How a rooflight is finished inside: linings, splays, plaster

The lining is the short tunnel between the rooflight frame and the face of your ceiling. On every rooflight there is a gap between the two, because the frame sits on the roof and the ceiling sits under the rafters, the insulation and, in many houses, a loft space. That gap has four faces: the head at the top of the opening, the sill at the bottom, and two sides, which the trade calls the reveals or cheeks.

Behind those four faces there is more going on than the finished surface suggests. The rafters either side of the opening have been trimmed, with new timbers doubled up to carry the load around the gap. Insulation has to wrap the frame and continue down the lining, or the lining becomes the coldest surface in the room. A vapour control layer has to be lapped onto the frame so warm, damp air from the house can't get into the roof build. Most roof window makers supply a collar for the insulation and another for the vapour barrier, sized to the frame.

Only once that is done does the visible lining go on. It is usually plasterboard fixed to a light timber frame, though factory-made lining panels exist for some roof windows. Either way, the face you see is the last few millimetres of a build that has to be right underneath.

A straight lining runs at right angles to the glass, so the opening in the ceiling is the same size as the frame. A splayed lining angles outwards as it comes down, so the hole in the ceiling is wider than the frame. The usual guidance from roof window makers is to set the head of the lining horizontal and the sill vertical, and to splay the two sides, because that geometry lets more daylight leave the opening rather than hitting the lining and stopping there.

The difference is easy to picture. With a straight lining through a deep roof, daylight comes down a box and lands in a patch under the window, with the lining faces in half shadow. Splay those faces and the angled plaster catches daylight and reflects it on into the room, so the ceiling around the opening looks lit instead of dark. The glass is the same size, and so is the rooflight you paid for. You simply use more of what it lets in.

A splay isn't always the right call. Under a shallow sloping ceiling the lining might only be a hand's width deep, and splaying it gains almost nothing. We go into that trade-off in more detail on what a splayed rooflight lining is.

How deep the lining runs depends on the roof, and that one measurement shapes most of the finishing decisions.

In a loft room, or any room with a sloping ceiling fixed to the rafters, the reveal is short. It is the depth of the rafter plus insulation and board, so often well under 300mm. The lining is quick to form, there is little to gain from a splay at the sides, and the finish is mostly about getting the four edges crisp against the slope.

A flat ceiling under a pitched roof with a cold loft above is another matter. Here the rooflight sits on the roof slope and the ceiling sits a metre or more below it, so the lining becomes a shaft, often called a lightwell, running through the loft. Its walls are now outside the warm envelope of the house on one side, which means they need insulating on the loft face as well as finishing on the room face. Shafts like this are where splaying earns its keep, because a long straight box throws away a good share of the glass. They are also where condensation shows first if the insulation is thin, so the finish has to start with the insulation.

Flat roofs sit between the two. The rooflight stands on an upstand or kerb above the roof deck, so the lining runs up through the joist depth and then up the inside of the kerb. That inner face of the kerb is part of what you see from below, and it needs boarding and finishing like the rest. Cob and stone cottages across the Culm and Exe valleys, and granite houses on the Dartmoor edge, can have irregular roof builds where the depth changes from one side of the opening to the other, so the lining has to be cut to the roof the house actually has rather than to a drawing.

In most houses built since the 1950s the ceilings are plasterboard, and the lining is finished to match. There are two ways to do it, and the right one depends on the ceiling it meets.

Boarded and taped

Plasterboard is fixed to the lining frame, the joints are taped and filled, and the surface is sanded ready to paint. It suits ceilings that were themselves taped and jointed, which is common in houses from the 1980s onwards and in new builds such as those around Cranbrook.

Boarded and skimmed

The board is fixed, the corners are beaded, and a thin coat of finishing plaster, usually 2 to 3mm, is trowelled over the whole lining. A skim gives a harder, flatter surface and blends better into a ceiling that was skimmed originally, which describes plenty of 1930s and postwar semis in Heavitree and the like.

Lime plaster

Older houses, roughly pre-1900, were plastered in lime, often over timber lath, and lime behaves differently from gypsum. It is softer and more open, it lets moisture pass through it, and it moves a little with the building. Gypsum plaster laid directly against old lime can fail at the join, and a hard gypsum patch in a lime ceiling tends to crack around its edge. Where the ceiling is lime, the lining and the making good around it are better done in lime too, built up in coats: a scratch coat keyed into lath or a backing, a float coat, and a thin setting coat. It takes longer because each coat has to firm up before the next goes on, but the finished edge moves with the ceiling rather than against it.

