Guide · Roof Window Installation
Rooflight reveals and why splaying them matters
The reveal is the lined shaft between the ceiling of the room and the underside of the glass, and it is the cheapest improvement available on the whole job. Splaying it, so the lining opens outwards towards the room rather than running square, spreads daylight across the ceiling instead of dropping it in a hard rectangle on the floor. It costs a little more plastering and nothing at all in glass.
It is also the part most often left to whoever turns up last. On a great many otherwise competent installations the unit is correct, the flashing is correct, and the internal lining is a plain box that gives away a third of the benefit. This page covers the geometry, the angles that are worth using, and the detailing that keeps a reveal crack free and dry. It applies to pitched roof windows and to flat roof units alike, though the two produce different shapes.
Where the reveal sits in the build-up
Between the room and the sky there is more material than people picture. The frame of the unit sits in the roof structure. Below it are the rafters or joists, insulation, a service void in some cases, and finally the plasterboard ceiling. The reveal is the lining that closes off all of that around the perimeter of the opening.
Its depth is therefore set by the roof, not by choice. A modern warm roof build-up with 150mm of insulation between and below the rafters gives a reveal 250mm deep or more. An older shallow roof might give 150mm. The deeper the reveal, the more the splay matters, because a deep square shaft acts as a light baffle.
The square reveal, and what it does to daylight
A square reveal has four vertical sides running straight up from the ceiling to the frame. Light entering the unit can only leave the shaft within the solid angle the shaft permits, so the beam that arrives in the room has hard edges and covers roughly the plan area of the opening.
The result is a bright rectangle on the floor and a dark ceiling around it, which reads as a contrast problem rather than a brightness problem. The eye adapts to the bright area of floor, the surrounding ceiling then looks gloomier than it did before the window went in, and the room feels less evenly lit than the daylight measurement suggests it should.
The splay, and the geometry behind it
Splaying opens the reveal outwards, so the aperture at ceiling level is larger than the aperture at the glass. Two things happen at once.
The first is straightforward: a wider opening at ceiling level passes a wider cone of light, so the beam spreads further into the room and reaches the corners. The second matters more. The splayed surfaces themselves are lit, and being pale and angled they act as reflectors, bouncing light onto the ceiling around the opening. The transition from the bright aperture to the surrounding ceiling becomes gradual rather than abrupt, and the room reads as evenly daylit rather than spotlit.
A splayed reveal makes the opening look roughly twice the size for the price of a bag of plaster.
The standard detail: vertical below, horizontal above
On a pitched slope the conventional reveal is not splayed equally on all four sides. The usual detail runs the lower lining vertically, plumb down from the sill of the unit to the ceiling, and the upper lining horizontally, level from the head of the unit into the room.
There is a reason for each. The vertical lower lining lets a person standing back in the room see out and down through the glass, which preserves the view of the garden rather than only sky. The horizontal upper lining throws light back across the ceiling and prevents the head of the opening cutting off the sky at a steep angle. The two sides of the reveal are then splayed, and the whole thing forms a shape that is neither a box nor a funnel but does both jobs.
Why that shape beats a square box on a pitched roof
Consider the geometry of a unit at 35 degrees. A square reveal on a 35 degree slope produces a parallelogram of light aimed at the wall beneath the window, which is the least useful direction: that wall is usually the one under the eaves and often the lowest part of the room.
The vertical and horizontal detail redirects it. The vertical lining below aims light out into the room rather than at the wall under it, and the horizontal lining above catches the sky higher up the slope. Rooms in the roof benefit most from this because the ceiling geometry is already working against them, a subject covered on rooflights for a loft conversion.
How much light a splay actually adds
The honest answer is that it adds less measured illuminance than people assume and more perceived brightness than any number captures. The glass area has not changed, so the total light entering the room is the same. What changes is where it lands.
The perceptual gain comes from removing the contrast between a small bright zone and a large dark one. The eye judges a room by the brightest and darkest surfaces it can see at once, and a lit ceiling around the opening lifts the whole space. The change is most dramatic in a deep reveal, in a small room, and where the ceiling is white.
Splay angles, and how far it is worth going
There is a practical range. Below about 15 degrees off vertical, the splay reads as a mistake rather than an intention: it looks like a wonky box. Beyond about 45 degrees the reveal starts eating floor plan and the plasterwork becomes awkward to form and to paint.
| Splay from vertical | Reads as | Where it suits |
|---|---|---|
| 0 degrees, square | A shaft | Shallow build-ups under 150mm |
| 15 to 25 degrees | A soft opening | Most rooms, most build-ups |
| 30 to 45 degrees | An architectural funnel | Deep reveals, vaulted ceilings, single large unit |
| Over 45 degrees | A light scoop | Very deep build-ups, designed as a feature |
For a typical 250mm reveal, something in the region of 25 degrees on each side gains around 230mm of aperture width at ceiling level. That is a meaningful change in a 1140mm wide unit.
Splaying all four sides
On a flat roof unit, where there is no slope to work with, splaying all four sides symmetrically is the usual approach and it produces a shape that reads deliberately. Light comes down and outwards evenly in every direction, and the reveal becomes part of the ceiling design rather than a hole in it.
On a pitched slope the four-sided splay is less common because it fights the vertical and horizontal detail described above, but it is used where the unit is high on the slope and the view out matters less than the light. The decision is worth making at design stage because it changes the plasterboard cutting, and it is not something to be settled by a plasterer on the day.
Insulating the reveal, and the cold line at the frame
The reveal is the point where the insulation line has to arrive at the frame of the unit without a break. If the insulation stops short and the plasterboard carries on to meet the frame, there is a thermal bridge running right round the perimeter of the opening.
