Rooflight reveals and why splaying them matters

What the reveal actually is, measured rather than described

Stand under a roof window and look up. You are looking through a shaft. Its walls are the four faces of the reveal, its depth is the distance from the ceiling line to the underside of the frame, and its top is the glass.

On a typical trussed roof at Quedgeley with 100mm of insulation between the rafters and 50mm below, that depth is around 220mm. On an interwar rafter at Hucclecote upgraded to current standards it is 250 to 300mm. Under a Cotswold stone roof with a breathable build-up on counter battens it can reach 350mm. The deeper the shaft, the more the splay matters, because a deep square shaft blocks the sky from anywhere except directly underneath it.

The four faces, and the different job each one is given

The convention on a pitched roof window is not symmetrical, and it is not arbitrary either. The head lining, above the window, runs vertical. The cill lining, below it, runs horizontal. The two sides are splayed outward.

Each of those does something specific. The vertical head throws light deep into the room. The horizontal cill lets warm air rise up the face of the glass. The splayed sides widen the beam sideways. Build all four square to the frame and you get a rectangular shaft that lights a rectangle of floor, which is the cheapest and the worst of the options.

Why the head lining runs vertical

Take the head lining perpendicular to the roof slope and it points the light at the floor immediately below the window. Take it vertical, plumb down from the top of the frame, and the light is thrown at a shallower angle across the room.

On a 35 degree slope that is a difference of 55 degrees in the direction the top face is aimed, and it moves the far edge of the lit area by a metre or more on a room of normal depth. It also means the head lining reads as an extension of the wall behind it rather than as a slanting soffit, which is why a vertical head looks correct even to people who cannot say what has been done.

Why the cill lining runs horizontal

The cill lining is the one with a thermal job as well as an optical one. Run it horizontal, level out from the bottom of the frame, and it forms a shelf with clear air above it.

Put a radiator on the wall under that shelf and its warm plume rises straight up the inside face of the glass. Moving air over a cold surface raises its temperature and carries vapour away, which is the cheapest condensation control available and it costs nothing to build. Splay the cill downward instead and you create a pocket of still cold air trapped under the glass, which is precisely where water forms first on a February morning.

Splaying the sides, and the angle worth using

The two side faces are where the daylight gain sits. Splayed means the reveal is wider at ceiling level than it is at the frame, so the opening flares outward into the room.

A useful working angle is 30 to 45 degrees off the perpendicular. Below about 20 degrees the splay is barely doing anything and it just looks like a badly set out square reveal. Beyond about 50 degrees the plasterboard starts fighting the rafter behind it and the finished angle looks mannered rather than deliberate. Somewhere near 40 degrees is where most well built reveals land, and it is a decision to make on site with a straight edge held up against the opening rather than on a drawing.

How much wider a splay makes the opening, in millimetres

The arithmetic is simple and it is more persuasive than the description.

A 780mm wide roof window in a 250mm deep reveal, boarded square, gives an aperture at ceiling level of 780mm. Splay both sides at 40 degrees and each side gains 250mm multiplied by the tangent of 40 degrees, which is about 210mm. The aperture at ceiling level becomes roughly 1,200mm. That is a 54 per cent increase in the opening the room sees, from the same window, the same glass and the same hole in the roof.

On a 300mm reveal the gain per side rises to about 250mm and the aperture reaches 1,280mm. This is why splaying is worth arguing about on a narrow unit between two trusses, where the window itself cannot be made any wider.

Roof thickness sets the limit on how much splay is available

You cannot splay more than the construction allows. The side faces are cut into the trimmed opening, and the opening is bounded by rafters carrying load.

Where the window sits between two rafters at 600 centres with 550mm of clear width, the splay can only start below the underside of the rafters, because nothing is coming off a structural member. That gives a splay that begins at the bottom of the rafter depth and runs from there to the ceiling line, so the effective splay depth is the depth of the ceiling build-up plus any counter batten, not the full 250mm. Where a rafter has been trimmed and doubled, the splay starts below the trimmer instead. This is worked out with the trimming design rather than after it, and the structural side of that is on the trimming rafters for a pitched roof rooflight page.

Pitch changes the shape of the box

A reveal on a 45 degree Forest of Dean slate roof is a different object from one on a 22 degree vale lean-to, even with the same unit.

The steeper the pitch, the taller the head lining and the shorter the cill lining, because the vertical drop from the top of the frame to the ceiling grows while the horizontal run from the bottom of the frame shrinks. At 45 degrees the head lining on a 1,180mm long window can be 600mm tall, which is a substantial plastered face and worth treating as a design element. At 22 degrees the situation inverts: the cill lining becomes a long, near-horizontal shelf and the head lining is barely there. On very shallow slopes the horizontal cill can project so far that it starts to shade the glass, and at that point the sensible move is to reduce it and accept a shallow angle instead of a true horizontal.

