Plastering is a process used to produce an acceptable final wall or ceiling finish to a building prior to decoration.
A substrate is the surface to which the plastering system is to be applied.
Backing coats which cover the substrates are between 9mm and 40mm thick under normal circumstances and the overall thickness can be built up in coats as is necessary.
Ceilings and Partitions
In older building, these areas are covered in a layer of timber laths which are plastered over.
When stregnth is required, a thin steel sheet mesh called expanded metal lath is fixed in place and then plastered over.
In contemporary fast-track work plasterboards are fixed in place, the joints taped with a fibre mesh tape (scrim) to prevent cracking and then skimmed over with a finishing coat of a gypsum plaster.
A basic plasterboard is a flat sheet of gypsum between 9mm and 12.5mm thick, sandwiched between sheets of durable paper. Boards are also available with various backings of differing thickness to improve insulation, restrict the passage of water vapour and to protect against fire.
It has become common practice to bond or fix plasterboards to substrates to serve as the backing plaster coat. This method is known as dot and dab or drywall.
Finishing coats are usually between 3mm and 5mm and bring the backing coats to a smooth skim finish. Finishing plasters used to produce these setting coats are available as premixed gypsum plasters.
Finishing coats can also be lime putty and fine sand or a mixture of lime, sand and gypsum known as gauged setting stuff.
Solid plastering denotes backing coats of a paste or mortar-type consistency comprised of an aggregate and a binding matrix. Binders include lime, gypsum and cement. Aggregates include sand, vermiculite and perlite.
The Plasterer is a skilled craftsman who can spread plastering materials over a substrate producing a finish as required by the client.
Although plasters can be projected onto walls with pumps or sprays and one-coat plasters are available, most high class work still applied by the plasterer using hand tools in 3 coats consisting of a scratch coat to even out suction, a float coat to give an even surface and a finishing coat to produce a smooth finish.
Choosing a Plastering Material
Lime plasters are compatible with old buildings containing slight dampness. They are said to breathe, which means they allow moisture to evaporate from the wall before it shows as damp patches. It is always worth considering why moisture is present in the wall in the first place?
Gypsum plasters set much quicker than lime plasters, so speed up waiting times between coats. In cold conditions gypsum backing plasters can take a long time to dry. Gypsum backing plasters (and this includes standard plasterboards) should never be used on damp walls.
Gypsum plasters are compatible with dry brickwork or blockwork, preferably to internal surfaces of cavity wall construction.
Cement-based plasters or renders have their uses, they can be gauged with lime and used externally, and can be useful for waterproofing and tanking. Strong mixes can pull away from weak backgrounds and are brittle – cracking if slight movement of the building occurs, they can also be cold and attract condensation.
Decorative plasterwork has a long history. Mouldings were originally run in-situ with ornaments cast in carved boxwood, pear-wood or hard plaster and gelatine moulds using composition materials and gauged setting stuff.
At a later date fibrous plastering techniques arrived, first used in France, the technique involves the use of Plaster of Paris with hessian/canvass scrim to reinforce casts and timber laths to assist fixing.
With some timber, sheet steel, moulding compounds and a few bags of casting plaster, all manner of ornamental plasterwork can be created.
There are times when consideration should be given to not applying backing plasters to certain substrates.
Aggressive salts can damage plasters and can absorb moisture from the air, these salts are said to be hygroscopic.
Areas with contaminated walls can be farm buildings, fertiliser stores, coal stores or flues – where combustion of sulpherous fuels has taken place. In some instances dampness from ground water will leave problematic salts in the wall.
In extreme cases, most backing plasters are unsuitable for direct application to these areas. Chemical treatments are available which are said to neutralize these aggressive salts, but analysis and treatment can be difficult and costly – and even unreliable.
It is sometimes preferable to isolate the contaminated substrate with a meshed plastic membrane and plaster over the top.
Lovers of lime will not like the membrane method but it works and plasterers and surveyors with experience of such contaminated areas often have bitter experience of backing plaster failures of all kinds in such situations.
Article Written by John Bradshaw
John Bradshaw is a Director of High Peak Remedial Service with over 25 years experience as a remedial treatment surveyor. John holds a CSRT Surveyors Qualification as required by the PCA, the CIOB certificate and diploma in site management and has received training from the Phil Hewitt School of Waterproofing.