Repair plaster walls & ceilings
This 'How to' shows how to repair minor and major faults in plasterwork. You'll learn the techniques for patching and finishing the different types of plaster surfaces: plastered masonry, brick in-fill stud walling, lathe and plaster and plasterboard.
Introduction
What you’ll need
Materials
Decorator's filler – powder Expanded metal, available in: • 100 and 150mm width rolls • 600 x 200mm sheets Galvanized clout nails (large head) 25mm Galvanized jagged plasterboard nails 30mm and 50mm Masonry nails 50 and 65mm Plaster Plaster bead Plaster of Paris – for repairing moulded ceilings and decorative mouldings Plaster repair scrim or hessian scrim Plasterboard offcuts Plasterboard screws 32mm 25mm x 75mm prepared timber batten Sawn timber 50mm x 100mm for making ceiling props and folding wedges (secondhand material is ideal). Alternatively, screw jack 'Acrow Props' may be hired. Sawn treated timber batten 25mm x 38mm String Panel adhesive
Tools & equipment
Block Surform Bolster chisel Broad knife 200mm Claw hammer – 16oz or 20oz Clean bucket Club or lump hammer Cork sanding block Craft knife Decorator's brush – 100mm or 125mm Dust sheets Dustpan and brush Empty plastic bottle for water Filling knife 100mm Gauging trowel Hard point handsaw or jigsaw Hawk – for holding wet plaster Large sponge Metal shears Plasterboard saw (utility saw) or padsaw Plasterer’s small tool – leaf and square Plasterer’s trowel Scratcher – a piece of thin timber 18mm x 100mm x 200mm into which nails have been partially driven, so that the points just appear. See How to Prepare walls and ceilings. Spirit level – minimum 900mm Spot board – 750 x 750 x 18mm shuttering ply with 3 x 25 x 50mm battens fixed to underside. Steel tape measure 2 Step ladders with tool platforms and 1 scaffold board – these items may be hired. Stirring stick 1200mm straight edge Wire brush Workbench
Types of plaster • Thistle Hardwall An undercoat plaster with quick-drying properties, pre-mixed with impact-resistant aggregates. Bag weight: Approximate coverage per bag: Approximate setting time: 40kg 4.6m2 2 hours
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HOW TO: Repair plaster walls & ceilings
What you’ll need (cont.)
• Thistle Multifinish A final smooth-coat plaster used over Hardwall. Can also be used as a finish on plasterboard. Bag weight: Approximate coverage per bag: Setting time: 40kg 10.4 to 16m 2 1/2 hour
Hardwell and Multifinish are not the easiest plasters to use. But they are by far the most economical, particularly if you have a large areas to fill.
fig. 2
• Undercoat plaster Similar to Hardwall, but available in a range of smaller quantities. Bag weights: Coverage 11mm thick – 12.5kg: 8mm thick – 12.5kg: Setting time: • Finishing plaster Designed to complement undercoat plaster. Use 2mm thick over undercoat plaster and 3mm thick onto plasterboard. Bag weights: Coverage 2mm thick – 12.5kg: Setting time: • One-coat plaster More expensive than Hardwell and Multifinish plaster. Formulated for easy use by an amateur and doesn’t require a separate finishing plaster. It can be built up in layers and the final layer can be polished smooth. Use 13mm thick on brick and blockwork, 10mm thick on concrete blockwork and 5mm thick on plasterboard. Maximum thickness is 50mm. Bag weights: Coverage: 13mm thick – 12.5kg: 10mm thick – 12.5kg: 5mm thick – 12.5kg: Setting time: • Patching plaster Available in 1.8kg boxes. Suitable for odd small patches. Doesn't require a finishing coat. Build up in 3mm layers. Setting time: 11/2 to 2 hours approximately 1.1m2 approximately 1.3m2 approximately 2.6m2 2 hours 2.5kg, 12.5kg, 20kg
fig. 3
7.5kg, 12.5kg, 20kg approximately 1.252m2 approximately 1.8m2 11/2 to 2 hours
2.5kg, 12.5kg, 20kg approximately 5m2 1 to 11/2 hours
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HOW TO: Repair plaster walls & ceilings
Patching a plastered masonry wall
1. Remove loose material
Use a bolster chisel and club hammer to remove all loose material round the area to be patched (fig. 2) and undercut the edges. Don't stint on this job. The cost of extra plaster is far outweighed by the benefit of a long-lasting perfect job. Wire brush the brickwork to remove all traces of loose material.