The symptom is predictable. In cold weather the internal lining goes cold near the frame, warm moist air from the room condenses on it, and a damp line appears in the corner of the reveal. It is read as a leaking window and it is nothing of the sort. Manufacturers supply insulation collars that wrap the frame for exactly this reason, and where a collar is not used the insulation is cut and fitted tight to the frame instead.
Vapour control, and the layer behind the plaster
A reveal punches through the ceiling, which on a warm roof is also the vapour control layer. Every service, fixing and joint around the opening is a potential route for moist room air to reach cold structure above.
The membrane is therefore taped to the frame around the full perimeter, not simply cut and left. Where a vapour control layer is discontinuous around a rooflight, moisture accumulates in the roof void over successive winters and shows up as staining on the reveal or damp insulation. It is invisible work behind the plasterboard, and it is one of the reasons the reveal is not a decorating job.
Ventilating the void around the shaft
Where the reveal passes through a cold roof void, as it does on a flat ceiling with a loft above, the shaft is a chimney standing in unheated space. The void around it still needs its ventilation path to work, and a large lined shaft dropped into the middle of a roof can block the cross flow between one bay and the next.
The fix is straightforward when it is anticipated: the ventilation path is maintained around or over the shaft. When it is not anticipated, the result is a dead pocket of humid air on one side of the reveal, which is where condensation appears first. It is worth raising before the ceiling is closed, because afterwards the only access is through it.
Plasterboard, beads and the cracks that appear at twelve months
Reveals crack at the corners more than any other plastered detail in a house, for two reasons. The angles are not right angles, so standard beads have to be worked, and the reveal is fixed to timber that moves with the roof while the ceiling is fixed to timber that moves less.
Good practice deals with both. The boards are cut so joints do not land in the corner of the opening. Angle beads are used at every splayed arris, and where a bead has to sit at an unusual angle a flexible bead is used rather than a standard one forced into shape. Around the frame itself, a movement joint, either a shadow gap or a sealed joint behind a bead, gives the two materials somewhere to move against each other. A hard plaster butt to a timber frame will crack, and it will crack in the first winter.
Finish, colour and how much the surface matters
The reveal is doing optical work, so its finish is a functional decision. A matt white surface with a high light reflectance value bounces the most light and does it without glare. A dark or strongly coloured reveal absorbs a large fraction of what hits it and undoes much of the benefit of splaying it in the first place.
Sheen matters too. A high gloss reveal produces a bright reflected image of the sky on one face, which is a glare source at eye level rather than useful ambient light. Matt or eggshell is the sensible choice. Where somebody wants a coloured reveal for design reasons, keeping the two splayed sides pale and colouring only the head or the deep face preserves most of the effect.
Where the reveal meets a wall or a ceiling junction
A reveal landing within a metre of a wall creates a decision. Running the splay right into the wall face, so that the lining and the wall become one surface, throws light down the wall and can look deliberate and generous. Stopping it short leaves a narrow strip of ceiling between the reveal and the wall, which reads as an error even when it is dimensionally correct.
The strip is the thing to avoid. Where a unit sits close to a wall, the choice is to run the reveal into it or to move the unit far enough away that the ceiling between them reads as ceiling. Anything under about 300mm looks accidental from below.
Reveals on a flat roof unit, and the kerb below
A flat roof rooflight sits on a kerb, so the internal shaft has a different shape: usually deeper, always rectangular in plan, and with the four sides equal. The splay is therefore symmetrical, and the depth is often greater because the kerb height is added to the roof build-up.
The greater depth makes splaying more valuable rather than less. A 400mm deep square shaft on a flat roof rooflight is a serious light baffle, and opening it out at 30 degrees changes the room noticeably. The same insulation and vapour control rules apply, with the addition that the kerb itself is insulated on its full height rather than only at the top.
Reveals with more than one unit
Where two units sit side by side, the strip of ceiling between them is part of the reveal design rather than leftover space. A narrow strip splayed on both sides forms a ridge between two funnels, which reads well and is very hard to plaster.
The usual resolution is either to widen the gap so each unit gets its own complete reveal, or to close it up and line the pair as a single opening with a plain flat soffit between them. The choice interacts with the flashing kit, since combination flashings fix the external spacing, and that relationship is covered on two rooflights side by side on one slope.
Sequencing, and who forms the lining
The reveal is the last operation and it is a wet trade, so it lands after the unit is in and weathered. That gap in the programme is where responsibility gets lost, because the installer has finished and the decorator has not started.
The way to prevent that is to settle it on the quote. The document should say who forms the lining, who boards it, who plasters it, and whether the splay angle has been agreed. A quote that stops at supply and fit of the window leaves the single most visible part of the finished job to a conversation on the day, and that conversation almost always ends with a square reveal because it is quicker.
What to ask for when the reveal is specified
Four things settle it. The reveal depth, measured as the finished ceiling to the underside of the frame. The splay angle on each of the four sides, stated in degrees or as a dimension at ceiling level. The junction detail at the frame, shadow gap or bead. And the finish, which for daylight purposes wants to be matt and pale.
Agreed before the ceiling is boarded, all four cost very little. Agreed afterwards, every one of them means taking plaster off again. The reveal is the part of the installation the household looks at every day for twenty years, and it is worth ten minutes at design stage.
Keep reading
More on roof window installation
Rooflights in a trussed roof and what cannot be cut
Read the guide →Soakers, aprons and head flashings explained
Read the guide →Two rooflights side by side on one slope
Read the guide →Rooflights on a shallow pitch below 15 degrees
Read the guide →Underlay and sarking around a pitched rooflight
Read the guide →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements