Flat rooflight reveal: lining, insulating and splaying the shaft

The reveal is the inside face of the opening, from the underside of the glass unit to the plasterboard ceiling. Designers draw the glass and the upstand. Nobody draws the shaft, so it is decided late, often by whoever is holding the boards.

A flat roof is built up in layers. Joists at the bottom, then insulation, then the deck and the waterproof layer, and finally the upstand that lifts the glazing clear of the surface. The opening cuts through every one of those layers. A skylight light well lining has to do two jobs across that whole depth. It has to finish the inside so it looks like part of the room, and it has to keep the insulation line unbroken so that the shaft does not become the coldest surface in the house.

The depth is larger than people expect. A typical extension has a joist of 150 to 200 millimetres, insulation that may add more above or between, and an upstand of 150 millimetres or more on top. The shaft can easily run to 400 millimetres or beyond. On a 1.5 metre rooflight, that is a lot of vertical wall, and all of it is lit only by what bounces down from the glass. Our page on flat roof rooflight upstands covers the outside of this same build, and the hub for flat roof rooflight installation shows where the lining sits in the whole job.

A thermal bridge is a path where heat leaves faster than it should. The shaft around a rooflight is a textbook case, and it is the detail most likely to show as a dark, damp line on a bedroom ceiling in January.

Start with the structure. The upstand is either a timber kerb, a pre-formed insulated kerb or a GRP moulded one. A bare timber kerb conducts heat readily, and a hollow moulded one is only as good as what fills it. Whichever is used, the insulation in the roof should meet the insulation in the upstand, with no gap where the two join. On a warm roof, the insulation sits above the deck and the kerb is wrapped or filled so that the layer runs on up the outside and across behind the lining. On a cold roof, the joist void is ventilated and the detail is more delicate, which is one reason we look hard at whether the roof is insulated as a warm roof before we agree a rooflight position.

Inside the shaft, the lining itself should not be the only thing standing between the warm room and a cold kerb. Rigid insulation board, cut to fit, goes against the structural walls of the opening. A foil faced PIR board is a common choice, taped at the joints. Foam from a can is useful to fill small gaps at the edge, and it is no substitute for board. The aim is a continuous layer from the roof insulation, down the shaft walls, to the ceiling insulation beside the opening.

Part L of the Building Regulations sets limits on heat loss through the building fabric, and it expects thermal bridging at junctions to be controlled. The rooflight unit has its own performance value, and the glass choice is covered in our answer on what glass a flat rooflight should have. The shaft is where the rest of that energy performance is won or lost.

Insulation keeps the lining warm, but the lining also needs a stable, flat, dry backing. The order of the layers, from outside in, is what stops moisture collecting in the wrong place.

Warm, moist room air is always trying to move into colder construction. If it reaches a cold surface inside the wall of the shaft, it condenses, the timber behind takes the water up, and eventually the plaster surface shows it. A vapour control layer on the warm side of the insulation stops that journey. It should lap onto the vapour layer in the ceiling, so that the seal carries on around the opening, and it should be taped at every join.

Then comes the backing. Timber battens, usually treated softwood, are fixed to the structural face of the opening and to the insulation board through to the framing. They give the lining boards something to bite into and they hold the boards true at the angle you chose. Where the boards are splayed, the battens are cut to the same angle, and the corners are packed out so that the finished surface runs flat from top to bottom.

The lining boards themselves are usually plasterboard. A moisture resistant grade is sensible in a kitchen or bathroom, because the shaft sits directly above the steam. Some builds use a fibre cement or a pre-finished liner panel instead, which can be quicker on the day but asks for neater corner detailing. Whichever is chosen, it is fixed to the battens with the heads set slightly below the face, taped and skimmed the same as the ceiling, so the shaft and the ceiling read as one surface.

• Insulation board against the opening, taped at every joint.
• A vapour control layer on the warm side, lapped to the ceiling layer.
• Battens cut to the finished angle of the reveal.
• Moisture resistant plasterboard, taped and skimmed.

