Guide · VELUX Window Installation
Roof window cill height and the view out
The bottom edge of a roof window normally wants to land between 800mm and 1100mm above the finished floor. That band is not a comfort convention. It is the only range that puts glass across the eyeline of someone standing and someone sitting in the same room, and a roof window that misses it becomes a light source rather than a window.
Cill height is settled on the slope, in millimetres, before a saw touches a rafter. It decides what you see, whether the handle can be reached, how much the reveal crops the view, and how the unit reads from the garden. This page works through how the number is arrived at on Essex roofs and where the standard figures stop applying.
What cill height actually means on a sloping window
On a wall window the measurement is obvious: floor to the underside of the frame, straight up. On a roof window the frame lies back at the pitch of the slope, so three different numbers get called the cill height and they are not the same.
There is the bottom of the structural opening cut through the rafters. There is the bottom of the window frame itself, which sits a little inside that. And there is the bottom of the finished internal reveal, which is what you actually stand in front of. The reveal edge is usually the number that matters to the person living in the room, and it can sit 40mm to 70mm above the frame once plasterboard and skim are on. Agree which one is being quoted before anybody writes it down.
The 800 to 1100 range, and where it comes from
Manufacturers publish this band and the logic behind it is physical. At 800mm the cill sits just below the hand of a standing adult and roughly level with the eyeline of an adult sitting on a chair. At 1100mm it is still below standing eye height on most pitches, so the view is uninterrupted from your feet.
Go above 1100mm and the glass rises out of the seated sightline entirely. The room gets brighter and the window stops showing you anything except cloud. Go below 800mm and two things happen: the internal lining starts eating into wall you wanted for furniture, and on a floor with a drop beyond it the guarding question arrives.
Pitch, and how it moves the cill for a given head height
The pitch of the roof changes everything about a fixed length of window. A 1400mm long unit rises a different vertical distance on every slope, and that vertical rise is what governs where the head lands once the cill is set.
| Roof pitch | Vertical rise of a 1400mm unit | Horizontal run into the roof | What it means inside |
|---|---|---|---|
| 15 degrees | about 360mm | about 1350mm | Reads almost as a ceiling panel, sky only |
| 22 degrees | about 525mm | about 1300mm | Sky and treeline, ground not visible |
| 30 degrees | about 700mm | about 1210mm | Middle distance appears from standing |
| 40 degrees | about 900mm | about 1070mm | Garden visible from standing, sky from seated |
| 45 degrees | about 990mm | about 990mm | Behaves close to a steeply set wall window |
The consequence is that a cill height copied from a friend’s 45 degree Victorian rear slope will not give the same view on a 22 degree trussed estate roof. Same window, same height, different room.
Sitting, standing and the two sightlines that matter
Eye height for a standing adult runs roughly 1500mm to 1700mm. Seated on a dining chair it drops to around 1150mm to 1250mm, and on a sofa lower again, nearer 1050mm.
Draw those two lines out through the glass and the picture becomes clear. On a shallow pitch the seated line hits the ceiling long before it reaches the window, which is why a low pitched roof window shows the sky to a standing person and nothing at all to somebody in a chair three metres away. Moving the cill down helps, but only up to a point. Moving the whole window closer to where people sit helps more.
The near view, the middle distance and the sky
A roof window gives you a slice of the outside world, and the cill decides which slice. Low cill on a steep slope and you get the fence, the shed roof and the bottom of the garden. Mid cill and you get the middle distance: the neighbouring roofline, trees, the church tower if there is one.
High cill gives you sky and weather, which is not nothing. In a room where the ground level view is a car park or a flank wall three metres away, sky is the better product and the cill should go up rather than down. That judgement is made standing in the room, not on a drawing.
Where a low cill starts to feel exposed
Below 800mm from the finished floor, glazing in a position where a person could fall against it engages Part K. The answer is not to abandon the low cill, it is to specify the inner pane as laminated so that it holds together under impact rather than giving way.
The situation that genuinely needs thinking about is a low cill on a steep slope over a stairwell or a mezzanine edge, where the floor beyond the window does not continue. There, guarding is a design item rather than a glass specification, and it is settled at the same time as the opening itself.
Stand in the room, mark the proposed cill on the wall with tape, and sit down in the chair you actually use before agreeing the height.
Rooms with a knee wall, and the eaves line below
Almost every loft conversion has a knee wall, a short vertical partition where the sloping ceiling meets a usable head height. In Essex conversions it is commonly built at 900mm to 1200mm above the new floor.
That wall is a hard constraint. If the window cill lands below the top of the knee wall, the wall blocks the lower part of the opening and the window is effectively shortened. If the cill lands well above it, a band of dead plasterboard sits between the two and the view starts high. The comfortable answer is usually a cill within 100mm to 200mm of the knee wall head, so the glass appears to continue where the wall stops.
Head height, and the ceiling that stops you going higher
The top of the window is limited by whatever the ceiling does above it. In a conversion the sloping ceiling usually runs up to a flat collar at around 2.3m to 2.4m, and the window head has to finish below the point where the slope meets that flat.
