Replacing a rooflight without damaging the ceiling

A rooflight that has been in place for twenty years has become part of the ceiling. The plaster has been run up to it, painted against it and, in many houses, pressed or screwed onto the very timbers the frame is fixed to. Pull the frame and you can pull the plaster with it.

Three things are worth understanding before the first screw comes out.

The frame is fixed to the structure, the lining is fixed to the frame. On a roof window, the lining board usually slots into a groove on the inner face of the frame, and the plaster or plasterboard is fixed to a small timber carcass behind it. Remove the frame carelessly and the carcass shifts, and the joint between lining and ceiling opens up.

Old sealant and paint bridge the joint

A skin of paint and mastic often runs from the frame across onto the ceiling. If it is cut cleanly with a knife before the frame moves, the ceiling keeps its edge. If it isn't, it tears a strip of paint and sometimes the top layer of plaster along with it.

Insulation and vapour control are doing a job

The collar of insulation around the frame and the vapour layer lapped onto it keep warm, damp room air out of the roof build. Disturb them and put them back badly, and the lining turns cold, then damp, then stained.

Everything else depends on whether the new rooflight is the same size as the old one. A like for like swap is the lowest risk to the ceiling. A larger or differently shaped rooflight is a new opening, and a new opening in a ceiling is a different job.

Whether the new unit can use the old opening at all is a question for the survey. Frame sizes change between generations of roof window, and an opening cut for a unit from the 1990s may not suit a modern frame without small adjustments. If you are weighing that up, will a new rooflight fit the old opening goes through what has to line up.

A flat roof rooflight on a kerb behaves differently again. The old kerb may be reused or replaced, and the inner face of the kerb is part of the lining you see from below, so a new kerb means new finishing on the inside. A roof lantern sits on its own upstand and brings its own internal reveal. The same principle holds for both: find out what stays and what changes before the day, not during it.

A good survey for a replacement includes looking up, not just out. Someone needs to stand under the rooflight with a torch and a notebook, and check what the ceiling is telling you.

• What the ceiling is made of: plasterboard, gypsum plaster on lath, lime plaster on lath, or something older still.
• Existing cracks near the opening, and whether they were there before the work. These are noted and photographed so there is no argument later.
• Stains at the sill or head, which point to a cold lining or old condensation.
• Whether the lining is plasterboard on a frame, a factory panel or a plastered shaft.
• How much of the ceiling around the opening is loose, hollow when tapped or flaking.
• What is above: loft space, a floor, or a sloping ceiling fixed straight to the rafters.

All of that is put in writing with the quote, so the plan names the ceiling, the lining and the finish before a ladder goes up.

The room is part of the job, and it is prepared before the roof is opened. Furniture is moved or covered. Floors are protected with a proper layer, not a thin dust sheet that a boot will drag aside. A dust screen goes up across the doorway or around the work area, and a ladder or scaffold tower is set on boards where it will stand on a finished floor.

Dust is the part people underestimate. Cutting back an old plaster edge and lifting a lining throws fine grit into the air, and it settles on shelves, in curtains and on the tops of door frames. A screen, a vacuum used as the work goes, and a final clean of the room at the end are what separate a tidy replacement from one you are still finding grit from months later.

Weather matters too. The roof covering is lifted only as far as the new unit needs, and the opening is closed in before anything is left overnight. For timing, can you replace a rooflight in winter covers what changes with the season.

The order of removal is what protects the plaster. Done in the wrong sequence, the same old rooflight does a good deal more harm than it needs to.

1. The sash or glazing is removed first where the design allows, so the frame is lighter and easier to control.
2. Internal paint and sealant lines are scored with a sharp blade along the frame and the lining, so any tearing stops at the cut.
3. Exterior flashing, tile or slate and the roof side fixings are released and set aside, so the frame is free from above.
4. From inside, the frame is supported while the last fixings go, rather than dropping or levering.
5. Insulation collars and the vapour layer are lifted carefully, kept if sound, and noted if not.

The frame comes out, in most cases, upwards through the roof. It is not hammered out sideways through the ceiling, and the lining is not used as a lever. If the lining is staying, it is supported with a temporary batten so its joint to the ceiling does not shift. If it is going, it is taken out in sections rather than snapped, because the plaster edge at the ceiling is what you are trying to protect.

Where cutting cannot be avoided, a straight edge and a fine blade or oscillating saw give a cleaner line than a lump hammer. The cut lands on a joist or noggin so the new edge has solid timber behind it.

