Answered · Skylight & Rooflight Installation & Replacement Chelmsford

What happens to the plaster around the opening?

The short answer

Some of it comes off, and how much depends almost entirely on whether the new frame sits in the same place as the old one. A like for like unit going back into the same opening usually disturbs a band of plaster forty to eighty millimetres wide around the reveal, which is filled, taped and skimmed. A change of size or type takes the whole lining down and rebuilds it.

This is the part of the job householders care about most and quotes describe least, because it is the bit that is visible from the sofa afterwards. This page sets out what is disturbed, who does what, how long the drying takes and where the opportunity in it sits. It runs alongside the wider skylight replacement work Sunspire carries out across Chelmsford and the thirty miles around it.

What actually gets disturbed inside

Three things hold the internal finish together at a rooflight: the plasterboard or lath lining the reveal, the plaster skim over it, and the bead or trim where that finish meets the frame. The frame comes out through all three.

Even where the new unit is dimensionally identical, the fixings, the packers and the insulation around the frame have to be got at, so the finish breaks back to the lining board. On an older unit the trim is often bedded into the plaster rather than fixed over it, and that band comes away in one piece as the frame is released. It looks worse than it is, and it is entirely routine.

Where the old lining meets the new frame

The junction is the whole question. Frame depths have changed across generations, and a modern roof window sash sits differently in the opening from one made in the nineteen eighties even where the outer frame size matches.

That means the point at which the plaster meets the frame moves by ten or twenty millimetres, which is enough that the old edge cannot simply be picked up. The lining is either packed out or cut back to suit, and a new bead is set to the new line. Where the frame has moved outward, a strip of new board goes in. Where it has moved inward, the old board is trimmed and the joint taped.

Plasterboard, and the older stock that is not plasterboard

How the reveal behaves when it is broken back depends on what it is made of, and Essex housing gives you three answers.

Plasterboard and skim, on anything from the nineteen sixties onwards. Cuts cleanly with a knife, takes tape and a two coat skim, and is the easy case.
Lath and plaster, in Victorian and Edwardian rear additions across Chelmsford, Colchester and Maldon. Brittle, keyed on nibs behind the lath, and it fractures well beyond where you cut. Expect the disturbance to run further and plan for a wider made good area.
Board on battens with a sand and cement float, common in interwar semis. Heavy, sound, and it comes off in slabs rather than crumbs.

Skim, tape and the joint that shows later

A made good reveal is only as good as the joint between old plaster and new. Get it wrong and it does not show on handover day, it shows in about eight months as a hairline crack tracking round the reveal.

The method that avoids it is to cut the old plaster back to a clean square edge rather than feathering into loose material, fix new board tight with no more than a three millimetre gap, scrim tape every joint including the corner between the reveal and the ceiling, and skim in two coats over the whole reveal face rather than trying to blend into the middle of a flat surface. A skim that stops halfway across a ceiling will always be visible under a rooflight, because the light rakes across it.

Taking the chance to splay a square reveal

This is the opportunity in the whole business and it is regularly missed. A square reveal, with the lining running straight up to the unit at ninety degrees to the ceiling, throws a hard edged rectangle of light onto the floor and leaves the ceiling around it dark.

A splayed reveal opens outward towards the room, usually with the head splayed hard and the sill left vertical or nearly so. It spreads light across the ceiling and roughly doubles the apparent size of the opening. On a deep roof build-up the difference is dramatic. Since the lining is coming off anyway and the plasterer is already booked, the extra cost is a few boards and half a day, and it is the cheapest improvement available on the entire job.

Who does the making good, and when

This is the single most common gap between two quotes for the same work, so it belongs in writing. A quote that says supply and fit and nothing else has usually left the reveal to somebody else.

Sunspire’s quotes state who forms the lining, who tapes and skims it, and where the finish stops. The sequence on site is that the frame is set and weathered on day one, the lining is boarded once the frame is packed and insulated, and the skim goes on after. Decoration is left to the householder, because a fresh skim wants time before paint goes near it and nobody should be brushing emulsion onto damp plaster to finish a job on schedule.

Drying times, and the wait before painting

New plaster is wet and it wants to dry from the inside out. A two coat skim on board in a normally heated room is touch dry in a day and ready for a mist coat in about a week. On a solid backing, or in a cold room in February, two weeks is more realistic.

The tell is colour. Wet plaster is dark brown, and it goes uniformly pale pink as it dries. Patches of darker colour mean moisture is still coming out and paint over them will trap it and flake. The first coat is always a thinned mist coat of matt emulsion, because plaster is porous and full strength paint sits on the surface and peels off in sheets.

Dust, coverings and the state of the room

Plaster dust is fine, it travels, and it gets into everything soft in the room. The protection that actually works is a taped polythene screen across the opening before the ceiling is touched, floor coverings out to two metres beyond the opening, and soft furnishings either removed or fully sheeted rather than draped.

Old plaster dust is also worth respecting on its own terms. Lath and plaster ceilings in Victorian houses carry a century of soot and dust above them, and a rear addition that once had a coal fire will drop black material the moment the lining is broken. That is a reason to screen properly, not a reason to worry.

Textured ceilings and the caution they need

Plenty of Essex ceilings from the nineteen sixties to the mid eighties carry a textured coating, and coatings of that period may contain a small percentage of chrysotile asbestos. That changes how the material is handled, not whether the job can be done.

Where a textured finish of that age runs up to a rooflight opening, it is either tested first or treated as if it contains asbestos, which means the disturbance is minimised, the material is not sanded or dry scraped, and it is removed and disposed of under the appropriate procedure. It is worth flagging at survey rather than discovering with a scraper in hand, because it changes both the method and the making good specification for that ceiling.

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