Carefully remove skirtings from wall and adjacent walls and set aside. Run utility knife along the cornice where it joins ceiling and wall to cut old paint. Using 100mm broadknife and a hammer, loosen cornice and remove. Knock off any remaining cornice cement with old chisel so wall and ceiling are smooth. Have an electrician disconnect switches and power points, if any.
Remove other fittings from the wall, if any. Here, there was a balustrade butted into the wall, so we cut through the handrail 20mm from the wall post. This is the thickness of plasterboard and adhesive, and how much shorter the balustrade will be with the new wall linings.
Attaching the sheets
Measure length of wall at bottom and middle. Transfer measurements to 1 plasterboard sheet and draw a cutting line on its face. Cut sheet by scoring along cutting line with utility knife. Bend sheet until it snaps, then run knife along other side of sheet to complete cut. If you require 2 sheets to cover a wall, as we did here, use 1 full sheet, then cut second sheet to fit rest of wall. Make cut end go into corner of wall. To keep sheets 10mm off floor, put offcuts on floor next to wall. Check fit of sheets on wall prior to mixing adhesive.
Using drill fitted with mixing paddle, mix masonry adhesive into a thick paste. Apply to wall in blobs about 15mm thick. Space them every 230mm at sheet ends and 450mm along and up the wall. Keep adhesive 50mm away from sheet edges.
Bring in first sheet and press against wall. Use long straightedge to ensure sheet is plumb and face is flat. Repeat for second sheet on lower half of wall, if using. Ensure tops of sheets are flush. Press long straightedge across join to ensure faces of sheets are aligned.
Measure length of wall at top and from sheets up to ceiling. Transfer measurements and cut sheets to suit, allowing for a 20mm gap to the ceiling. Aim to stagger the sheet joins. Notch corner of sheet to fit around existing cornices, if necessary. Check fit on wall before mixing a new batch of adhesive.
Repeat process of mixing and applying adhesive to wall for top sheets. Lift sheets up next to wall and press them against it. Ensure you do not slide sheets along adhesive blobs when applying. Use long straightedge to make sure top sheets are aligned with bottom sheets and with each other.
Installing a cornice
As cutting angles on cornice can be tricky, it helps to create cutting templates. Take a 400mm length of cornice and place it in cornice mitre box, which is specially designed for cutting cornice. Using an old handsaw, cut at a 45° angle. Take another short piece and cut it the other way. Hold pieces together as they would sit on the wall to form an internal corner (or external corner, if required). You may need a few tries to get it right.
In order for the new cornice to join correctly, the existing cornice will need to be trimmed accordingly. Place corresponding cornice template on the new wall so it butts into existing cornice. Mark line of the cornice on existing one. Use wallboard saw to trim existing cornice on this line. Try fitting template into corner and keep trimming until it fits easily. Stretch chalk line between undersides of existing cornice on adjacent walls and flick to create a guide for new cornice.
Measure between adjacent walls to get length of cornice. Deduct 3mm to allow for easy fitting. Use cornice templates as a guide to get angled cuts on ends right. Cut cornice.
Mix cornice cement with water into a thick paste. It sets quickly, so only mix enough to do 1 length of cornice. Load cornice cement on 1 edge of plasterer’s trowel and run trowel along both edges of cornice, leaving a 15-20mm bead of cement. Also put a little at the ends where it will join.
Lift cornice into position, gently bending in the middle to get corners in first. Line up the bottom with chalk line and press into wall so cornice cement oozes out. If it starts to sag, hold in place with temporary nails along edges. Using 50mm broad knife, scrape off excess cornice cement. Use plasterer’s tool and excess cement to fill and trim corner joints. Remove temporary nails and fill holes. Finish by wiping a damp sponge over the work to smooth the cement, so you don’t have to sand it later.
To set joints between sheets, mix up base coat and spread along joint. Press paper jointing tape into wet compound and work your way along joint, removing any wrinkles or bubbles in tape. Finish with a skim coat that just covers tape. Internal corners are set the same way. Fold paper jointing tape along crease in centre and use internal corner tool to press tape into base coat. Leave to set, then apply a second, wider skim coat to joints and corners, feathering edges to plasterboard. Leave to set.
Apply a coat of finishing compound 300mm wide to joints, feathering edges to sheets. On butt joint where ends of sheets meet, make compound 600mm wide to properly blend joint. Use internal corner tool to apply finishing compound to corners. Allow to dry for at least 24 hours.
Wearing dust mask, carefully sand with 150-grit sandpaper. Check for smoothness by running your hands along the join, feeling for any ridges. Remove all dust, then paint with a sealer undercoat followed by a topcoat in your selected colour. Allow to dry, then apply a second coat.
Apply construction adhesive to back of skirtings and put on wall. Use 30mm nails to hold skirting to sheets while glue dries. Cut skirtings on adjacent walls to suit new walls. Replace any other fittings, such as the balustrade post here, and nail in. Have an electrician reconnect power points and switches, if required.
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