Insulation around the frame, and the void that must not exist

Everything above is about light. What follows is about whether the reveal stays sound, and it is decided before a single sheet of plasterboard goes up.

Insulation runs continuously around the outside of the frame, tight to it, with no gaps at the corners. The usual failure is a 20 or 30mm void left at the head of the frame where the insulation was cut a fraction short and nobody went back to it. That void is a cold bridge with warm room air on one side of the plasterboard and outside air on the other, and it announces itself as black spotting in the top corners of the reveal in the second or third winter. Rigid board cut tight, with the joints taped or foamed, is the standard. Mineral wool stuffed roughly into the same space settles and leaves the gap back again.

The vapour control layer, taped to the frame rather than trimmed short

Behind the plasterboard on the warm side there is a vapour control layer whose job is to stop humid room air getting into the construction. At a rooflight it has to be carried right up to the frame and taped to it.

Left trimmed 50mm short, which is what happens when it is cut with a knife against a straight edge and never taped, it gives warm moist air a direct route into the rafter zone around the opening. The water it deposits there is out of sight, it does not appear on the glass, and it turns up years later as a soft edge to the plasterboard at the head of the reveal or a stain along one side. The tape is the cheapest item on the job and the one with the longest consequences.

Plasterboard, beads, and the angle a plasterer can finish

A splayed reveal has four internal angles at the ceiling and four external arrises at the frame, and every one of them has to be beaded and set.

Angle beads on a splay are cut to a compound angle rather than a mitre, which takes longer than a square reveal and is one of the reasons a splay is quoted separately. The board is best fixed to noggins set at the splay angle rather than bent, because plasterboard held at an angle by nothing more than its own screws waves visibly under a skim in raking light, and raking light is exactly what a rooflight produces. Where the reveal is deeper than 300mm it is worth using two layers of 12.5mm board on the head lining, because that face is the one people look at directly from the doorway.

The finish, and the colour that does the most work

A reveal is a light reflector before it is a decorative surface. A matt white with a high light reflectance value, somewhere at 85 per cent or above, returns most of what hits it back into the room.

A mid-grey or a strong colour on the reveal faces absorbs a large share of the daylight the splay was built to collect, which is a strange decision after paying for the splay. Where a coloured reveal is genuinely wanted, put it on the head lining only and keep the two splayed sides white, since the sides are doing the work. A matt finish scatters light evenly; a silk or eggshell throws a visible bright band across the reveal on sunny days.

Blinds, and the clearance a splay has to leave

Most manufactured roof windows take a blind fixed to the frame or to a pair of side channels, and those channels need a clear run from head to cill.

A splay that starts too close to the frame fouls the channel or the blind cassette, and the usual outcome is a blind that has to be surface mounted below the reveal instead, which looks like an afterthought. The rule is straightforward: the reveal linings stop short of the frame by whatever the blind system needs, typically 30 to 50mm, and the splay begins from that line. Deciding the blind at the same time as the reveal, rather than a year later, is what keeps the two compatible. Fitting choices for blinds on particular units are covered on the VELUX window installation page.

When a square reveal is still the right answer

Splaying is not automatic, and there are rooms where it is the wrong move.

Where the window sits directly over a knee wall in a loft conversion, the lower splay has nowhere to go and a square cill reads more cleanly. Where two units sit close together on one slope, splaying both can leave a plaster fin between them so narrow it cannot be finished properly, and a square pair with a proper pier between them is better. In a room with a strongly geometric ceiling, a very deep splay can dominate. And on a listed roof, a conservation officer may take a view about the internal reveal, since listing covers the inside as well as the outside. Those constraints are worth raising early rather than at boarding stage.

Gloucestershire roofs where the reveal decides the room

Three situations in this county turn the reveal from a finishing detail into the main event.

On a Cotswold stone roof through Painswick, Bibury or Northleach, the construction is deep and the permitted glazed area is modest, because a conservation officer will accept a small flush unit and not a large one. A 350mm deep reveal, fully splayed, is how a modest conservation rooflight lights a room properly, and the depth is described further on the conservation rooflight installation page. On an interwar hipped roof at Longlevens or Charlton Kings, the hip shortens the usable run of slope so the unit is small by necessity, and every millimetre of aperture the splay adds is aperture the roof would not otherwise give. On a trussed estate roof at Bishops Cleeve, where the clear width between webs is 550mm, the splay is the only way to widen anything at all without a designed trimmer.