Fixing & Stopping

The process of installing plasterboard essentially comprises fixing of the sheets and stopping of the joints. Nails or screws are used to secure the sheets at times in combination with daubs of adhesive. The stopping process comprises of three stages. The first entails the application of a base coat into which tape is bedded to reinforce the joints. A further coat of compound is applied over the first followed by a topping coat to complete the stopping. The final coat is sanded when dry to give a smooth finish.

Proper fixing practice will ensure long term serviceability and minimise maintenance. Poor application of plasterboard can result in joints that are impossible to conceal, sagging ceilings, popped fixings etc.

The commencement of plasterboard installation is better left at least until all outside cladding is complete. That should also include the installation of ridge capping. gable ends etc., thus avoiding the possibility of weather damage to both framing and linings.

Plasterboard should not be fixed to timber substrates unless the moisture content is lower than 16% (range of 12-16% for NZ).

The positioning of joints in relation to artificial and glancing light sources is most important. Often the contractor will have no choice in the direction of ceiling joints without the cost of installing battening (See leaflet - GLANCING LIGHT).

 
KEY POINTS TO CONSIDER
Usually fix ceilings first. Butt joints should be kept to a minimum.

Stud adhesive and fixing points to be as per manufacturer's instructions.

All boards should be fixed a minimum of 6 mm above floor level.

Ensure adequate fixings around all openings and the perimeter of the boards.

No joint should be placed on window or door studs (i.e. in line with reveals). Use archi-traves rather than grooved reveals.

All sheets must fit snugly together - but not forced.

METAL PLASTERERS ANGLES
Internal Plasterers angles, although not standard manufacturer's specification, can, if used correctly, add strength to internal corners and accuracy to the final job.

BACK BLOCKING
Back blocking is a system where pieces of plasterboard are laminated on site, to the back surface of the sheets, behind joints using either base coat or cornice cement.

Back blocks are used on recessed joints in ceilings as reinforcement against stressed joints resulting from building movement and / or timber shrinkage which can cause peaking and cracking.

Back blocking is recommended by all plasterboard manufacturers. The Australian and New Zealand Standard 2589.1 has a minimum requirement that it shall be used for all butt joints and in any area with 3 or more recessed joints. The decision as to back blocking of all joints not just the minimum requirement in the Standard is best made by the builder after consideration of factors such as:

• moisture content of timber
• maintenance history for similar installations
• type of roof framing and timber used
• severity of temperature and humidity conditions to which the installation will be subjected.

STOPPING OF PLASTERBOARD
A three coat system of jointing is recommended for normal work and complete drying between coats is essential for air drying compounds.
Manufacturers produce a wide variety of compounds for this purpose. The use of the appropriate material is important with material applied in accordance with manufacturer's recommendations.
Paper jointing tape is recommended for the strongest possible joint reinforcement. The use of mesh tape is not recommended.

FINISH
Where a wall or ceiling is subject to severe lighting, extreme care must be taken when finishing joints and fixing holes etc. Wall and ceiling joints should run at right angles to a natural light source wherever possible.
Special attention should be given to the fixing of metal accessories: - arch beads, external angles etc, to avoid future maintenance problems. Any reliance on jointing compounds for the securing of these components may lead to eventual problems. Ensure that fixings are adequate.

JOINT SHRINKAGE
Be aware that as water evaporates from air drying compounds there may be slight shrinkage. This shrinkage is directly proportionate to the amount of water used in mixing.
Do not use thinned compounds, otherwise, starved or hollow joints may result.

KEY POINTS

1. Any gaps in joints or angles exceeding 3mm should be pre-filled with a setting type compound before taping.
2. Retarders must not be used in basecoats.
3. Remove any loose paper from butt joints and ends of boards, internal angles etc.
4. Fibreglass tape will not resist movement of the framing timbers. (Its use in ceiling and butt joints should be avoided).
5. Care must be taken when fixing metal external angles to minimise build up on skirting line etc.
6. Selection of the appropriate grade of sandpaper to suit particular jointing materials is important.

THE FOLLOWING LEVELS OF FINISH ARE AVAILABLE FOR SPECIFIC FINAL DECORATION CHOICES

Standard AS/NZS 2589.1 Gypsum linings in residential and light commercial construction - Application and finishing Part 1: Gypsum plasterboard provides a choice of different levels of finish, which allows flexibility of application.

Unless otherwise specified compliance with the Standard is achieved by a Level 4 finish for all gypsum plasterboard except for those areas which are non-visible and generally non habitable (e.g. non walk in cupboards, concealed storage areas and non livable attics) where a level 3 finish is acceptable.

LEVEL 4
This is generally the accepted level of finish for domestic construction. It is used where light textures or wall coverings and smooth textured finishes are illuminated by critical lighting and where smooth textured finishes and flat/low-sheen/satin paints in light colours are illuminated by non critical lighting.

DESCRIPTION OF LEVEL 4
All joints and internal angles shall have tape embedded in the joint compound and additionally two separate coats of joint compound applied over all tapes, angles, fastener heads and accessories (3 coat system). All joint compound should be finished smooth and free of tool marks and ridges.

OTHER LEVELS OF FINISH
The Standard does provide other options for levels of finish to suit specific circumstances. These are summarised as:

Level 0 - Temporary Construction

Level 1 - Usually Concealed

Level 2 - Surface appearance not of primary concern

Level 3 - Heavy Coatings

Level 5 - Gloss/Semi-gloss finish or dark colours specified or critical light conditions occur on flat, low-sheen or satin paints (see separate leaflet SPECIFICATION CHOICE - LEVEL 5 FINISH).