The paint follows the plaster. New gypsum plaster needs a thinned mist coat before the top coats. Lime needs a breathable finish, such as limewash or a mineral or clay paint, because a sealed modern emulsion stops it drying out and can lift.

Georgian and Regency terraces in Southernhay and St Leonards, Victorian villas in Torbay and older town houses across Devon often still have their original lath and plaster ceilings. These are thin strips of timber nailed across the joists, with lime plaster pushed through the gaps so it curls over the back and grips. Those curls are called nibs, and they are what hold the ceiling up.

A rooflight in a lath ceiling is entirely possible, but the cut has to respect how the ceiling works. The area around the opening is supported from below before anything is cut, so the ceiling can't sag while the laths are free. The plaster is scored and cut cleanly rather than broken out, and the laths are cut back to a joist or a new noggin so their ends are fixed. Loose sections near the opening are screwed back with plaster washers. Where the ceiling has old cracks running towards the new opening, those are noted before the work starts, so everyone knows what was there.

The new opening is then framed, and the edge where old ceiling meets new lining is the most delicate join on the job. It is usually built up in lime, sometimes over new lath, and feathered into the existing ceiling so there is no visible ridge. If you are unsure about your own ceiling, our answer on whether a skylight can go in a lath and plaster ceiling covers the questions to ask first.

The edges are what you notice. A lining can be flat and well plastered and still look unfinished if the corners are soft or the line against the frame wanders. There are three edges to think about, and each one has its own detail.

Lining to ceiling

This is the outside corner where the reveal turns into the ceiling. On a straight lining it is a right angle, finished with a metal or plastic angle bead under the skim. On a splayed lining the angle is wider than 90 degrees, so a standard bead won't sit right; a flexible paper-faced bead, or an adjustable angle bead, follows the true angle and gives a sharp arris.

Lining to frame

Most roof windows have a groove or rebate in the inner frame that the lining board slots into. That gives a tight, straight joint with no visible filler. Where there is no groove, a stop bead finishes the plaster just short of the frame, leaving a fine, straight shadow line.

Trims and architraves

Some homes suit a timber trim around the ceiling opening, much like the architrave around a door. It can match period joinery in an older house, or hide a ceiling that is too uneven to plaster to a crisp line. In most rooms a plastered return looks cleaner.

The paint line is the last decision. It is where the painted plaster stops and the frame begins. It should run dead straight along the frame edge, with a thin flexible sealant joint where the materials meet so a hairline crack doesn't open up as the timber and plaster move at different rates. Whether we leave the lining ready for your decorator or paint it ourselves is agreed before the job starts, and written down.

Replacing an old rooflight brings its own finishing work, because the inside was formed for the old frame. If the new frame matches the old opening, the existing lining can often stay. Its edge against the frame is cut back cleanly, the new frame's lining groove or stop bead is used to close the joint, and the old paint line is redone. That keeps disturbance to the ceiling small.

If the new rooflight is a different size, or the old lining is stained, cracked or was never insulated properly, the lining is stripped and formed again. That is also the moment to splay it if it was straight before, or to add the insulation it never had. Old condensation staining at the sill is a common sign that the lining was cold, and simply painting over it brings the stain back.

Around the opening, the ceiling is checked for cracks the old rooflight left behind and for any plaster loosened during removal. That is made good in the material the ceiling is made of, lime or gypsum, and blended in before the room is decorated. Dust sheets and dust screens stay up until the plaster work is done. Our guide to replacing an old skylight covers the roof side of the job, and there is more on protecting the room in replacing a rooflight without damaging the ceiling.

All of these choices are easy to agree in conversation and easy to forget by fitting day. So every Brightview quote comes with a finish schedule, a one-page written note for the room the rooflight is going into. It records:

• the room, and the ceiling and roof build above it, for example lath and plaster under slate, or plasterboard under concrete tile;
• how deep the lining runs, and whether it will be straight or splayed, and why for that roof;
• how the lining is finished: boarded and taped, plasterboard and skim, or lime plaster;
• which beads or trims are used at each edge, and where the paint line sits;
• whether the surface is handed over bare for your decorator or already decorated, and in what kind of paint;
• how dust is managed and the room protected while the work is done.

With the skylight reveal finish written down, you can check it against the finished room. Read it before you agree the quote, ask about anything that doesn't match how you picture the room, and keep it for when the work is done.

The rest of the job, from choosing the right rooflight to the Building Regulations side, is on our skylight and rooflight installation homepage. When you're ready, ask for a quote and a finish schedule for your room.