Plaster and Render

CONTENTS

Mineral-based plasters can be divided up into plasters and renders for outdoors and plasters for indoors, and into hand-applied or machine- applied plasters.

Plasters and renders for outdoor use have to be weatherproof, withstand moisture and temperature changes, and provide adequate protection against driving rain. These requirements are met particularly well by cement-based and lime-based plasters, and by synthetic resin and silicate dispersion plasters.

Plasters for indoor use should, above all, be breathable, abrasion-resistant and suitable for decorating or covering with coatings, wallpaper and tiles. Interior plasters are usually based on gypsum or gypsum- lime or lime-cement or gypsum cement skim, but recently, clay-based plasters have been making a modest impact on the market owing to their excellent environmental credentials.

1 Components

 

Basic Components

Binding agent

• When mixed with water, binders become adhesive and hold the components of the plaster together
• Binders are responsible for a plaster’s strength, moisture resistance and permeability.
• Binder materials include, cement, lime, gypsum and clay

Structural filler

• The filler makes up for most of a plaster’s volume and is the strongest component
• Sand is the most common structural filler, though other aggregates can be used.

Water

• When water is added to a plaster / render mix, binding agents cause the filler particles to adhere to each another, creating a pliable, cohesive material that spreads smoothly and bonds onto walls. As the water evaporates, plasters set, or cure, and the binding agents create a hard, protective finish.

 

Other Components

Fibre

• Provides tensile reinforcement and improves elastic performance
• Adds bulk to the plaster, enabling it to be applied in a thicker coat
• Fibre materials include: Hair (traditionally horse hair), hemp, straw, fibreglass fibres and mesh, plastic fibres and mesh, reed fibres and mat, steel fibres and mesh or ribbed mesh.

Additives

• Additives are added to improve plasticity, workability, durability, curing time or provide colour.

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2 General Characteristics

 

Airtightness

External rendering or internal plastering in the form of ‘parge coating’ on brick / block backgrounds can be useful in effecting an air seal - providing there is continuous coverage. The application of a ‘skim’ coat to plasterboards can also prove effective.

Moisture control

Where structures are sensitive to moisture, it is important to understand the dynamics of plaster permeability. Tests conducted by John Straub (see download below) revealed these performance characteristics:
• Cement:sand renders are relatively impermeable – to the extent that at 38mm it can act as a vapour barrier.
• The addition of lime to a cement render may increase permeance dramatically.
• Pure lime:sand renders are very vapour permeable.
• Clay renders are more permeable than lime plasters. The addition of straw increases the permeability further.
• Lime washes appear to be somewhat useful in reducing water absorption while not reducing vapour permanence.
• Siloxane (a plastic coating) appears to have little or no effect on the vapour permeance of cement, cement:lime, lime and clay plasters whilst almost eliminating water absorption.
• Sodium silicate does not seem to have much of an impact on water absorption or vapour permeance.
• Applying a coat of oil paint to render, plaster or plaster skim can provide an effective vapour barrier. (Vapour barriers are normally best applied internally to prevent internal moisture entering the construction, rather than externally preventing it from escaping).

Hygroscopicty

Materials characterised by the expression ‘Hygoscopic’ are those that absorb moisture, into the material itself store it, and at a later time, release it.

Moisture mass

Materials characterised by the expression ‘Moisture mass’ (aka ‘Moisture storage capacity’) are those that absorb moisture, store it, and at a later time, release it. This is a particularly important characteristic in inadequately insulated buildings where intermittent heating and cooling can leave condensation on surfaces where it can activate ever present spores which grow into mould. Moisture mass will draw the moisture from the surface preventing the spores being reactivated.

Thermal mass

Materials characterised by the expression ‘Thermal mass’ (aka ‘Thermal storage capacity’) are those that absorb heat, store it, and at a later time, release it. Large surface areas of relatively thin, dense internal clay plasters or cement renders can provide thermal mass at the surface to help store passive heat gains.

Sound absorption

Plasters generally have a low sound absorption factor of 1 – 3%, though ‘acoustic plasters’ are available that can increase this level to 25%.

Fire protection

Plaster finishes are non-combustible. Combinations of plasterboards and particularly vermiculite or perlite gypsum plasters are used in fire protection construction.
 

 

3 Types of plaster / render

Cement render

• Application: used internally and externally, particularly in applications where an extra-hard or water-resistant surface is required. Cement render is usually a cheaper alternative to Lime or Polymer renders.
• The rigidity of cement render usually limits it's application to older buildings, where flexibility of finish is usually required to overcome structural movement.
• Usually 'Portland' cement is made from calcium carbonate (limestone) and small amounts of silica and alumina, typically from clay or sand.
• Portland cement is a specific type of hydraulic lime.
• Cement render is currently the most common form of render.
• It is mixed as a ratio of cement:lime :sand and often includes admixtures to enhance its flexibility.

Characteristics

• Much harder and stronger than lime render.
• It is has a low vapour permeability.
• Cement plasters are generally inflexible. Using them on 'flexible' backgrounds such as wood and straw will invariably lead to cracking.
• Cracking at junctions such as openings and corners can be avoided by using reinforcing lath such as expanded metal.

Environmental considerations:

 

Polymer modified cementitious / proprietary renders

• Application: used externally as a more modern alternative to traditional cement render. Adding silicone fibre (to the cement) imparts a high degree of water repellence whilst allowing water vapour to pass through the render and lets the substrate breathe.
• Backgrounds / substrates: block, lightweight block, modern brick walls, insulation boards, metal lath and SIPs.
• Usually 'Portland' cement is made from calcium carbonate (limestone) and small amounts of silica and alumina, typically from clay or sand.
• Portland cement is a specific type of hydraulic lime.
• Often used in conjunction with a nylon reinforcing mesh to provide high-strength base coat.
• Polymer renders are available as 'single-coat' renders - eliminating the need for a base coat - but backgrounds in such applications are limited to, usually, lightweight / aircrete blockwork.

