Can a skylight go in a lath and plaster ceiling?

A lath and plaster ceiling is built from thin timber strips nailed across the joists, with lime-based plaster pressed through the gaps to form keys on the back. The keys hold the whole ceiling up. Over a century or two, they dry, crack and break away in places, so an old ceiling is often more fragile than it looks from below.

Cutting through one sends vibration across the surface. A saw or a hammer on the wrong spot can drop a patch of ceiling several feet from the opening. That is why the method matters more than the tool: the plaster is supported first, then cut.

Plasterboard behaves differently. It is a sheet, so it can be cut to a line with a clean edge and little disturbance. A lath ceiling has to be dealt with section by section.

The work usually starts from above, where the roof is opened and the position of the new unit is marked through to the room. The ceiling is then cut from below, not knocked through from above. Before the cut, the plaster around the line is held up so it can't sag, often with plywood or battens screwed up to the joists. The lath is then cut along the line with a fine blade, with the plaster face supported throughout, and the edges are secured to the new framing so nothing is left unsupported.

Where the ceiling has already lost its keys, or the plaster has cracked around the opening, the loose areas are made secure or replaced before the lining goes on. If a hole is found where old plaster has come away, we'd rather tell you at the survey than discover it on the day.

• Cuts are made in a controlled order, with the plaster supported on both sides.
• Loose plaster is fixed back or removed, not covered over.
• New trimmers carry the joists or laths round the opening.
• The lining frame is fixed to the new framing, not hung from the old plaster.

The edge is where an old ceiling and a new opening meet, and it shows. In many Georgian and Victorian houses, the plaster is lime-based and has a slightly soft, uneven surface. Matching it with hard gypsum can leave a visible line, and in some houses the two don't bond well to each other. Where the ceiling is lime, the junction is often built up in lime plaster, so the old and new surfaces move and breathe in the same way.

The lining itself can be boarded and skimmed, or plastered in several coats, depending on the ceiling. In period rooms with splayed window reveals, a splayed rooflight lining often looks more natural than a straight box, and our answer on what a splayed rooflight lining is explains the choice. A bead is run along the arris for a crisp edge, and the cornice or ceiling rose, if there is one, is kept clear of the cut.

The finish schedule records which of these applies to your ceiling: lath and plaster under slate, say, with a lime plaster junction and a splayed lining, ready to paint.

Some situations call for a closer look before any quote is final.

Decorative plasterwork

A moulded cornice or ceiling rose close to the opening may limit where the unit can go, or need specialist support while the cut is made.

Listed buildings

Internal plasterwork can be part of what is protected, so listed building consent may be needed for the work as a whole. Check with the local planning authority first.

Heavy ornament or paper layers

Many old ceilings have been papered or painted repeatedly, and the extra weight can strain the keys.

Previous damage

Sagging, damp stains or a patched area suggest the ceiling needs more than a quick cut.

Many Devon houses have a mix. A Georgian terrace in St Leonards or a Victorian villa in Exmouth may have lath and plaster downstairs and plasterboard in a later loft room, and a 1930s semi in Heavitree often has plasterboard or fibre board throughout. Cob cottages in the Culm and Exe valleys have thick walls and sometimes ceilings of lath and lime, sometimes of boards with a plaster skim. The only sure way to tell is to look at it.

A few signs help. A lath ceiling often has a slightly wavy surface, fine cracks following the lines of the laths, and a hollow sound when tapped lightly. Plasterboard is flat, with joints at regular intervals, and is often lined along the edges. In the loft, laths show as narrow timber strips with lumps of plaster pushed between them. The loft view tells us which ceiling we are cutting before the day arrives.

The answer to the question is the same either way: yes, it can be done. What changes is the method, the care taken and the finishing time.

If your house has lath and plaster ceilings, a survey is the way to find out how sound yours is. We look at the ceiling from the room, and from the loft where we can, so we can see the keys and the condition of the laths. Our finish schedule records the ceiling and the roof build, and sets out the shape and finish of the lining. There is more on the whole sequence in how a rooflight is finished inside.

Timing is also affected by a lath ceiling, since the finishing is slower. We cover the days involved in how long it takes to fit a skylight. When you're ready, ask for a quote and mention the age and type of your ceiling in the form.