Back-blocking is required in Level 4 and 5 Finishes where three or more recessed joints occur in a continuous ceiling area. Cut back-blocks of at least 200mm width and long enough to fit loosely between the framing members.

Apply Gyprock BASE COAT or Gyprock CORNICE CEMENT to the back-blocks with a notched spreader to form 6mm x 6mm beads at approximately 20mm centres at right angles to the joint, over the entire face of the back-block. Apply Gyprock sheets with the long edges at right angles to joists or battens. Place back-blocks along the full length of the sheet edge.

As soon as all the blocks are in position, install the adjoining sheet. Back-blocks must be adhered in position before the joints are finished. Back-blocks may also be applied by working above the ceiling after the sheets have been fixed.

Back-Blocking of Butt Joints on Ceilings

Where butt joints are positioned between framing members, the sheet ends are reinforced and depressed, forming a recess to allow jointing.

• Sheet ends should be neatly cut and butted together within 50mm of the centre line between the ceiling joists or other main support members.

• Bend sheet ends upwards using temporary battens and packing such as nails to form a 2mm deep depression (as illustrated).

• Reinforce the butt joint by back-blocking with pieces of Gyprock plasterboard cut to fit neatly between the framing. Back-block along the full length of the joint, extending a minimum 50mm past each end of the joint over the adjoining sheets.

• Fix back-blocks with Gyprock BASE COAT or Gyprock CORNICE CEMENT as described earlier.

• Allow the adhesive to set for a minimum of 24 hours before removing the temporary battens. A hollow formation suitable for jointing remains.

Jointing and finishing of Gyprock plasterboard installations is to be carried out in accordance with the Gyprock Installation Guide and the following details, to provide a smooth surface for decorating.

Where stopping and external corner beads are required, these are to be applied to all edges subject to damage.

• A Level 4 Finish is generally the accepted level of finish for domestic construction (as detailed in AS/NZS2859.1:1997 Clause 6.6) and requires a three coat system, consisting of:

• tape (or base) coat

• second coat, and

• finish (or topping) coat.

Gyprock jointing compounds are classified as either setting type or drying type. Setting type compounds produce stronger joints and reduce installation delays and shrinkage associated with drying type compounds. They are recommended for experienced trades people and have a defined setting time e.g. 40-60 minutes.

Setting type compounds are:

BASE COAT 45, BASE COAT 60. Additional coats may be applied over setting type compounds once they have gone hard (set), usually 40 minutes to an hour. A drying type compound must be used as a finish coat and must be completely dry before sanding. This may take up to 24 hours.

Drying type compounds are:

WET AREA BASE COAT, JOINTMASTER TOPPING, TOTAL COAT-LITE, TAPE and TOPPING, EASY FINISH, and PRO-LITE TOPPING. These products are premixed and TOTAL COAT-LITE is also available dry.

Drying type compounds will shrink and harden with evaporation of water. The joints must be allowed to set and appear completely dry before re-coating or sanding. Actual drying times will be extended in low temperature and high humidity conditions. Do not use a setting type compound over a drying type compound.

All compounds can be applied by hand or with mechanical jointing tools.

Jointing Tape

Gyprock Perforated Paper Tape has been developed to enable the preparation of strong joints and should be used on all butt and recess joints

Site Mixing of Compounds

The first step to achieving good jointing is proper mixing of the compound.

• Always use clean containers and mixing equipment.

• Always use clean water of drinking quality.

• Never mix different compounds together or mix old batches with new ones.

• Follow mixing instructions printed on each bag

• Use compounds before the printed Best By date printed on the packaging.

When setting type compounds are used during hot, dry conditions, rapid evaporation of water and increased absorption by the lining surface can prevent the compound from setting correctly. This will result in the compound being soft and weak.

If jointing must be carried out under severe drying conditions, only small quantities of compound should be mixed. The compound should then be left standing for approximately 15 minutes to ensure that it sets soon after application to the joints. Additionally, depending on the severity of the drying conditions, the surface of the area to be jointed may require wetting with a brush before applying the compound.

Drying-type compounds should not be used when the interior temperature is less than 10C. Jointing Hand Applied.