How to repair a ceiling
In old houses the ceilings could well be constructed from lath and plaster, while in modern houses they're more likely to be made of plasterboard. It's not difficult to repair small areas of damage on your ceiling, and it's well worth the effort - particularly before you start decorating a room.
Lath-and-plaster ceilings in older houses often crack with age, and the plaster can bulge where it begins to break away from the laths. If only a small area of your ceiling is affected, you can make a simple repair with a plasterboard patch using dry-lining techniques that you won't need plastering skills to use. Or it may be easier to overboard the entire ceiling with plasterboard sheets.
Always wear safety goggles and a dust mask.
Both, lath-and-plaster and plasterboard ceilings are fixed to the joists that support the floor above. The depth of the joists will vary depending on the age of the building (older ones are often deeper than modern ones) and their length.
Lath and plaster
This was a time-consuming method of building walls and ceilings, as individual laths (thin strips of wood) had to be nailed to the joists and then covered with layers of plaster. The plaster was squeezed between the laths so that 'nibs' formed - ridges of plaster that squeezed through the laths and set hard, holding the ceiling plaster in place. The room above would traditionally have had floorboards.
Most modern ceilings are built from sheets of plasterboard nailed to the joists. These joints are taped with scrim - a very open-weave, self-adhesive tape. Wet plaster is then added, or the ceiling is finished with jointing tape and compound - which seals the joints and creates a surface ready for decorating. The floor of the room above a plasterboard ceiling may well be laid with floorboards or chipboard.
Very high ceilings can sometimes be lowered by fitting a suspended ceiling. Panels or tiles are fitted to a lightweight framework that consists of an outer frame (called the angle section). This is fixed to the walls, a main bearer section spanning the width of the room and a cross-bearer section which fits between the main bearers. Wires fixed to angle brackets that are attached to the original ceiling hold the framework in place. Panels can be decorative and translucent, or they might have soundproofing and/or thermal properties.
If the laths are still intact, you can repair damaged plaster with new plaster.
Carefully cut back the damaged area, removing all loose material, until you reach sound plaster. Dampen the laths and spread a thin coat of bonding undercoat plaster.
Score the surface of the plaster in a diamond pattern with the side of a trowel to create a key for the next coat. Allow it to set and then apply a second coat of undercoat plaster. Scrape it back 2mm below the surface and lightly score it. When this has set, put on a finishing coat of plaster to bring it down level with the rest of the ceiling.
If you have good access to the upstairs floor, you can repair an area of plaster that's sagging away from the laths from above.
Prop up the sagging plaster using a flat piece of chipboard or plywood nailed to a length of 38mm-square timber. This should reach from the floor to the ceiling (this is known as a 'deadman'). Lift the floorboards in the room above and vacuum between the joists over the bulge to collect up the loose plaster.
Mix fairly runny bonding undercoat plaster and pour it over the damaged area to replace the broken plaster. Leave the supporting prop in place until the plaster has dried and bonded to the laths.
Locate the joists each side of the hole with a stud detector. Next draw a line along each joist, half-way across its width, with a straightedge and pencil. Then join them up to mark a square or rectangle around the damaged area.
Cut away the laths and plaster or plasterboard inside the marked area with a pad saw, and remove any protruding nails in the joists with a claw hammer. Then cut two noggins from 100mm x 50mm of sawn softwood to fit between the joists and form the other two sides of the opening. Half the thickness of the noggins should project into the exposed area.
Drive nails into the noggins at an angle to secure them to the joists. Using plasterboard that's slightly thinner than the depth of your ceiling, cut a piece the same size as the hole. Fix it in place with plasterboard nails along the joists and noggins.
Fill the join with bonding undercoat plaster. When it's dry spread multi-finish plaster over the patch to bring it level with the rest of the ceiling. You can put on two thin coats rather than one thick coat if you need to.
It's a good idea to take off the old plaster before you buy the plasterboard to make a repair patch. Then you can choose the thickness closest to the depth of your plaster.
You'll need to deal with areas of loose, bulging plaster quickly, as they're not only ugly but dangerous. But make sure you put on a dust mask, protective goggles and preferably, a hard hat. Start by opening up one of the cracks with a filling knife or similar bladed tool until you can put your fingers into it.
Next, pull away the loose sections of plaster to expose the wooden laths. Continue until you've reached sound plaster and you've exposed the joists to which you can nail the repair patch.
Undercut the edges of the plaster slightly with a filling knife. Then tape a sheet of newspaper over the hole - making sure you stretch it and trace the edge of the plaster with a crayon or felt-tip pen.
Remove the newspaper and cut around the outline of the hole. Then lay it on a sheet of plasterboard and draw round it to get the shape of the patch.
Lay the plasterboard onto a firm surface (like a portable workbench) and cut out the patch. You can use a pad saw (which is a powered jigsaw) or a plasterboard pad saw. Gently rub down any ragged paper edges with abrasive paper.
Check the fit of the patch. If you need to, scrape away more of the ceiling plaster to leave a gap of about 3mm. Pop the patch into place (it's easier if you have someone supporting it), hold a straightedge across the ceiling and measure the vertical gap between it and the patch. This will show the thickness of packing you'll need to set the patch in line with the ceiling's surface.
You can make the packing out of stiff card (but check it can't be squashed), hardboard, plywood or even plasterboard. You'll need to nail it to the joists.
You can hide and reinforce the edges of your repair by using plasterboard jointing tape and compound.
Top tip - Containing the mess
Removing damaged plaster creates a lot of dirt and dust. To reduce the mess, cut two holes in a rubbish sack and put your arms through them. That way, the sack will catch most of the debris as you take it out.
Start by holding the patch in position and checking it's level with the ceiling. Just adjust the packing if you need to. Next, nail the patch to the joists with plasterboard nails. It might help if you mark the positions of the joists with pencil on the surrounding plaster.
Brush a PVA solution onto the edges of the patch and surrounding plaster and let it dry. Then fill in the gap around the patch with a ready-mixed filler, making sure you press it in well. The undercut edges of the plaster will help keep it in place. Once it's dry, you can sand the filler lightly.
Spread a layer of plasterboard jointing compound over the joint and press jointing tape into it. Then put another wide layer of compound on with a coating knife.
Smooth the compound with a damp sponge, feathering the edges. When it's dry, you can sand it smooth before you redecorate.
Removing crumbling plaster from a ceiling is a very messy and dusty job. If your ceiling isn't sagging and most of the plaster is still intact, it's much easier to overboard or dry-line the whole ceiling using plasterboard sheets. If you'd like to improve your sound insulation, try acoustic check plasterboard.
First, find the joists by using a joist, pipe and cable detector, or by measuring the distance between joist centres from the floor above (they're usually 400mm or 600mm apart). Check carefully for pipes and cables, then hammer nails into the ceiling in line with each joist centre along opposite walls.
Snap chalk lines across your ceiling to mark the joist centres. It's a good idea to extend the lines a few centimetres down the wall, so you can still see them as you fix the boards.
Next, secure tapered-edge plasterboard sheets with their long edges at right-angles to the joists - the ivory side should be facing downwards. Join the board ends in line with a joist and stagger the joints.
Make sure you use plasterboard nails long enough to drive securely into the joists. Remember to check for pipes and cables at each fixing point and be aware that you may need to scribe sheets to fit along the wall edges.
Finish your ceiling with a skim coat of plaster, or with jointing tape and compound.