9.2.3 Plastering
Plastering shall provide an adequate substrate for the decorative finish.
The design, preparation and application of plasters should be in accordance with BS EN 13914-2 and BS 8481. 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. Definitions and requirements’ |
BS 5270-1 | ‘Bonding agents for use with gypsum plasters and cement. Specification for polyvinyl acetate (PVAC) emulsion bonding agents for indoor use with gypsum building plasters’ | |
Metal laths and beads | BS EN 13658-1 | ‘Metal lath and beads. Definitions, requirements and test methods. Internal plastering' |
BS EN 13658-2 | Metal lath and beads - definitions, requirements and test methods. External rendering' |
The background should be:
- given an appropriate treatment before plastering, in accordance with BS 8481 and BS EN 13914-2
- 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, eg 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, concrete or calcium silicate bricks and blocks and dense concrete (including soffits) | Depending on suction and bond; may require use of a spatterdash pre-treatment or bond with polymer-modified cement mortar or metal lathing |
Lightweight clay block | The suction should be checked to ensure that it is similar to that of normal clay bricks. If not, special precautions may be necessary |
Mixed backgrounds, eg concrete with bricks/blocks | In accordance with BS EN 13914-2. May require either a carrier system or reinforced undercoat to reduce the effect of differential movement |
Lightweight concrete blocks | A polymer-modified spatterdash or bonding agent is recommended, depending on the plaster and the smoothness and suction of the blockwork |
Autoclaved aerated concrete blocks | Plastering should be conducted in accordance with the manufacturer’s recommendations, accounting for the moisture content of the blocks. Depending on the absorption characteristics, the use of a suction reducing treatment may be required |
Normal clay brickwork | May require raked joints or the use of keyed bricks |
Concrete and calcium silicate brick - or blockwork | Dependant on the suction and bond, may require use of a spatterdash pre-treatment or metal lathing |
Plasterboard | Guidance is contained in BS 8000-8 |
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.
At localised areas 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 |
Last updated: 2nd January 2024