What happens to the plaster around the opening?
On a like-for-like swap done from outside, usually nothing. The old unit leaves through the roof and the new one comes in the same way, so the plastered reveal is never disturbed and the decoration stays as it is. That is the outcome the sequence is designed to produce. Where the reveal does have to come off, it is cut back to a straight line at the nearest framing member, new plasterboard is fitted, the joints are taped and the whole reveal is re-skimmed to meet the existing plaster, then left to dry before anybody paints it. There are three findings that force that second route and all three are looked for at survey rather than discovered on the day. The wider job is on the replacing a failed or misted rooflight page, and the geometry of a reveal and why its shape matters is at rooflight reveals and why splaying them matters.
Why the reveal usually survives untouched
The plastered reveal is the box between the frame and the room: a vertical lining above the window, a horizontal lining below it, and two sides. It is built inside the structural opening and it stops short of the frame, which sits up in the plane of the covering. That geometry is the reason it can be left alone.
A replacement done from above lifts the covering, removes the flashings, releases the fixing brackets and takes the frame out onto the roof surface. Nothing crosses the reveal at any point. The new frame comes down into the same opening from above, is packed square and fixed through into the same trimmers, and the flashings and covering go back over it. The plaster and the paint never see a tool. Where a fitted wardrobe or a window seat has been scribed up to the reveal in a finished loft room, that is preserved too, which is the whole point of working from the outer face.
The three findings that open it up
The reveal comes off for three reasons, and knowing which one applies before the work starts is what stops the surprise.
- The opening is changing size. A larger or a taller unit means new trimmers in different positions, so the reveal is rebuilt by definition. That is a choice rather than a finding, and it is discussed at when the opening has to change size.
- Insulation is missing or compressed at the perimeter. Flattened or absent for 100mm around the frame is the normal condition of a 1980s or 1990s installation, and it cannot be reinstated through 12.5mm of plasterboard.
- The board itself has been wet. Plasterboard that has taken water goes soft, the paper face delaminates from the gypsum core and the skim loses its key. It does not dry back to a sound board.
The survey looks for all three: a moisture meter into the reveal corners, a knuckle rapped along the linings listening for the dull note of a soft board, and a small probe or a borescope through an unobtrusive hole to see whether insulation actually reaches the frame rather than assuming it does.
What a dark mark in the top corner is telling you
Before anybody touches the plaster it is worth reading it, because the position of a mark says where it came from. A grey or black bloom in the two top corners of the reveal, worst in February and better by May, is a cold bridge. Insulation stops short of the frame, that strip of construction runs cold, room air condenses on the cold surface behind the paint and mould grows on the condensate.
A brown or tea-coloured stain with a defined tide line, worse after rain from one direction, is water that crossed the covering and arrived at the head trimmer, which is a weathering matter at the underlay or the head flashing. A general dampness across the whole lining with no defined edge is more often room humidity meeting an uninsulated box. The three want different answers and painting over any of them changes nothing. Telling them apart is set out at telling a failed unit from a failed installation.
Cutting back to sound board, and where the cut lands
Where the reveal comes off, it comes off properly. The cut is made along a straight line on the framing member behind the board, not at the edge of the damage, because a board edge with no timber behind it has nothing to screw into and no support for the joint. On a reveal that usually means taking the whole lining out rather than a portion of it, since the linings are small and the framing is at the corners.
Board is cut with a knife scored through the paper and snapped, then the back paper cut, rather than sawn, which keeps the dust down in an occupied room. Fixings come out rather than being punched through. Where the old board was fixed with nails, which was normal into the 1990s, the nails are pulled and the timber checked for splitting where they went in. And the exposed framing is measured for square before new board goes back, because a reveal built off out-of-square timber will show it in the light along the head.
What goes back in, and which board goes where
The board specification depends on the room and it is not a detail worth economising on above head height. Standard 12.5mm tapered edge plasterboard covers most reveals in a bedroom, landing or living room. Moisture resistant board goes into a bathroom, a shower room or a utility, and on a rooflight above a shower it is the only sensible choice given what the air in that room does.