The ceiling decides how careful the work needs to be and what it is made good with. The three common types in Devon homes behave very differently.

Plasterboard ceilings

These are in most houses built from the 1950s onwards, and in extensions and conversions of any age. They cut cleanly, and the damage you want to avoid is the board tearing at its paper face or the joint tape lifting. Board is fixed back to timber with screws, joints are taped and filled, and the edge is skimmed or sanded to match. It is the forgiving case.

Gypsum plaster on lath or board

Common in interwar and postwar houses, such as the 1930s semis around Heavitree. The plaster is hard and brittle, and it cracks along lines of stress, so a cut or a vibration can send a crack across a metre of ceiling. The answer is to support the area before cutting, cut on a line, and to bridge the new edge with scrim or tape before skimming.

Lime plaster on timber lath

Found in Georgian and Regency terraces in Southernhay and St Leonards, in Victorian villas, and in older town houses across the county. The lath is held by plaster nibs on its back, and those nibs are the ceiling's grip. Jolting the structure can snap them and let a section sag. For old ceilings, the area is held up from below before any cutting, the opening is cut back to a joist, and loose lath is screwed back with plaster washers. The new edge is built up in lime and feathered into the existing ceiling.

Cob cottages in the Culm and Exe valleys add thick walls and uneven rafters to the mix, so the opening has to follow the roof as it actually is. If your ceiling is old, can a skylight go in a lath and plaster ceiling sets out what is worth asking at the start.

The test of a replacement is the join between old ceiling and new lining. You should be able to stand in the room and not locate it.

That starts with the substrate. The lining is formed square and solid, with insulation behind it and a board face fixed to timber that will not move. The edge of the existing ceiling is cleaned back to sound material. Loose plaster is cut away to a firm line, not left to fail later under the new coat.

Then the material follows the ceiling. In a plasterboard ceiling the join is taped and skimmed. In a gypsum ceiling the new plaster is laid in thin coats and feathered out past the old edge. In lime, the coats are built up in the traditional order, each left to firm before the next, because a lime edge that is rushed will craze. A hard gypsum skim laid against soft lime tends to crack at its margin, which is why the material matters as much as the technique.

Beads and edges come next. A metal or paper-faced bead gives a sharp arris where the lining meets the ceiling, and a stop bead or a groove in the frame gives a tight, straight line against the window. For how those details are chosen and why they matter, how a rooflight is finished inside has the full set.

New plaster needs time. Gypsum skim dries in days, and needs a thinned mist coat before the top coats go on, or the paint soaks in unevenly and shows as dull patches. Lime takes longer, and wants a breathable finish such as limewash or a mineral paint, because a sealed emulsion can stop it drying out and begin to lift.

Because a strip of fresh paint on a ceiling shows up against older paint, it is often worth decorating the whole ceiling face, not just the new edge. This is a decision for you to make with the quote in front of you. The finish schedule records whether the lining is left ready for your own decorator or painted by us, and what paint is used, so the room is not left half done.

The paint line against the frame is the final detail. A thin flexible sealant joint where plaster meets timber lets the two move at their own rates without a hairline crack opening. It should run straight enough that your eye reads it as a shadow, not a seam.

Taking out the whole rooflight and fitting a new frame and glass is notifiable building work. Windows are controlled fittings, so the new unit has to meet Part L of the Building Regulations, and the replacement is notified to Building Control or self-certified by an installer registered with a competent person scheme. That notification is ours to make. A new sealed glass unit alone, in a frame that stays, is a different case with nothing to notify, and you can read about that route in replacing just the glass in a rooflight.

Where a new, larger opening means trimming rafters, that is a structural alteration and needs to be dealt with at the outset. In a conservation area or on a listed building, the external appearance of the new unit may need consent, so put the question to the council's planning team before the order is placed.

All of this goes into the quote, and the quote comes with the finish schedule. By the time the first tile is lifted, you have in writing which ceiling you have, what is being kept and what is being formed again, how the edge is being made good, and how the room is being protected.

If your rooflight is misted, stained or failing and you are working out what the replacement involves, the misted and failed rooflight replacement page covers the whole route, from choosing between a new glass unit and a whole new frame through to what happens on the day. Signs a rooflight needs replacing is a useful starting point if you are not yet sure which one applies.

When you are ready, ask for a quote. We will look at your ceiling as well as your roof, and the finish schedule will say, in plain words, how the room will be left.