How do I overboard an existing plasterboard wall?

Overboarding is the answer to a plasterboard wall that is structurally fine but cosmetically finished — repeatedly patched, covered in old adhesive, textured, papered into oblivion, or simply too damaged to be worth filling. It converts a plastering problem into a carpentry problem, it produces no dust to speak of, and it is reversible. The price is 9.5 or 12.5mm off the room in each direction and a set of detailing consequences that are much easier to plan than to discover.

Safety

What you'll need

• Plasterboard, 9.5mm or 12.5mm
• Drywall screws long enough to reach 25mm into the studs
• Stud detector, and a cable and pipe detector
• Plasterboard saw or a knife and straightedge
• Screwdriver with a depth-setting nose, or a collated screw gun
• Chalk line and a long level
• Scrim or paper tape and jointing compound
• A board lifter or a second pair of hands (optional)

Step by step

1. Check that overboarding is the right answer first.It suits a wall that is sound but ugly. It is wrong where the existing board is loose, water damaged, mouldy, or hiding a problem, because you would be sealing the problem in. Push the wall firmly at several points: movement means fix the cause first.
2. Find and mark every stud, floor to ceiling.Detector, confirmed by a small pilot hole. Mark the centre of each stud on the floor and on the ceiling in pencil, and snap a chalk line down each one before you start. Once the new board is up, those marks on the floor and ceiling are the only way to find the timber.
3. Isolate the circuits and map the services.Sockets, switches and any light fitting on that wall. Scan for cables and pipes and mark them. Assume vertical and horizontal runs from every accessory, and stay out of those zones with screws.
4. Deal with the accessories before boarding.Sockets and switches have to come forward by the board thickness. Isolate the circuit, remove the faceplate, and either fit an extension ring to the existing box or plan to refit into a dry-lining box in the new board. Electrical work has rules — replacing an accessory like for like is one thing, altering circuits is another.
5. Decide board thickness and orientation.Nine and a half millimetres is lighter, cheaper and adequate for an overboard where the substrate is continuous. Twelve and a half is more robust. Run the boards horizontally on a wall so the long joints do not coincide with the old ones, and so a full sheet spans more studs.
6. Set out so the new joints miss the old joints.Start the first board a half-sheet offset from the existing layout if you can. New tape directly over an old joint is the one place a crack will find its way through both layers.
7. Cut the boards to fit with a small gap at the floor.Score the paper face with a knife against a straightedge, snap, cut the back paper. Leave a 10mm gap at the floor — the skirting covers it — so the board is not sitting in any moisture and can be levered up tight to the ceiling.
8. Fix at the correct centres, into the studs.Screws roughly every 200 to 300mm along each stud, kept about 15mm in from the board edge so they do not break out. Drive them just below the paper surface without tearing it — a screwdriver with a depth nose or a drywall gun does this consistently and a normal driver does not.
9. Support the board edges.Every board edge needs timber behind it. Where a horizontal joint falls between studs with nothing behind it, either move the joint or add noggins through the old board into the stud bay before you cover it.
10. Tape the joints and finish.Tape and joint for a paintable finish with no plastering, or scrim and skim if you want a plastered surface. Tapered-edge board is designed for taping; square-edge is designed for skimming.
11. Re-fit the trims forward.Skirting, architraves, coving, window boards, curtain poles and radiator brackets all now sit on a surface 10 to 13mm further into the room. Reveals around windows and doors will need packing or re-lining so the return is not left as a step.

Tips

• Photograph the wall with the stud lines, cable runs and socket positions marked before you cover it. This is the cheapest insurance in the whole job.
• Screw length is the number people get wrong. You need the thickness of the new board plus the old board plus any skim, plus at least 25mm into the timber. On a 12.5mm-over-12.5mm build-up that is a 50mm screw at minimum.
• If you are overboarding to reduce noise, use a dense acoustic board and seal the perimeter with acoustic sealant — a second layer of standard board fixed straight through to the studs gains far less than people expect, because the studs still carry the vibration.
• Cutting a 10mm gap at the floor and at the ceiling, then sealing both with a flexible sealant, is what stops sound and draughts flanking around the new layer.

Common mistakes

• Fixing only into the old board rather than the studs — You have created a heavy layer hanging on the paper face of a board that was already tired. It will sag, the joints will crack, and nothing can be fixed to the wall afterwards.
• Overboarding a wall that is damp or mouldy — The moisture is still there and now it is in a warm sealed cavity. The new board grows mould from behind and the problem is invisible until it is much worse.
• Landing new joints on old joints — Both layers move together at that line, and it is the most likely place for a crack to telegraph through.
• Forgetting the reveals and the skirting until the boards are up — Every internal return, window reveal and door lining now has a step in it. Planning it takes minutes; retrofitting it takes a day.

If it doesn't work

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