Skimming a ceiling

At first glance a ceiling is just another surface, like a wall. In practice it is one of the most demanding surfaces in the home, for two reasons: gravity and light.

Skim on a ceiling has to hold against its own weight. On a wall a thicker coat can still be spread and levelled. On a ceiling a heavy coat tends to sag, slowly lose its grip and crack as it dries. That is why ceilings are done in thinner coats, and why good adhesion to the background matters even more than on walls.

The second reason is light. Daylight from a window hits the ceiling at a very shallow angle, almost parallel to the surface. Light like that picks out every ripple, every trowel mark and every edge that was not sanded properly. So why does a ceiling look perfect during the day and suddenly bumpy in the evening under a lamp? Because the lamp shines from a different angle and reveals what the daylight hid, or the other way round. A good decorator checks a ceiling from several directions.

Skimming the ceiling is one of the last wet trades before painting. If the order goes wrong, the fresh skim gets damaged or has to be redone.

• Wiring in the ceiling is finished and chases are filled with plaster or patching compound
• Windows are fitted and reveals are made good, so the room can be ventilated and heated
• Wet floor layers such as sand and cement screed or anhydrite screed have partly dried, otherwise they keep the room humid and the skim dries slowly
• The room is usually at least around 10 °C, ideally normal room temperature, with no strong draughts
• Furniture is out of the room, or gathered in the middle and well covered
• Light fittings are taken down and the cable ends are made safe

Taking down light fittings and making the cables safe is not a minor detail. Electrical work belongs to a qualified electrician, not the decorator and not you. The electrician can also tell you whether anything else should go into the ceiling now, such as an extra light point or cabling for downlights. Adding it after skimming means cutting into a finished surface.

In older properties, especially those with textured coatings applied before the 2000s, have the old finish tested for asbestos before anyone scrapes or sands it. If asbestos is found, removal or encapsulation is a job for a specialist contractor, not for the plasterer or a homeowner.

In a block of flats, agree with the building management or owners' association on the times for noisy work, especially sanding. Skimming itself is ordinary surface work. If you also plan changes to the ceiling structure, such as new openings, check with your local building authority what the building regulations require.

Preparing the background

The most important part of the job is the part nobody sees later. The old ceiling is tapped and scraped first. Anything loose has to come off: flaking paint, loose bits of plaster, old polystyrene ceiling tiles and leftover adhesive.

Older houses often have distemper or limewash on the ceilings. Distemper is treacherous under a skim coat, because it softens when wet and the skim will not hold on it. The test is simple: wipe the ceiling with a wet sponge. If the paint washes off or smears, it is distemper. It has to be washed off back to the background, not painted over.

On a concrete slab ceiling the joints between slabs are checked. Loose joint filler is raked out, the joint is refilled and covered with reinforcing tape or fibreglass mesh. Cracks in plaster are treated the same way.

Priming

Once clean, the background is primed. Primer evens out suction, so the skim does not dry fast in one spot and slowly in another, and it binds a dusty surface. On very smooth concrete, such as some precast slabs or poured concrete ceilings, a bonding primer with fine grit is used instead of an ordinary primer. It leaves a rough key for the skim to grip. The primer has to dry according to the manufacturer's instructions before the next coat.

Levelling and the first coat

Larger dips, hollows and chases are filled first with a thicker bonding plaster or patching compound. Then the first full coat of skim goes on. On a ceiling the work runs in strips, usually from the window into the room, so the trowel strokes follow the direction of the light and show as little as possible.

If the ceiling is cracked in several places, fibreglass mesh is pressed into the first coat across the whole surface. It spreads the stress and lowers the risk of the cracks coming back.

Second coat and sanding

When the first coat has dried, it is lightly sanded and a second, thinner finishing coat goes on. For a higher standard of finish a third coat is added. Sanding is done with fine mesh or paper, by hand or with a long-reach drywall sander connected to an extractor. Dust extraction matters a lot, because gypsum dust gets everywhere.

