Substrates

Plasterboard can be applied to metal and timber framing or to masonry. Each of the substrates has its own individual fixing specification and compliance with correct fixing procedures is vital to achieve a satisfactory job. Additionally, each of the substrates behaves differently and requires separate considerations to enable it to provide a stable substrate. Timber is most affected by moisture, whilst metal may be affected by temperature.

Regardless of substrate the Australian and New Zealand Standard requires a minimum level of straightness of the substrate for the application of plasterboard. In all substrates, dimensional accuracy is the key to success. Plasterboard will follow the plane of the substrate to which it is fixed. Valleys or hollows may generally be easy to hide, but high points are very difficult to conceal. Plasterboard will not straighten walls or ceilings. No amount of joint compound can make a poorly framed job acceptable.

MOISTURE CONTENT OF TIMBER FRAMING
Timber is the original framing material and may be in the form of plantation softwood, hardwood or fingerjointed timber. Generally, plantation softwoods and finger jointed timber framing is kiln dried to achieve a stable moisture content. Hardwood framing Is likely to be partially green and subject to substantial drying shrinkage. Plasterboard lining, to be defect free, requires a relatively stable substrate and therefore careful maintainance of moisture content is as vital as dimensional accuracy.

CEILING FRAMING
Changes in direction of roof trusses, girder trusses, etc, are now common construction practice. Fixing of plasterboard to some roof truss configurations is almost impossible if manufacturer's specifications are to be met.

RESULT
The end result can mean more ceiling joints, board sagging and a general deterioration in the overall appearance of the ceiling. The introduction of trussed rafters has meant a significant increase in the amount of ceiling problems. Frequently, these complaints reflect on the plasterer and not on the construction in general.

AS/NZS 2589:2007 REQUIREMENTS
Where the design of the ceiling is such that framing members must change direction the Australian and New Zealand Standard has the minimum requirement that trimmers be installed. The best solution is to install metal furring channel systems.

LARGE CEILING AREAS
In open plan designs in which large ceilings flow into passageways or where the ceiling flows from room to room, the plasterboard will be subject to differential movement.

RESULTS
The joints at the junction of two separate ceiling areas will invariably crack. It is generally pointless to simply repair the crack, as this kind of failure normally recurs as the ceiling framework responds to normal building movement.

RECOMMENDATION
Either introduce a bulkhead, door head or control joint, to cater for the unavoidable movement and continuous cracking that will occur in these areas.

WALL FRAMING
All studs, plates and noggins must be in alignment. Metal bracing if used must be checked into the framing to achieve the same alignment as the framing.

RESULT
Failure to achieve alignment will result in valleys and high points in the finished wall sheeting, with subsequent misalignment of skirtings and cornice when fixed later.

Irrespective of the condition of timbers when delivered to site, it is important that when the plasterboard lining is applied, the moisture content of the timber is below 16%. The practice, common in some parts of Australia to apply plasterboard immediately following framing, but before external brickwork is erected, is not acceptable as the framing may have absorbed moisture during construction as a result of ambient humidity and/or inadequate weather protection.

TIMBER FRAMED WALLS ON SAME ALIGNMENT & FIXED TO MASONRY WALLS
Masonry external walls are becoming a common construction method, in the contemporary house. Frequently, an internal timber/metal framed wall will abut an external wall on the same alignment. The Plasterboard is often shown to be continuous over the two different substrates.

RESULT
The finished wall will have an approximate 5mm step in it at the junction of the two dif-ferent substrates, as the use of adhesive fixed plasterboard over masonry will build out the plasterboard by approximately 5mm. Cracking will also tend to occur at the junc-tion due to the differing expansionary move-ments of the two substrate materials.

METAL FRAMING
These recommendations are not intended to be used as a specification but if followed will greatly help in satisfactory installation with metal-framed construction. Metal frames are prone to greater thermal movement than timber but tests have shown that such movement more closely mirrors plasterboard thermal performance than timber framing.

INSTALLATION
Plasterboard fixing instructions are generally similar to timber frame construction. The following good practice recommendations apply for both framing systems.

FINISHING
Paper tape only to be used for jointing when applied to framing.

FOIL BACKED MEMBRANE
In Australia apply foil backed building paper to roof areas and to external walls.

EXPANSION JOINT TO BE USED

• Where areas of passageways adjoin large rooms or at room junctions where no door heads are used.
• In stairwells at the floor line, covering gap with moulding fastened to one edge.
• Where plasterboard is installed in patios or soffits.

BACK BLOCKING
It is recommended that back blocking be used on plasterboard ceilings as it produces the strongest possible joints and dramatically reduces the incidence of joint cracking and distortion.

For a full description of back blocking see 'FIXING AND STOPPING'

RECOMMENDATION

• Select light colours (reflective) for final roof finish.
• Ventilate roof space with soffit vents.
• Consider ridge or gable ventilation.
• Mechanical ventilation may be considered.

In Cooler Climates condensation and fungal growth must be considered. Roof space ventilation is again an important step in eliminating problems. Insulation should be installed in order to keep materials above dew-point temperatures

MASONRY WALLS
Drywall Masonry Systems, popularly known as "stick-ons", are alternatives to solid plastering and provide significant advantages over the use of wet trades.

The success of this system relies totally on adherence to manufacturer's recommendations.

The following comments should be used as a guide to ensure proper bonding of the boards to the wall.

• Ensure boards are stacked flat prior to use.
• Remove any bond breaker or release agent from cast concrete faces.
• Provide a ventilation gap top and bottom of approximately 10mm.
• Use temporary fixing or support to hold sheets in position until the adhesive sets.
• The packers used for keeping sheets off the floor are not to be removed until the adhesive is completely dry.
• Minimise the movement of boards when levelling after initial contact. Once contact with the adhesive has been made the boards should not be pulled back.
• Excessive porosity of the background may cause premature dryout of compound and result in bond failure.
• The use of a bonding agent is recommended over porous surfaces.
• Maximum wall height of 3 m.