A straight shaft throws light onto the floor in a hard patch and leaves the walls dim. Angling the sides out towards the room lets the daylight fan across the ceiling and down the walls instead.

The idea is simple geometry. Light comes through the glass and lands on the shaft walls first. If those walls stand vertical, the light that strikes them bounces back up or straight down, and the ceiling around the opening looks dark by contrast with the bright glass. If the walls lean outwards as they come down, they act like a wide funnel, and the bright surface now faces the room. The eye reads the opening as larger, the contrast softens, and the glare near the glass eases.

A splay of around 45 degrees is a common starting point. That means the opening at the ceiling is wider than the opening at the glass by roughly the depth of the shaft on each side. A shallow shaft can take the full angle. A deep one may need a gentler splay, because a steep angle on a long drop would push the ceiling opening out very wide and eat into the flat ceiling band around it. A compromise is often a splay on the two long sides and a straighter finish on the short ends, or an angle that changes part way down.

The same thinking applies to the sides that face the window or the brightest wall. A strongly splayed reveal on those sides gives a softer transition to the ceiling, because the light has a gentler surface to land on. Our guide to laying out several flat rooflights on one roof explains how two splayed shafts side by side can overlap their glow, and the answer on lighting a stairwell from a flat roof shows a case where a deep splayed shaft is the whole point.

There is no single right angle. The depth of the shaft and the width of the opening do most of the choosing, and these pairings are starting points we test against the real roof at survey.

The table reads across as a general rule. A very deep shaft, such as one passing through a first floor joist zone and a flat roof build, may want a tapered liner made as a single fitted component, so it is straight and true from top to bottom.

Paint colour and sheen do a great deal for the light. The shaft is the largest bright surface in the room, so it should be treated as a reflector.

A matt white or a very pale warm white on the reveal bounces the light evenly and hides small imperfections in the plaster. A gloss finish looks sharper in a sample, but it throws back hot streaks, shows every trowel mark and turns the reveal into a mirror at certain times of day. A light eggshell can suit a bathroom, where a wipeable finish is useful. Dark colours are a choice for a designed feature, and they cost daylight at the very point where it needs help.

Where the reveal meets the ceiling, the corner is best left crisp or with a small shadow gap. Coving, cornice and architrave all create ledges that catch dust and cast small shadows. If the ceiling has a plaster skim, the reveal is skimmed in the same pass, and the paint is cut in cleanly at the glass, where the frame meets the lining. A neat line against the frame matters because it is the one spot the eye goes to every time the sun is out.

Blinds add another layer of finish. A blind needs a fixing point and often a power or control route, and both are easier to plan before the boards are up. The answer on fitting a blind to a flat rooflight explains the options, and the shaft lining is the place to leave a solid timber block for the bracket.

The junction between the glazing frame and the top of the lining is a small area with a large effect on both draughts and sound. It is worth getting right in the first fix.

The frame of a flat rooflight sits on top of the upstand, and the shaft continues down from the inside face. A gap around that junction is a route for air. The frame should be sealed to the upstand with the compression seals or gaskets made for it, and the inside of the joint is closed with a flexible sealant or an airtight tape before the lining goes on. A rigid fill in a moving joint tends to crack, so a sealant that stays flexible gives a better long term result.

This is also where our quiet approach shows up in the detail. Sound finds the same gaps that air does, and it is carried by hard, thin surfaces. A tight seal at the frame, a dense, well fixed lining board and an insulated upstand all cut down the route for noise to travel into the room from the glass above. Rain drumming on flat glass is partly the pane and partly the frame. A heavier laminated inner pane, which our guide to quieter glazing over a bedroom explains, does the main work, and a well packed reveal stops the shaft acting like a drum skin beneath it. The written quiet spec that comes with every quote records the seals and the upstand finish for your room.

A lined reveal can still show a damp corner if the detail above it is wrong. Condensation is almost always a sign of where the cold is, and the pattern tells you which fix to ask for.