Finally the surface is dusted down and primed again, or given a mist coat, so the paint soaks in evenly.

The corner between ceiling and wall

The corner is where the ceiling and walls move slightly differently, so hairline cracks tend to appear there. It is treated either with reinforcing tape bedded in the skim, or by running a flexible decorators' caulk along it after painting. In concrete buildings caulk in the corner usually works better, because it takes up small movements that rigid plaster cannot.

The right material depends on the background, the room and whether the work is done by hand or by machine.

• Gypsum finishing plaster or powder filler: the most common choice for living rooms and bedrooms, sands well, sets quickly, suits concrete and plaster
• Ready-mixed paste fillers or joint compounds from a tub: no mixing, easy to spread thinly, good for the final coat and for plasterboard, dry more slowly and are less suited to heavy levelling
• Lime-based finishes: breathable, suited to older houses with lime plaster and to vaulted ceilings, slower to work with
• Cement-based or moisture-resistant fillers: for bathrooms and other damp rooms where gypsum would suffer
• Spray-applied fillers: for large areas, sprayed on by machine and then smoothed, they speed up work in new builds

By type of ceiling

A concrete slab ceiling is usually flat, but it has joints and sometimes such a smooth surface that skim struggles to stick. The bonding primer and careful joint treatment decide the result here.

A plastered ceiling in a masonry house tends to be less flat, with ripples and local cracks. It needs more levelling before the finishing coats.

A plasterboard ceiling is not skimmed the same way as plaster. Joints are taped and filled and screw heads are spotted first. A full skim over the whole board follows only if you want a high standard of finish. In Europe the finish levels Q1 to Q4 are often used, where Q4 means a fully skimmed surface suitable even for strong side light.

A lath and plaster ceiling on timber joists, typical of older houses, is a separate story. These ceilings flex and move, so skim cracks easily. First check that the plaster is still keyed to the laths, by tapping and pressing gently. If there is a large hollow or sagging area, a skim coat is not enough. The plaster needs repairing, or the ceiling is overboarded or replaced with plasterboard.

On NUMIQ ceiling skimming and wall skimming are separate items, because they are different jobs even when the same material is used.

• Overhead work: a ceiling is skimmed from a platform with arms raised, which is physically harder and slower per square metre
• Access: walls can often be done from a step ladder and the floor, a ceiling needs a mobile tower or a solid platform that is moved as the work goes on
• Coat thickness: ceilings get thinner coats and more passes, heavy levelling is riskier overhead
• Light: a ceiling faces strong side light from the windows, so it usually needs a smoother finish than a wall that will partly sit behind furniture
• Obstacles: on walls you work around sockets, switches and reveals, on a ceiling around light points, hooks and the corner along the whole perimeter
• Wear: walls get knocked by furniture and people, a ceiling does not, so appearance and adhesion matter more
• Cleanliness: on a ceiling plaster and dust fall onto the whole floor, so protection has to be more thorough

When should you order only the ceiling? For example when the walls are fine after new plaster and the problem is just the slab joints overhead, or when the ceiling needs redoing after rewiring. When only the walls? When the ceiling is flat and just needs repainting, but the walls are damaged after removing tiles or old wallpaper. If the whole room is being done, the order is clear: ceiling first, then walls, because ceiling work messes up everything below it.

Ceiling area is measured in square metres and in a rectangular room it roughly matches the floor area. For walls, windows and doors are deducted instead.

How demanding ceiling skimming is, and so how much work it involves, depends mainly on the condition of the background. The easy case is a flat concrete or newly plastered ceiling with no old paint, in a dry room with a clear floor.

The job takes longer mainly with:

• Old distemper that has to be washed off
• Several layers of old paint that are flaking
• A textured coating that has to be removed, tested or covered
• Large dips, sagging and wiring chases
• A cracked ceiling that needs mesh across the whole surface
• A high ceiling, as in many older houses, where taller access equipment is needed
• An occupied home with furniture that has to be covered and moved
• A cold or damp room where each coat takes a long time to dry
• A demand for the very smoothest finish under strong side light

It helps if the room is empty, the light fittings are down and you have said clearly what finish you expect. The difference between a smooth ceiling for ordinary emulsion and a ceiling without a single ripple under raking light is quite large in terms of coats and sanding.

