Wall and ceiling finishes

9.2

9.2.3Plastering

Plastering shall provide an adequate substrate for the decorative finish.

Materials for plastering should be in accordance with BS 8481 and those listed in Table 1.

Where plaster is intended to contribute to fire resistance or sound insulation, overall performance should be in accordance with the building regulations.

Table 1: Materials for use in plastering

Plasters	BS EN 13279	‘Gypsum binders and gypsum plasters’.
	BS 5270-1	‘Specification for polyvinyl acetate (PVAC) emulsion bonding agents for indoor use with gypsum building plasters’.
Metal laths and beads	BS 405	‘Specification for uncoated expanded metal carbon steel sheets for general purposes’.
	BS EN 13658-1/2	‘Metal lath and beads. Definitions, requirements and test methods’.

The background should be:

given an appropriate treatment before plastering, in accordance with BS 8481

suitably finished to provide an adequate key

checked to ensure adequate and even suction

sufficiently even to provide a reasonably flat plaster finish (excessive ‘dubbing out’ should be avoided).

Mixed background materials and associated differential movement can lead to cracks and should be avoided. Suitable precautions should be taken, e.g. using metal lathing.

Metal beads should be used to provide edge protection, and be fixed with zinc-plated fasteners, as recommended by the manufacturer.

Table 2: Recommended treatments for substrates

Surface	Treatment
High-density clay, or concrete bricks and blocks and dense concrete (including soffits)	Suitable bonding treatment, hacking, spatterdash, or stipple.
Mixed backgrounds, e.g. concrete with bricks/blocks	May require expanded metal to provide key for plastering and to reduce the effects of differential movement.
Lightweight concrete blocks	Plaster should not be stronger than recommended by the blockwork manufacturer.
Autoclaved aerated concrete blocks	Plastering should be conducted in accordance with the manufacturer’s recommendations, accounting for the moisture content of the blocks.
Normal clay brickwork and concrete block	May require raked joints or the use of keyed bricks.
Plasterboard	Guidance is contained in BS 8212.

Where services are to be concealed by plaster, they should be:

completed and tested before plastering

protected against the adverse effects of chemical action or thermal movement.

To avoid surface cracking; metal lathing or wire netting should be used where there is an insufficient depth of plaster.

The plaster mix should be:

as specified, or as recommended by the plaster manufacturer for the particular location and use

appropriate for the strength and surface characteristics of the background

an appropriate quality for the intended finish

checked to ensure undercoats and finishing coats are compatible

applied by suitably trained operatives (specifically where plastic compound finishes are used)

of a type that does not include Portland cement and gypsum plaster in the same mix.

When plastering:

completed work, especially timber, chipboard and glazing, should be protected from damp and damage

in cold weather, follow the guidance in chapter 3.2 ‘Cold weather working’ (plasterwork damaged by frost should be removed and replaced)

dubbing out should be conducted well in advance of the application of the first coat

surfaces should be dry, clean and free from laitance, grease, loose material or substances likely to prove harmful to the bond or the intended finished appearance of the plaster

ensure plaster is thoroughly mixed but avoid prolonged mixing

avoid mixing excessive quantities of plaster (plaster should not be retempered)

the background surface of each coat should be fully set (the surface should not be overworked, and adequate time should be left between coats to allow strength and suction to develop)

the number of coats should be sufficient to achieve a reasonably plane finish

finished surfaces, reveals, soffits to openings, external angles, etc. should be in accordance with Chapter 9.1 ‘A consistent approach to finishes’

the plaster should be applied to a thickness, excluding dubbing out, in accordance with Table 3.

Table 3: Plaster thickness

	Surface to be plastered	Minimum number of coats	Thickness of plaster
Walls	Metal lathing	3	13mm (nominal from lathing)
	Brickwork	2	13mm maximum
	Blockwork	2	13mm maximum
	Plasterboard or concrete	1	Sufficient to provide a crack-free surface
Ceilings	Concrete	2	10mm maximum
	Plasterboard	1	Skimcoat