2. Prepare plaster
Mix the plaster according to the manufacturer's instructions. Use the gauging trowel to sprinkle powder into the water, while stirring with a clean stick (fig. 3). When the plaster is a thick, creamy consistency without lumps, turn it onto a dampened spot board.
fig. 4
3. Fill the patch
Thoroughly dampen the area to be plastered. Use the plastering trowel to slide a large sausage of plaster onto the hawk (fig. 4). Standing close to the patch, tilt the hawk towards you and, with a continuous movement, lift off half the plaster onto the trowel (fig. 5).
fig. 5
Hold the trowel horizontally but slightly angled towards the wall. Press the plaster into the patch and move the trowel up the wall, gradually flattening it towards the brickwork. Don't completely flatten the trowel as this will cause suction and pull the plaster off the wall (fig. 6). Repeat this procedure until the patch is slightly overfilled. Don't overwork the plaster as this will weaken it and cause it to fall to the floor.
4. Ruling off
Hold the straight edge tool horizontal to the floor, but angled slightly down on its front edge. Start below the patch and move it from side to side up the patch as you go (See main image pg. 1). If there are any hollows, flick a dab of plaster onto the bare patch and re-rule. Once the plaster has set, apply a finishing coat. You won’t need a finishing coat if you're using one-coat plaster or patching plaster. The plaster will start to set after approximately 45 minutes. Go over it lightly with a dampened trowel to smooth the surface (fig. 7). After a further 20 to 30 minutes, splash the surface with cold clean water then trowel to a fine smooth surface. Keep the trowel damp and don't over-trowel. Deep patching If the patch is deep you may need to apply two layers of plaster. Apply the first layer to half the depth of the patch then, when it has partially set, scratch up the surface and leave it to set fully without drying out. Apply a second coat and rule it off. When set, apply a finishing coat.
fig. 6
fig. 7
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HOW TO: Repair plaster walls & ceilings
Patching lathe & plaster
1. Remove loose material
Tap the surface to determine the extent of any loose plaster. Mark the perimeter of the bad area with chalk then score this line deeply with a heavy-duty craft knife. Remove the loose plaster and wire brush the lathes and studs. Dampen the lathes and edges of the surrounding plaster, then apply a coat of plaster to half the thickness of the old plaster. Use enough pressure to force plaster between the lathes while still leaving a covering of plaster on them. When this first layer is partially set, scratch up the surface and leave it to set fully without drying out.
fig. 8
2. Ruling off
See above Patching a plastered masonry wall: step 4 Repairing brick in-fill stud walling Treat in the same way as other walls, except where a wooden stud is exposed. Use galvanized clout nails to fix 150mm wide galvanized expanded metal to the studs so that the metal extends over the brickwork (fig. 8). Safety tip Wear thick leather gloves when cutting and handling expanded metal. It can tear your skin.
Repairing plasterboard – small holes
1. Smooth edges
Feel inside the hole to check for concealed pipes or cables. If you find one, push it to one side. Trim round the hole with a heavy craft knife. If there are no pipes or cables, use a plasterboard/utility saw or pad saw to clean up the damaged edges of the hole. Remove a 25mm wide strip of finishing plaster from round the hole. Safety tip Turn off the electricity while you check for pipes and cables.
fig. 9
2. Fix backing
Cut a piece of plasterboard 25mm longer than the hole and pass a piece of knotted string through a hole in the centre. Holding onto the string, check you can feed the patch through the hole and pull it into position behind the plasterboard. Remove the backing patch and coat the contact surface with panel adhesive. Poke the patch through the hole and pull firmly into position (fig. 9). Tie off the string until the adhesive has set. Scrape off any excess adhesive. When the adhesive has set cut off the string flush to the patch.
