BS 5492:1990 download free.Code of practice for Internal plastering.
1 Scope
This code of practice gives recommendations for internal plastering on all types of background for use under normal conditions. Fibrous plasterwork is included.
It covers materials, types of background, preparation of the surface to be plastered, choice of a suitable plastering system, methods of application (including renovation work) and maintenance. Guidance on the use of projection plasters, lighting for plastering, types of proprietary plaster and lead times for fibrous plasterwork is given in appendices. Recommendations for plastering over warming panels. playing surfaces of squash courts and external rendering are not included in this standard.
NOTE The titles of the publications referred to in this standard are listed on page 42.
2 Definitions
For the purposes of BS 5492 the definitions given in BS 6100-6 apply, together with the following.
fibrous plasterwork
preformed section of plasterwork made from casting plaster, hemihydrate class A, and reinforced with scrim and timber or metal lath
3 Exchange of information
Drawings and specifications, and bills of quantities where appropriate, should be prepared in sufficient detail to afford proper guidance in the preparation of estimates and the execution of the work.
An exchange of information between those responsible for the execution of the backgrounds, the internal plastering and the subsequent decoration, and with other trades whose work will affect or be affected by the plastering, should take place as soon as possible after acceptance of contract. When preparing plastering details the designer should take into account the following points.
a) The choice of types of plaster (including plaster for specialists’ work) to be used in various parts of the building depends on functional requirements and the nature of the background. The nature of the background and its plaster finish should therefore receive consideration together. Particular information should be given where blockwork or brickwork is the background to be plastered. The manufacturer and type of block or brick and its density should be specified due to the effect on suction and the need to specify the correct type or grade of plaster and whether a bonding agent is required.
5 General
Materials and accessories should comply with
British Standards where these are applicable.
Clauses in this section either refer to appropriate
British Standards or give recommendations for
materials and accessories not covered by British
Standards.
6 Plasters
6.1 Gypsum plasters
6.1.1 Gypsum plasters, excluding premixed lightweight plasters and preinixed gypsum plasters
Gypsum plasters complying with BS 1191-1 may be divided into the following classes:
a) class A. Plaster of Paris, suitable for gauging lime final coats for patching and repair work and for fibrous plasterwork;
b) class B. Retarded hemihydrate plaster. Final coat plaster (finish, board finish and thin coat plasters).
Finish plasters are suitable for final coats, neat or gauged with not more than ¼ part of lime to 1 part of plaster by volume in two or three-coat work. Board finish plasters are suitable for neat work on board backgrounds.
6.1.2 Premixed lightweight plasters Premixed lightweight plasters complying with BS 1191-2 consist of retarded hemihydrate plasters incorporating suitable lightweight aggregates and other additions to impart desirable properties. They may be divided into the following types:
a) type A. Undercoat plasters (browning plaster, browning plaster suitable for high suction backgrounds, metal lathing plaster and bonding plaster;
b) type B. Final coat plaster (finish plaster).
6.1.3 Prenzixed gypsum plaster Premixed gypsum plasters are undercoat plasters consisting of retarded hemihydrate gypsum plasters incorporating aggregates and other additives to impart desirable properties.
6.2 Premixed lightweight cement plasters Premixed lightweight cement plasters are proprietary plasters based on cement and lime, premixed with lightweight aggregate.
6.3 Renovation plasters
Renovation plaster systems offer increased resistance to moisture and salts, e.g. during the drying of walls after the installation of a new effective damp-proof course.
21 Factors influencing the selection of a plastering system
The primary function of a plaster finish is to mitigate unevenness in the background and provide a reasonably smooth surface which is compatible with the required decorative finish and resistant to damage under the conditions to which it is likely to be subjected. Other functions that a plaster finish may be required to perform are to improve the fire resistance of a structural element or the acoustic or thermal properties of a surface or partition.
In selecting a plastering system for any particular purpose, consideration should first be given to the relative importance of these various functions.
Consideration should be given to the use of a cement-based final coat in some locations, i.e. walls to be tiled or walls in public areas liable to damage.
The background to which the plaster is to be applied should then be considered in relation to its strength, suction, bonding properties, liability to shrinkage or thermal movement, and probable content of water and soluble salts. ‘Fhe need for bonding treatment should be considered. It may be necessary to consult the blockwork manufacturer’s recommendations.
Other factors are the availability of suitable sand, the weather conditions that are likely to prevail during the process of plastering and subsequent drying before decoration, and the length of time available for these processes.
The characteristics of plaster undercoats and final coats are described in Table 1 and Table 2. Recommendations for suitable plastering systems are given in section 4.
Where non-traditional detailing, e.g. no architraves, is proposed, careful consideration should be given as to how this should be achieved.
Grounds should always be specified to ensure that plaster does not come into contact with the floor slab or screed.
Large areas of walls and ceiling, e.g. those found in school halls. should preferably be sub-divided into smaller areas by the use of design joints.
25.2.2 Gypsum plasters
Fully set, hardened and dry, neat gypsum plasters are among the least troublesome surfaces. Passage of salts from backing materials may, however, give rise to efflorescence and saponification difficulties. BS 1191 distinguishes the following classes of plaster (see 6.1.1):
a) class A. Plaster of Paris. This is not normally used for internal plastering. It is used in fibrous casting. For painting purposes it may be considered with class B;
b) class B. Retarded hemihyd rate plasters. These are generally neutral in reaction and even if gauged with lime, when dry, should not affect paints. The porosity may sometimes vary according to the nature of the background:
patchiness can usually be overcome as described in 25.2.1. In continuously damp conditions, retarded hemihydrate plasters are liable to the defect known as “sweat-out”, shown by softening or disintegration. Dampness caused either by condensation or by water from the background is best avoided by removal of the source. Under conditions of condensation upon walls or ceilings an impervious paint film is some protection for the plaster but will not eliminate condensation. Moisture held in the plaster by a relatively impervious paint film may be an alternative cause of “sweat-out”.
25.2.3 Plaster surface on boards
When retarded hemihydrate plasters are used on board backgrounds they present no risk of alkali attack, and thicknesses up to 5 mm can be expected to dry out rapidly and permit early decoration.
25.2.4 Premixed lightweight gypsum plasters These are based on retarded hemihydrate gypsum plasters with additions of lightweight aggregates,
e.g. vermiculite or perlite (see 6.1.2). They may he treated as for class B gypsum plasters (see 25.2.2) as there is little risk of any alkaline reaction. They have a more open texture but contain more water and they therefore take longer to dry out than do class B plasters.
25.2.5 Projection plasters
Projection plasters are based on a blend of calcium sulphate plasters and are applied neat in one coat. They present no difficulties in painting provided the background is dry when plastered. (See 6.6 and Appendix A.)