Red plasterboards are fire rated
Plasterboard consists of a gypsum core sandwiched between paper liners. Various types are designed to take plaster, and it is advisable to read the manufacturer’s instructions regarding fixing and plastering before you start work as there can be small variations to the instructions given below. Most plasterboards, however, have square edges and these need ‘scrimming’, or reinforcing, after the boards are placed in position. Most can also be plastered with one ‘thick’ (about 5mm coat of finish plaster.
The plaster must be a hemi-hydrate. Various sizes of boards are available and it is sometimes useful to obtain a variety to minimize wastage. But as it is easy to cut, one of the standard sizes, say 1.2m x 2.4m (4ft x 8ft) is a convenient size to work with. Plasterboards also come in two thicknesses 9.5mm and 12.5mm. The first is suitable for most situations, but where the distance between the centres of the ceiling joists exceeds about 350mm, the latter should be used. Some ceilings are very uneven and it may be necessary to counter batten them to provide a new level surface on which to nail the plasterboards. The first battens are nailed at any convenient, spacing and are used to form fixing points for a second set of battens which are then fixed at right-angles to the first ones. This second set of battens are fixed at centres to suit the width of the plasterboard and they are levelled by driving thin wooden packing pieces between the battens where necessary.
The ends of the boards can be butted up to the walls or the plaster can be chipped away so that the boards can go right up to the brickwork. In the former method jute scrim is applied to the angle to reinforce the plaster. When flxing the plasterboards to the joists do not use ordinary galvanized clout-head nails as these large, flat heads cut the paper covering of the boards.
If you are not using plasterboard screws, use only the proper plasterboard nails which have a slight bevel underneath the head rather like a countersunk screw head. These small-headed galvanized nails should be 30mm long for 9.5mm thick boards and 40mm long for the 12.5mm boards. Place the nails 12.5mm from the edge and at 150mm centres.
Fixing to the ceiling
Plasterboards over which you intend to plaster can normally be nailed either across or along the joists. For large areas it is often desirable to position them in both directions so that long joins, which may cause cracking, are avoided wherever possible. It is particularly important to ensure that the joints are adequately nailed to the joists. You should leave an 3mm gap between the boards for scrimming. Before starting work, it is a good idea to draw a plan of the ceiling and work out roughly how your boards will be placed. This will help you to arrange them to minimize cutting, and also to ensure that they are sufficiently staggered.
If you are working alone you will need the help of a ‘dead man’s hand’ in addition to the normal tools and working platform. This is simply a long piece of straight-edged wood, about 50mm x 25mm to which is fastened a cross piece about 600mm wide. The bottom of the batten rests on the floor and the cross piece is wedged against the plasterboard to hold it in position. Your hands are then free to nail or screw the board to the joists. The easiest way to cut plasterboard is to score the face side deeply with a knife along a straight edge and then lay the board, with the cut side uppermost, over the edge of a table or bench so that the cut is in line with the edge. Snap the core by pressing down sharply, turn the board over and cut the paper on the other side along the crease. Alternatively, you can use an old saw, but this is slower and more tedious.