Plasterwork

Internal Finishing

Blockwork finishes – Internal

There are 2 methods that can be used for internal wall finishing, namely wet plastering techniques and dry-lining with plasterboard. With wet plastering techniques, a certain amount of drying time must be allocated before decorating. This is not the case with dry-lining. Usually, the type of insulation being used will have a bearing on the method used, e.g. in hollow block construction where the insulation must be placed on the inside, dry-lining must be used.

The sound requirements for party walls in terraced houses or semi-detached houses should also be taken into account. Technical Guidance Document E of the Building Regulations defines the minimum density of a wall as 415 kg/m^2^ and the minimum thickness of plaster on both sides as 12.5 mm. The document states that concrete blocks of 1900kg/m^3^ combined with 13 mm of lightweight plaster each side will meet this requirement.

Alternatives

Please see detailed alternatives in the Technical Guidance Document E included in this app.

Finishing walls and ceilings

Using plasterboard-based finishes

Commonly internal wall finishes are provided by plastering methods incorporating plasterboard. Along with wall finishes, plasterboard also forms the basis of most ceiling finishes in domestic construction. Thereare 2 types of plasterboard-based methods, namely “dry” systems and “wet” systems.

“Dry” system

In this system, large plasterboard sheets that are 12.5 mm thick are fixed to the walls and joint-filling methods are used to leave the boards ready for painting/decorating.

Finishing of ceiling

Scratch coat of bonding plaster to a thickness of at least 8 mm; scratch to form a key and finish ceiling with a 2 mm nominal thickness of finish coat. Finish smoothly using a steel trowel.

How to avoid cracking of plaster in ceilings

  1. Moisture content should not exceed 20%; ensure timber is properly dried.

  2. For the type of board being used, all joints should be caulked, taped, or scrimmed appropriately.

  3. Provide adequate support, noggings etc.

  4. In cut roof construction, ensure struts that prop purlins are supported adequately.

  5. Ensure sizing of joists is appropriate to their spans.

Alternatives

1) Tape and fill all joints and internal angles and leave ready for painting/decoration using a proprietary priming agent or a slurry of joint finish.

2) Tape and fill all joints and internal angles and finish with a 2 mm coat of skimcoat plaster finish suitable for dry-lining boards.

Alternative procedures for wet plastering to blockwork

1)

  • Scud blockwork using a thick slurry of 1:2 sand cement ratio to a depth of 3-5 mm.

  • Apply a scratch coat of 1:1:6 (cement:lime:sand) or 1:6 (cement:sand) with plasticiser to a depth of 10-16 mm. Scratch the surface thoroughly to form a key, then, once dry, finish with a 2 mm coat of gypsum plaster.

Failure of the gypsum plaster finish may occur if the scratch coat is not allowed sufficient time to dry.

2)

Apply a coat of proprietary gypsum base coat plaster to a depth of 9 mm; scratch well to form a key. Finish with a 2 mm coat of gypsum final coat plaster once the base coat has set adequately. Ensure base coat has dried fully before plaster finish is applied. Correct application of the base coat is essential to ensure finishing coat does not fail.

Cracking in sand/cement base coat due to shrinkage causes delamination/boasting of the finishing coat. Poor scratching; no key provided for finishing coat. Good scratching; good key formed for finishing coat.

Fixing

Fixing Plasterboard to Walls

Fix the boards with the bound edges vertical to vertical battens located at less than 600 mm centres. A continuous horizontal treated batten fixed at no more than 450 mm centres should be provided at the top and bottom of each sheet to act as a fire stop for the wall to ceiling junction and for proper fixing. Window or door reveals should be reinforced with angle bead or with flex corner tape.

Table E1 - Maximum joist spacing for ceiling plasterboard

Wet system – 1 coat

Finishing of ceiling

Install tapered edge wall board, then tape and fill all joints as outlined. Finish the ceiling with a 2mm finish coat; smooth with a steel trowel.

Finishing of walls

Apply a coat of gypsum board finish to a thickness of 3 mm. Finish smooth using a steel trowel and ensure that all reveals are accurately formed. Never install slabs until house is closed to avoid damage resulting from dampness; i.e. ensure the roof is completed and doors and windows are installed and sealed.