Plasterboard is heavy and care must be taken to support edges where they abutt the wall. Two methods are suitable for this job. The more satisfactory and stronger method is to plaster the board into the wall plaster surface.
Before starting to plaster the ceiling, cut back a 50mm wide strip along the top of the wall edge where it joins the ceiling. Butt the board tightly up to the unplastered wall surface. Strengthen the joint with scrim. Fill in the gap when applying the finish plaster coat.
The other method, which is not so strong, is to abutt the board edge to the plaster surface of the wall and reinforce the right-angled joint between wall and ceiling with scrim.
At all times, use jute scrim unless the boards are not to be plastered. In this case, use cotton scrim.
Plasterboard can be cut by scoring the face side, using a sharp knife held against a straight edge. Lay the board, face side upwards, on a bench or table, the cut edge aligning with the edge of the support surface.
The core is snapped by pressing down sharply on the overhanging edge. The board is turned over and the paper backing is cut through with a knife.
Plasterboards are nailed either across or along the joists, using either 30mm or 40mm galvanized plasterboard, dependent on the thickness of the board. Boards should be put up in both directions, particularly over a large area. This will avoid long joins which may cause later cracking of the finish coat.
Nailing should be at least at 150mm intervals and nails should be positioned no less than 13mm in from the edges. Nailing any closer will cause the boards to split at the edges. Drive the nails in firmly but without damaging the paper covering.
A 3mm gap should be left between the boards. This allows for scrimming, with 90mm-wide jute scrim, before plastering.
If you are working single handed, a device called a ‘dead man’s hand’ will be necessary. This is a pole, or piece of straight-edged timber with a platform 600mm wide fixed to the top. The pole or timber support stands on the floor, with the platform resting beneath the ceiling supporting the plasterboard, leaving both hands free for working.
Measure out and cut the required lengths of scrim. Mix about half a bucketful of finish plaster. Transfer this to the hawk and, using a laying-on trowel, work along the joins between the boards. Press the plaster well into the gaps.
Position the scrim at one end of the joint and guide it into position over the gap, using the trowel to press it firmly into the plaster. Leave the joints between the ceiling and wall until last.
After scrimming, start laying on the plaster coat, treating each board as a separate area. Scrim a thin layer of plaster, using the steel trowel, up to but not over, the scrimmed joints.
Use the wood float to apply further plaster until the ceiling surface is covered to a depth of 5mm. With the steel trowel, lay on a small amount of fresh plaster to fill in gaps, and even off where necessary.
The strip between the wall and ceiling, if the plasterboard abutts the unplastered surface, is next filled in, using an angled trowel. It is smoothed off with a steel trowel.
Once the ceiling has started to set, smooth over the plaster with a clean trowel. A small amount of water, ‘flicked’ on from the brush, aids this process but do not make the surface too wet.
An angled trowel is used to smooth the corner between the walls and ceiling. Plaster is a quick-drying compound so the job, once started, must be finished rapidly.
10. November 2011 by admin
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