Cill height and the view out

The working range

Between about 900mm and 1100mm above the finished floor gives a view standing and from most seated positions. That is the default and it suits most rooms.

Below about 800mm the sash starts to interfere with furniture and, on a top hung window, with anybody standing at it. There are also safety considerations with low glazing.

Above about 1200mm it stops being a window you look out of and becomes a source of daylight. Legitimate in a bathroom or a hallway; disappointing in a bedroom.

The escape constraint

Where the window is a means of escape, the bottom of the clear opening must be between about 800mm and 1100mm above the floor. That narrows the choice considerably and it takes precedence.

Pitch changes everything

On a steep roof, a window at a given cill height sits close to vertical, and the view is like a normal window: horizon, garden, street.

On a shallow roof, the same cill height angles the window towards the sky. You see cloud rather than garden. Neither is wrong, but they are different products in practice and people are often surprised.

The shallower the roof, the lower the cill needs to be for a view of anything other than sky.

Top of the window

The top matters for daylight distribution rather than view. A window whose head is higher throws light further into the room, which in a deep loft room is worth more than a lower cill.

Where both matter, a taller unit is the answer, and taller usually means wider too, which brings the rafter question back.

Decide in the room

With a tape measure, standing where you will stand and sitting where you will sit. Ten minutes doing that prevents the most common regret in this trade.

The decision that changes how the room feels

Where a roof window sits on the slope decides whether it is a window or a light source, and it is a decision people frequently leave to whoever is fitting it.

Set low, a roof window gives a view out at standing or sitting height, and light that reaches far back into the room. Set high, it gives sky, more even light across the space, and better privacy.

Both are legitimate. What is not legitimate is choosing by accident.

What low gives you

A view. On a loft room with something worth looking at, a low cill turns a rooflight into a window. You can stand at it, or sit near it, and see out rather than up.

Light deep into the room. Light entering at a low point on the slope travels further across the floor before it lands. In a room with a knee wall, a low window lights the useful part of the space rather than the ceiling above the far wall.

A sense of connection. A room where you can see out feels larger and less enclosed than one lit only from above.

Escape compliance. Where the window carries a means of escape duty, there is a maximum height for the bottom of the opening above the finished floor. That is a hard limit rather than a preference, and it frequently forces a lower position than you would otherwise choose.

What high gives you

Sky. More of it, and a better view of weather and light changing, which some people prefer to a view of a neighbour’s roof.

More even light. A high window throws light across the ceiling and down, distributing it more evenly than a low one that concentrates it on the floor near the window.

Privacy. No view in, and no view out at eye level, which in a bathroom or a bedroom overlooked from higher ground is worth having.

Wall space. A high window leaves the wall below it usable for furniture, which in a small loft room matters.

Less low sun. A low window on a west facing slope admits late afternoon sun at a shallow angle, straight into the room and into people’s eyes. A high one does not.

The escape constraint, which decides it in many rooms

Worth understanding before anything else, because it is not negotiable.

In a habitable room in a loft, Building Regulations frequently require the roof window to serve as a means of escape. That brings a minimum clear openable area, minimum opening dimensions, and a maximum height of the bottom of the opening above the finished floor, so a person can reach it.

Setting a window high for the view can put the aperture out of reach for escape purposes. That is a compliance failure discovered at inspection, by which time the window is in and the covering is back on.

Note that the floor level is the finished floor of the conversion, which sits above the original ceiling joists. A position measured against the old joist level is not the position it will occupy against the new floor, and that catches people out.

Where an alternative escape route exists, the duty may be satisfied elsewhere and the window is free to be positioned purely for daylight. Establish which situation applies at design stage, because it changes everything.

The other constraints

The purlin. On a traditional cut roof a purlin runs horizontally through the slope partway up, and it is not something to cut. An opening goes above it or below it, which frequently settles the question on an older roof.

The ridge. A rooflight cannot sit above the highest part of the roof, and practically it needs at least a course or two of covering above it for the head flashing to lap under. Ridge ventilation, where present, needs to remain clear.

The eaves. Similarly, the sill needs covering below it for the apron to sit on, and it must not interfere with eaves ventilation.

Reach. A high window on a steep slope is difficult to operate and difficult to clean. Above about two and a half metres to the handle, a pole becomes a chore and the window stops being opened.

That is an argument for motorising rather than for lowering the window, and it is worth deciding at the same time.

Pitch changes the arithmetic

On a steep roof, moving a window up or down the slope changes its height above the floor a great deal for a small movement.

On a shallow roof the same movement changes the height less, so there is more latitude, and the constraint is more likely to be the purlin or the truss spacing than the cill height.

Shallow roofs also give a longer reveal, which is where splaying it matters most: cutting the reveal back at an angle at the head and the sill opens the beam out and lets light reach further into the room, which partly compensates for a window that had to be set higher than ideal.

The compromise that usually works

On a loft room with two windows, one low and one high.

The low one gives the view and the escape provision. The high one gives even light across the room and reaches the parts the low one does not.

They should align with each other horizontally and be the same model and size, so from outside the pair reads as deliberate rather than accidental.

What to do before deciding

Stand in the room, or in the loft if it is not converted yet, and hold something at the proposed sill height.

Look at what you can see. Sit down and look again, because a room is used sitting as much as standing. Then consider what will be under the window: a bed, a desk, a wardrobe, or nothing.

Ten minutes doing that is worth more than any amount of discussion from a plan, and it is the point at which most people change their mind about where they thought the window should go.

Why the height on a raked roof is not obvious

On a wall, sill height is a single number. On a roof it depends on two things at once: where the window sits along the slope, and how steep the slope is.

The same window positioned identically on a 30 degree roof and a 50 degree roof will have its sill at very different heights above the floor, because the slope rises faster.

Which means sill height cannot be specified in the abstract. It comes out of the pitch and the position together, and it should be worked out on a section drawing rather than estimated.

What each height gives you

A low sill, at seated eye level or below, gives a view of the garden and the street from a chair. It makes a loft room feel connected to the outside rather than looking only at sky.

It also brings light lower into the room, onto the floor rather than onto the opposite wall.

A high sill gives sky and light without any view, which is what you want in a bathroom or where the outlook is a neighbour wall.

It throws light deeper into the room, which on a deep loft is a real advantage.

The traditional resolution on a deep room is two windows: one low for the view, one high for the light.

The escape window constraint

Where a loft room needs an escape window, the bottom of the clear opening has to be within a workable height above the floor: low enough to climb through, high enough not to be a fall hazard.

That is a genuine constraint and it interacts with pitch. A window set high on a shallow roof can have a sill well above the workable range even though the window itself is large.

Establish whether an escape window is required before choosing the position, because it constrains model, size and position simultaneously.

Reach and cleaning

A window whose head is a long way up the slope is one nobody opens and nobody cleans.

On a centre pivot unit the control bar comes down towards you as the sash rotates, which helps. On a top hung unit the handle is at the sill, which means a high sill puts the handle out of reach.

Where the position is fixed by structure and the reach is poor, a powered opener with a rain sensor is what turns a decorative window into a working one.

The knee wall

In most loft conversions there is a knee wall at the low end of the slope, and the window has to sit above it. That frequently sets the minimum sill height before any preference is considered.

Worth checking early, because it is the constraint that most often overrides the intention of a low window with a view.