Where the reveal is open anyway, insulated laminate board earns its place: plasterboard bonded to a rigid insulation backing, which turns the lining itself into part of the thermal envelope and takes the cold bridge out of the corner permanently. It is thicker, so the reveal reads slightly deeper, and that is a fair trade for a corner that stops going black. Behind whichever board is used, the insulation is reinstated tight and continuous to the frame with no voids, and the vapour control layer is taped to the frame rather than trimmed short of it. That detail is at vapour control layers sealed to the frame.
Skim, feather and the joint you can see in raking light
The finish is where a reveal is judged, because a rooflight throws light down the face of its own linings at a low angle and low-angle light shows every flat spot. Joints between new and existing board are scrimmed with self-adhesive tape or paper tape bedded in jointing compound, external corners take a metal or paper-faced angle bead, and the internal angle between the reveal and the ceiling is worked with a corner trowel.
Where only part of a reveal has been renewed, the sensible route is to skim the whole lining rather than to blend into an old surface, because a two-coat skim across the full face gives one continuous plane while a local blend gives a shadow line that appears every sunny morning for the rest of the room’s life. The existing surface is prepared for that: gloss or heavy vinyl paint is abraded, and a bonding agent is applied so the new skim keys to it. That is half a day rather than an hour, and it is the difference between a reveal that looks original and one that looks attended to.
Lath and plaster, which is a different job entirely
Older property in Gloucester’s Victorian terraces, central Cheltenham, the Cotswold villages and the Forest of Dean frequently has lath and plaster rather than plasterboard: riven timber laths nailed across the framing with a lime and hair coat pushed through them to form nibs, then a finish coat over that.
Working around it wants a different hand. Lath and plaster is brittle and the vibration of cutting nearby detaches the nibs from behind, so the plaster comes away in sheets well outside the area anybody intended to disturb. Where a reveal in an older house has to be opened, the cut is made with a fine blade rather than a heavy tool, the surrounding area is supported while it is done, and the finish is made in a lime-based plaster where the surrounding fabric is lime, because a hard band of gypsum set into a soft lime wall cracks along its edge within a season. On a listed building the internal finish is part of what the listing protects, so the approach to the reveal goes into the consent conversation rather than being decided on the day. That side is at conservation areas and listed buildings in Gloucestershire.
Drying, cracking and why decoration waits
Fresh plaster holds a great deal of water and it has to leave before paint goes on. A two-coat skim on a reveal wants somewhere between four and seven days in a normally heated and ventilated room, and longer in a cold unheated loft room in January. The surface goes from dark chocolate through patchy to a uniform pale pink, and the uniform pale colour is the signal rather than the calendar.
The first coat onto new plaster is a mist coat: emulsion thinned with water so it soaks into the surface and forms a key, rather than a full coat which sits on top and peels. Hairline cracking at the junction between new and old is normal as the skim shrinks and it is filled before the topcoats go on rather than being taken as a fault. Rushing this is the commonest way a good reveal ends up looking poor, and the honest position is that decoration is a separate visit a week later.
Dust, protection and how the room is left
Plaster work generates fine dust that travels, and a rooflight reveal is above head height so gravity is working against the room. Floors are covered and taped at the edges, furniture is moved out or sheeted, doorways to the rest of the house are screened, and where there is a loft void the hatch is sealed for the duration. Cutting is done with dust extraction on the tool where the tool takes it.
What is handed back is a reveal that is boarded, insulated, skimmed and ready for decoration, with the debris removed and the floor left clean. Whether the decoration itself is included is set out on the quotation rather than assumed, and it is one of the items worth checking on any figure you are given, as described at what a replacement quotation should contain. To find out which route your own reveal is likely to take, email info@severnskylightsgloucester.co.uk with a photograph of the reveal from inside, close enough to show the top corners, and the address. The general approach is on the skylight and rooflight installation page.
Next on this subject.
- Question Do rooflights need planning permission? Usually not. Putting a rooflight into the roof of a house is normally permitted development, which means the national rules…
- Question Do I need building regulations approval for a rooflight? Yes. Forming a new opening in a roof is notifiable work under the Building Regulations, and it is notifiable whether…
- Question Who notifies building control about a rooflight? In law the duty sits with the person having the work carried out, which means the homeowner, and in practice…
- Question How much does a rooflight cost? There is no single figure, and any site that publishes one is quoting for a job it has not seen.…