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Kiln-dried timber is used in the majority of house frames, although unseasoned timber may still be used in some areas. Since unseasoned timber shrinks across the grain as it dries, with shrinkage being proportional to the depth of the joist, an effort should be made to source seasoned timber, particularly where larger-size ceiling joists are used – e.g. greater than 125mm deep.
One of the initial steps that can be taken to reduce plasterboard peaking is the correct selection and installation of the ceiling support structure.
When timber is the preferred material, correctly stress-graded timber should be used, with sizes selected in accordance with the span tables provided in the National Timber Framing Code (AS 1684) or other approved span tables. Ceiling joists must be securely fixed at all connection points, such as the top plates of wall frames and the ends of rafters and attached to hanging beams by timber or metal ‘soldiers’. This will ensure that the ceiling framework maintains a flat, even surface to receive the plasterboard lining.
When installing plasterboard, the manufacturer’s recommendations and the recommendations of the Australian Standard AS/NZS 2589.1 Gypsum linings – Application and finishing should be followed. The Standard specifies six levels of plasterboard surface finish, with Level 4 being the generally accepted level of finish for domestic construction, and recommends the following construction practices to achieve a Level 4 finish.
Framing members supporting gypsum linings should be spaced at not more than 600mm centres. When framing members change direction within a room, trimmers shall be provided as part of the framing system. Linings can be either screw fixed, nail fixed or, as is most commonly adopted, a combination of adhesive and mechanical fastenings.
When using a combination fixing system the spacing between the permanent fastener and the adhesive should be no less than 200mm as closer applications can result in the fastener popping. The application of the adhesive fixing should be approximately 25mm in diameter and 15mm high and be placed at maximum 230mm centres.
When fixing with screws or nails only, these should be used with a recommended centre-to-centre fastener spacing of 300mm and 200mm respectively. Nail fixing is generally used where an adhesive cannot be used – i.e. over existing linings or over a vapour barrier.
It is also recommended that fastening be between 10–16mm from the edges and ends of the plasterboard for screw or nail applications.
To achieve a level 4 finish pursuant to AS 2589.1, butt joints on ceilings should be back blocked and recessed joints on ceilings should be back blocked in any area containing three or more recessed joints.
When back blocking, plaster-based adhesive/cement is to applied over the full face of the back block, the use of a notched spreader being recommended to give 6mm x 6mm beads at 20mm c/c. After the first board has been placed the back blocks should be placed centrally along the full length of the board edge. The next plasterboard sheet should then be placed.
Back blocks may also be cemented into position from above the ceilings after the boards have been fixed and before they are flushed.
When back blocking butt joints, battens and packers are recommended to support and deflect the sheet ends upwards about 2mm. This will allow a suitable depression for finishing of the joint after the back blocking has set.
Back blocking of joints in the plasterboard as detailed above will assist in reducing rotation in the plasterboard joints. This technique has proven effective in minimising joint peaking and cracking.
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