An experienced decorator can usually put one coat on an ordinary room's ceiling in a day, but the whole job takes longer, because every coat has to dry. A thin coat of gypsum filler usually needs roughly 12 to 24 hours, thicker coats or a cold, damp room need longer. Ready-mixed paste fillers usually dry more slowly than powder gypsum products.

For one room, allow several days from preparation to a surface ready for paint. The skim must be completely dry before painting. You can tell by an even, light colour with no darker patches. Gentle ventilation and heating help drying. Draughts, direct sun through the window and a fan heater pointed at the ceiling do harm. Drying too fast causes cracking and weaker adhesion.

Skim over a weak background. Skim spread over flaking paint or distemper only holds until the layer underneath lets go. After a while the ceiling starts peeling in larger flakes, often after it has been painted. The only fix is to remove everything back to a sound surface.

Missing primer or bonding coat. On smooth concrete the skim comes away after drying and sounds hollow when tapped. On a very absorbent background it dries out too quickly, the trade calls it burning, and ends up brittle.

Too thick a coat in one go. Trying to level a big dip in a single coat leads to cracks as it dries and a risk that part of the skim pulls away under its own weight.

Untreated slab joints. After a year a crack appears in exactly the same place, as straight as a ruler. Concrete slabs move slightly with temperature changes and plain filler cannot cope. Reinforcing tape or mesh and flexible caulk in the corner help.

Checking under one light only. The ceiling looks fine under the ceiling light, but once painted, waves appear in daylight from the window. Always check with side light as well.

Painting onto damp skim. Paint soaks in unevenly, leaving patches and shade differences, and sometimes it peels.

If the room has a normal ceiling height and the area is small, such as a hallway or a box room, you can skim it yourself. Work from a sturdy mobile platform or a stable work platform, not a ladder, because from a ladder you cannot draw the trowel in one continuous stroke. Leave stairwells, high ceilings and old lath and plaster ceilings to professionals. Light fittings must be taken down by an electrician before you start, and any old textured coating should be tested for asbestos first.

1. Clear the room, cover the floor with dust sheets or plastic and tape down the edges.
2. Test the ceiling with a wet sponge for distemper, and if you find it, wash it off back to the background.
3. Scrape off loose paint and plaster and remove the dust.
4. Rake out cracks and loose joints, fill them and cover them with reinforcing tape.
5. Prime the whole ceiling and let it dry according to the manufacturer's instructions.
6. Fill the deeper hollows and let them dry.
7. Mix only as much material as you can use within the working time printed on the bag.
8. Apply the first thin coat in strips, working from the window into the room.
9. Once dry, sand lightly, dust off and apply a second thin coat.
10. Sand the final coat, check the ceiling with a lamp held close to the surface and fix any unevenness.
11. Dust off, prime or mist coat, and paint only when everything is fully dry.

Wear a dust mask and safety glasses when sanding. Filler dust irritates the eyes and airways, and when you work overhead it falls straight onto your face.

Check the finished ceiling before it is painted, while faults are still easy to fix. The handiest tool is an ordinary lamp or a strong torch held close to the ceiling so it shines almost parallel to the surface. Light like that shows ripples, trowel marks and unsanded edges. A long straightedge gives a rough check of flatness.

• The skim is completely dry and has an even colour with no darker patches
• Nowhere does the ceiling sound hollow when tapped
• Under side light the surface is smooth, with no obvious waves, scratches or tool marks
• Slab joints and the original cracks are no longer visible
• The corner between ceiling and wall is straight and clean, taped or ready for caulk
• The area around light points is made good and the cables are safely terminated
• The surface is dusted off and primed, if that was agreed
• Floor, windows and other surfaces are clean and free of plaster splashes
• You know which material was used and when the ceiling can be painted