom, and that trimmer needs bearing, so the cill position interacts with where the existing rafters fall and what is already notched or supported nearby.

Purlins are the usual complication. A purlin running across the slope at 1,100mm above the floor sits exactly where the cill wants to be, and moving it is a structural exercise rather than a carpentry one. On older Oxford roofs with substantial purlins, the purlin frequently sets the cill height whether anybody likes it or not.

The honest answer in those cases is that a particular cill height is not achievable without structural work that changes the cost of the job entirely. We will say that at survey rather than after the ceiling is open.

Furniture, radiators and what sits under the glass

A low cill puts glass where furniture wants to be. A wardrobe under a roof window is fine at 1,200mm and impossible at 850mm. A desk under a low cill is excellent, provided the sash opening mechanism does not swing into the space above it.

Radiators are the other conflict. The traditional position under a window works badly with a low roof window cill, because the sloping ceiling above the radiator has no wall to sit against. Where the cill is low, the heating usually needs relocating, and that is a decision for the same design conversation.

Sketch the furniture layout onto the same drawing as the cill height. The two constraints are much easier to reconcile on paper than in a finished room.

The handle, and reaching the window at all

Cill height and handle position are connected. On a bottom-operated unit the control is at the cill, so a high cill puts the handle out of comfortable reach and a low cill puts it at a natural height. On a top-controlled unit the handle is at the head, so a low cill combined with a tall unit can push the control above 2,000mm and out of reach without a pole.

Reach is worth checking with a tape rather than assuming. Comfortable operating height for most adults is under 1,900mm, and anything above that means a control rod, a pole or an electric unit, all of which are fine if they were planned for.

This interacts with the opening mechanism, which is covered separately on our page comparing centre-pivot and top-hung units.

Recording the decision so it survives to site

Write the cill height on the drawing as a dimension from finished floor level, and say finished floor level explicitly. A number taken from the joists rather than the finished surface is wrong by the depth of the deck, the insulation and the floor covering, which can easily be 60mm to 100mm.

Mark the rafters as well. A pencil line on the timber with the letters FFL and the figure written next to it is understood by everybody who will touch the job, and it survives a change of trade on site in a way a drawing in a folder does not.

On our installations the cill line is agreed with the household, marked on the roof structure, and checked against the finished floor build-up before the opening is cut. Then it is fixed. The roof is not left open overnight, and the marking out is done before any covering comes off.

Getting the number right for your roof

There is no single correct cill height, and any figure quoted without a pitch and a room use behind it is a guess. What there is, is a method: establish who uses the room and how, project their eye line onto the slope, check what the knee wall and the purlins will allow, then let escape and guarding requirements trim the range.

We survey with a laser and a batten, mark the sightlines with the household in the room, and give a range by email first with a fixed price after the survey. The workmanship is guaranteed for ten years, and the detail that guarantee actually depends on is the perimeter: upstand, falls, flashing laps and where the covering terminates. Call 01865 704245 or email info@heritageskylightsoxford.co.uk before the cill height gets decided by default.