Small holes in plasterboard, up to about 130mm across, can be filled with a small backing piece and standard filler. If you need to fill a larger hole or a large damaged section of plasterboard, you will need to consider fitting a plasterbaord patch.
The first thing you need to do before you start is to find out how thick the existing plasterboard is. This can be done by measuring the thickness at the edge of the hole you need to patch. The existing plasterboard will be either 9.5mm or 12.5mm thick. You can now purchase a sheet of plasterboard which is a match.
Squaring Off the Hole
Pull away any loose plaster from the hole and check that there are no pipes or cables behind the board. Now use a trimming knife or padsaw to cut horizontally across the plasterboard on each side of the hole until you hit the timber studs on each side. Use the straight edge of a spirit level and mark the inside edge of the studs on either side, extending the line at least 50mm above and below the hole. Join up these two lines above and below the hole to mark out a square, making sure that the corners are right angles. This is the first section you need to cut out of the plasterboard, so do this now.
Studs and Noggins
On each side of the square, mark a line to show the middle of the studs (this would usually be 25mm). Using a trimming knife, score along this line and then cut away this small strip to reveal half of the stud behind. This is to give you something to nail the new patch to at the sides of the hole. To give you a fixing at the top and bottom, you will need to cut and fit two new noggins (horizontal pieces of wood) out of at least 50mm wide timber. Nail these noggins into place behind the plasterboard, at the top and bottom of the square hole so that 25mm extends past the edge of the plasterboard. Use a G-clamp to hold them in place while you hammer in the nails if you need to.
Accurately measure the area of the hole and cut a matching patch out of your new plasterboard sheet. Take your time and cut this carefully, as the neater the edges the neater the finished patch will be and the less time you will need to spend cleaning it up. Fit the patch into the hole with the ivory side outwards (so that it can be plastered) and fix it into place using plasterboard nails driven into the studs and noggins behind. Sink the nails into the plasterboard using a punch, but be careful not to damage the surface of the patch.
Clean up the joint around the patch using sand paper or a sanding block and carefully remove any burring if necessary. Apply joint compound to the joints and smooth it flat all the way around. Let this dry and sand it smooth and flat. If the wall has a layer of skim over the plasterboard, you will obviously need to skim the area of the patch. Use a straight-edge and the surrounding wall to get a flat and level finish. When the new plaster is dry, you can decorate to match the rest of the wall.