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HOW TO: Repair plaster walls & ceilings
Repairing plasterboard – small holes (cont.)
3. Fix patch
Cut another piece of plasterboard very slightly smaller than the hole. Glue in position with panel adhesive and leave to dry.
4. Finish
Apply strips of self-adhesive plaster scrim over the joints and finish off with finishing plaster filler.
Repairing plasterboard – large holes
1. Locate studs
With a steel rule or thin batten, find adjacent studs and mark them on the plasterboard. Safety tip Check inside the hole for pipes or cables.
2. Remove damaged plasterboard
Use a straight edge to mark horizontal lines above and below the damaged area. Use a plasterboard/utility saw or padsaw to remove the damaged plasterboard. Clean up the edges with glass paper. Remove a strip of finishing plaster all round the edges of the hole.
fig. 10
3. Fix battens
Use 50mm plasterboard nails to fix sections of 25mm x 50mm sawn, treated batten to the studs. Keep the battens flush with the front edges of the studs. Cut further lengths of batten and use panel adhesive to fix them horizontally behind the plasterboard. Hold the battens in place with G cramps (fig. 10).
4. Fix patch
When the battens have set, cut and fix a piece of plasterboard to fit the hole, leaving a 3mm gap round the edges. Fix the plasterboard using 32mm plasterboard screws.
fig. 11
5. Finish
See above Repairing plasterboard – small holes.
Repairing corners
Small patches
Cut away the damaged and loose plaster, see above Patching a plastered masonry wall. Use masonry nails to fix a length of 25 x 75mm prepared batten to one face. Position the edge of the batten in line with the surface of the plaster on the adjacent face (fig. 11).
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HOW TO: Repair plaster walls & ceilings
Repairing corners (cont.)
Dampen the brickwork and plaster edges, then apply plaster to fill the patch (Fig. 11a). See above Patching a plastered masonry wall. When the plaster has set, carefully remove the batten and re-fix it over the recently plastered face. Plaster the second face (fig. 11b) Remove the batten when the plaster has set. Make a small chamfer on the corner with either a block Surform or coarse abrasive paper wrapped round a block. Apply a coat of finishing plaster.
Large areas
Cut away the plaster from skirting to ceiling. Use Hardwall or Basecoat plaster to apply 'dots' to both surfaces at the corner of the walls. Cut plasterboard to length and press it into the dots until it's vertical and lines up with the adjacent old plaster surfaces (fig. 12). When the 'dots' are set, plaster and finish as described in 'Patching a plastered masonry wall'.
fig. 12
Repairing a sagging ceiling
Sometimes the ceiling sags where the plaster, usually because of a water problem, has separated from the lathes. If caught in time, you can easily and relatively cheaply avoid disaster.
1. Clean above the ceiling
Gain access to the space directly above the ceiling. If the roof space is above the ceiling, this will be relatively easy. Otherwise you may need to remove furniture, and lift carpet and floorboards. Use an industrial vacuum cleaner to remove all traces of dust, dirt, soot and old plaster from the joists, lathes and gaps in-between the lathes.
fig. 13
2. Support the ceiling
From below, place boards up against the ceiling and hold them there. Gently push the plasterwork back into place, using either jack-up, Acrow-props or wooden lazy man.
3. Fix plastering scrim
Have cut lengths of coarse plastering scrim ready. Thoroughly wet the lathes and plaster. Mix plaster of Paris to a thick, creamy consistency and pour it over the lathes. Spread it quickly and evenly, spreading 6-9mm over the lathes. Press the scrim into the plaster and 40-50mm up the sides of the joists. If necessary, pour over more plaster of Paris (fig. 13). Work fast, as Plaster of Paris sets very quickly (10 to 20 minutes). Leave the props in place until the plaster has hardened.
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