The glass itself is the coldest surface, and a little moisture on the lower edge on a cold morning is ordinary. That is a matter of the room's humidity. Moisture on the reveal walls is another matter. It points to a gap in the insulation line or a missing vapour layer, which lets warm air reach a cold surface behind the board. Moisture in only one corner usually means a void or a compressed section of insulation at that point.

Ventilation helps. Approved Document F sets ventilation rates for kitchens and bathrooms, and an extractor in a humid room reduces the water load on every surface, including the shaft. In a bathroom, the choice of a rooflight at all is worth thinking about, and our answer on whether a flat rooflight suits a bathroom covers the practical points. In a failing or aged unit, condensation inside the glass is a separate sign, and a misted or failed rooflight replacement is the route for renewing the sealed unit.

A shaft is also a void, and voids need a little thought. Anything that passes through the lining, or sits inside it, needs to be planned before the boards close it in.

Cables for a blind motor, a rain sensor or a light fitting need a route and a conduit, and a junction box that stays accessible. Recessed lights inside the reveal give a soft wash down the walls, though the fittings need to be a type suited to an insulated, enclosed space, and they should not break the vapour layer without being sealed to it. A small LED strip set into a timber cove at the top of the shaft can light the reveal at night, which gives the rooflight a second life as a lamp after dark.

Fire performance depends on the building. Part B of the Building Regulations deals with fire spread, and the plasterboard lining on the ceiling side of a roof is part of how the floor or roof meets its required period. Cutting an opening into that means the lining must be put back to the same standard of protection. Where the rooflight is near a boundary, the rules on distance and on the glazing material apply, and the answer on how close to a party wall a flat rooflight can go explains the position. The planning route and the regulations are checked for the address, and where the work is notifiable, we handle the Building Control notification.

The order of work matters more than the speed. A lining fitted out of sequence is the usual reason a neat design ends up with awkward joins and uneven corners.

1. Frame and trim the opening, then fit the upstand and check it is square and level.
2. Insulate the upstand and shaft walls, and lap the roof insulation into them.
3. Close the roof on the outside, so the waterproofing is complete and the shaft is dry.
4. Fit the vapour layer, battens and boards, cut to the chosen splay.
5. Seal the frame junction, tape and skim, then let the plaster dry before painting.

The glazing itself is usually craned or lifted on once the upstand is ready and before the lining is finished, so the room is weathertight while the plaster dries. The whole job follows the pattern in our guide to how a flat roof rooflight is installed, and the lining is the part that happens while you are living in the house. We protect floors, keep the dust to the opening and make good to the ceiling so the room is finished, not just watertight.

These are the practical points that come up when someone is choosing the finish for the shaft.

Can an existing flat rooflight have its reveal splayed afterwards?

Often yes. The lining is ordinary plasterboard on battens, so it can be stripped back to the structure and rebuilt at a new angle, if the ceiling opening and the joists allow it. The work is tidier when it is done with a rooflight replacement, because the shaft is open and the insulation can be corrected at the same time. Our answer on whether a rooflight can go into an existing flat roof covers the roof side of that question.

Does a deeper shaft make the room darker?

A deep, straight shaft does, because the walls soak up light before it reaches the room. Splaying the sides and painting them white recovers much of that. A shallower upstand also helps, within the limits that drainage and waterproofing set, which the answer on how high a flat rooflight upstand should be explains.

What should the reveal be painted?

A matt white or pale warm white, in a finish that can be wiped clean in a kitchen or bathroom. It bounces the light evenly and hides small flaws. Dark colours and gloss both work against the daylight the rooflight was fitted to bring in.

Does lining the reveal add to the cost?

It adds boards, battens, insulation and finishing time, and the amount depends on the shaft depth and the angle. The factors behind a whole flat rooflight quote are set out in what affects the cost of a flat roof rooflight, and a specific figure for your roof comes with the written quote. To start that, get a quote, or message us on WhatsApp.