In an unconverted roof a purlin often crosses the slope at mid height, and a purlin is a structural member spanning between walls. It sets a hard ceiling on how far up the opening can go without a designed alternative. Between the knee wall below and the purlin above, the available band on a Victorian rear slope is frequently only 1.2m to 1.6m of slope length, which is what dictates the unit rather than any preference about view.
Long windows, and where the extra length actually goes
Going from a 1180mm unit to a 1400mm one adds length up the slope, not down. Manufacturers set out their ranges so the cill stays where you want it and the head rises, because the cill is the ergonomic anchor and the head is the flexible end.
That matters when head height is tight. If the purlin or the collar caps the head, the way to gain glass is not a longer unit but a second one, either alongside or lower down the slope. Where a slope is genuinely short, two units on the same plane deliver more usable daylight than one unit forced into a length the roof cannot take.
Reveal depth, and how much of the view it crops
The reveal is the lining between the ceiling finish and the frame, and on an insulated rafter it can be 250mm to 400mm deep. Every millimetre of that depth crops the view the way a deep porch crops the sky, narrowing what you see from any position more than a metre back.
Splaying the head of the reveal outward opens the upward view and pushes light across the ceiling. Keeping the cill of the reveal vertical, plumb down to the floor, protects the downward view, which is precisely the part a low cill was bought for. Splaying the cill instead is a common finish and it quietly gives back the ground level view that the cill height was set to achieve.
Desks, beds and worktops under the glass
Furniture heights are fixed and worth designing to. A desk top sits at about 720mm to 750mm. A kitchen worktop at 900mm. A bed mattress at 500mm to 600mm.
A cill at 900mm clears a desk with room to open the window over it and gives a seated person at that desk a genuine view. A cill at 900mm above a worktop line means the opening starts exactly where the wall units would have been, which is often the reason the window went in. Above a bed the cill can go lower, because nobody stands there, and the view from lying down is a different geometry again.
Handle position, and how it interacts with cill height
Reach is the practical limit on how high a cill can go. Some units are operated from a bar across the top of the sash, others from a handle at the bottom, and the two arrangements are set out in detail on our page comparing centre pivot and top hung units.
The number to hold onto is that a comfortable operating height for a standing adult tops out around 2m. With a top control bar, the bar sits at the head of the window, so a 1400mm unit at 1100mm cill on a 40 degree pitch puts the bar at roughly 2m and that is the ceiling on cill height for manual operation. Above that, either the cill comes down or the unit gets a motor.
Cill height on a dormer against cill height in the slope
A dormer window is vertical, so its cill height behaves like any wall window and the standard 900mm is straightforward. The trade is daylight: a vertical dormer window collects light from about half the sky, while a window lying in the slope faces far more of it.
In practice a dormer gives the better view and the roof window gives the better light, which is why a converted Essex loft often ends up with both. Where the pair sit on the same elevation, aligning the dormer cill with the roof window cill makes the arrangement read as deliberate rather than accidental.
Essex loft conversions, and the heights that keep recurring
The stock repeats itself, and so do the numbers. Victorian brick terraces in Chelmsford and Colchester carry rear slopes at 40 to 45 degrees with a purlin mid slope and a converted floor that sits high, so cills tend to land at 900mm to 1000mm and the head is capped by the purlin.
Interwar semis with hipped concrete tile roofs run nearer 35 degrees and have generous slope length, so the 1100mm cill is comfortable and the head is rarely the problem. On the trussed estates at Great Notley, South Woodham Ferrers and Beaulieu Park, pitches of 22 to 30 degrees with trusses at 600mm centres force narrow units between webs, and the cill drops towards 800mm to recover a usable sightline on a shallow plane.
Marking the cill on site before anything is cut
The reliable method costs ten minutes. A batten is screwed across the rafters at the proposed cill line from inside the roof void, or a line of tape is run across the ceiling where the opening will start.
Then the room is used. Stand at the door, sit in the chair, lie on the bed if it is a bedroom. The line moves more often than it stays, usually downward, and it moves for free at that stage. Once the trimmers are in and the flashing is dressed, the cill is where it is. This check is part of every survey we run before a VELUX unit is ordered, because the manufacturer’s catalogue cannot see your knee wall.
What to send to get the height agreed
Three measurements settle most of it before anyone visits. Floor to the point where the sloping ceiling meets the knee wall. Floor to the flat ceiling above. And the pitch, which can be read off a phone level held against the slope inside the loft.
Add a photograph looking up the slope from inside and one of the roof from the garden, and a sensible cill height and unit size comes back by email. The survey then confirms it against the purlin, the rafter centres and where the internal lining will actually land, and that is the number that goes on the order.
Keep reading
More on velux window installation
Roof window escape openings and the rules
Read the guide →Roof window blinds and when to specify them
Read the guide →Roof window flashing kits by roof covering
Read the guide →Roof window sizes and how they are coded
Read the guide →Roof window ventilation flaps and filters
Read the guide →
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