Characteristics

• Low vapour permeability.
• Good weather resistance / waterproofing. Improved resistance to freeze-thaw over cement render.
• Better bonding characteristics than cement render
• Improved flexibility over cement render, but reinforcing lath still required at junctions and around openings. Expansion joints are usually required for larger areas and should be specified according to the manufacturer's recommendations.
• Wide range of 'through-colouring' in pre-blended batches

Environmental considerations:

 

Thin Coat renders

'Thin coat' render has become increasingly dominant in the render market through offering far superior performance and durability to cementitious renders . It is a form of polymer render, typically containing, in addition to silicone, quartz and calcite reinforcing fillers. Thin coat render requires sealing or coating once cured. The complete render system is applied in two coats with the first coat (base coat) incorporating a nylon or glass fibre reinforcing mesh, and a finishing coat typically based on an acrylic, silicate or silicone formulation. The total thickness ranges between 8mm and 12mm.

• Application: where flexible, crack resistant performance is critical eg metal / wood frame structural systems, ventilated facades, insulated render systems.
• Background / substrate: insulated render systems, mineral-based carrier boards, blockwork, insulation slabs. Other substrates may be suitable.

Characteristics

• High flexibility provides resistance to cracking
• Lightweight
• High durability
• Good water-shedding properties
• Systems can be produced in a wide range of textures and colours

Environmental considerations:

Lime render and plaster

• Application: External rendering; internal plastering
• Lime is derived from heating and calcining limestone (calcium carbonate)
• Limestone contains varying concentrations of magnesium carbonate, for the best plaster, this concentration should be kept to less than 5%
• Lime plaster is made from lime putty combined with sand and fibre
• The only significant difference between lime putty and hydraulic lime (aka ‘bagged lime’) is the amount of water that has been added to them.

Characteristics

• Lime plasters are less rigid and brittle than cement plasters
• A reduced tendancy to cracking makes lime ideal for flexible substrates.
• Hairline cracks are ‘self-healing’; larger cracks can be filled with limewash.
• Lime plaster is vapour-permeable – useful in ‘breathing wall’ construction. It can store water and dry out easily.
• Lime plaster is relatively sensitive compared with cement – it must be carefully stored and applied.
• Lime plaster is more difficult to work with than cement plaster. Skilled tradesmen are at a premium.
• Lime plasters are generally ‘sealed’ with limewash. This procedure must be repeated every 4-5 years on exposed facades and around 25 years on protected facades.
• Additives such as sand, powdered limestone, perlite, and others are used to improve the plaster’s technical characteristics depending on the product (less than 1%).

 

Gauged plaster

• Gauged plaster, for internal spaces, addresses the issue of the long curing period associated with pure lime plaster. Small amounts of gypsum are added into a lime: gypsum : sand mix to achieve a shorter curing and drying time.

 

Environmental considerations:

Gypsum plaster

• Application: internal surfaces only.
• Gypsum is a naturally occurring mineral composed of calcium sulphate and water (2CaSO₄.2H₂O).
• Gypsum can also be recovered as a bi-product of reducing the emission levels of coal-fired power stations. Currently categorised as ‘waste’ in the UK, Flue-Gas Desulpherisation (FGD), can be used as a construction material in its pure state.(*)
• Gypsum plaster is formed by heating the gypsum to remove around 75% of the water. This is known as a hem-hydrated gypsum or ‘Plaster of Paris’.
• ‘Plaster of Paris’ is referred to as a Class A unretarded plaster in BS 1191. It sets too quickly to be useful in wall plastering.
• Setting times are lengthened by the addition of retarders resulting in BS 1191 Class B ‘retarded hemi-hydrate plaster’. It is this plaster that is the basis for surface-covering gypsum plasters.
• Class B plasters are divided into four types:
• Type A: Undercoat plaster (plaster with sand) including ‘browning’ and ‘metal lath’ plaster.
• Type B: Final coat plaster (using finer powder than Type A) including ‘finishing’ and ‘board finish’ plaster.

Characteristics

• Gypsum plaster is vulnerable to moisture - it can have a porosity over 50 per cent by volume. It is not suitable for external applications or internal applications where dampness occurs.
• Plasters can be formulated to suit a wide variety of background types. (Primer/sealers can deal with some backgrounds)
• Gypsum plaster expands as it dries which reduces cracking.

Environmental considerations:

• The UK gypsum plasterboard sector is engaged in continuing to make a concerted effort to develop schemes to divert gypsum plasterboard offcuts products away from landfill. Recovered gypsum can be combined with fresh gypsum or FGD to produce new gypsum plasterboard.

 

Lightweight Plasters

Lightweight plasters, weighing as much as 60% less than sand-based plasters, substitute lightweight aggregate for sand. Aggregates include exfoliated vermiculite and expanded perlite. These aggregates are mixed with Class B gypsum plasters for undercoat and finish coat applications. Characteristics of lightweight plasters include improved fire resistance and insulation performance.

(*)’Flue gas desulphurisation (FGD) gypsum in plasterboard manufacture’ – Dr AM Dunster, BRE, 2007
 
 

Clay plaster and render

• Application: external render in protected conditions and where structural humidity protection is very good; internal plaster.
• Clay is the oldest form of render / plaster
• The binder is clay which, unlike cement and lime, does not chemically change during drying and can be reactivated by water.
• Clay plasters have traditionally been derived from on-site materials, but in recent times it has been more common to use manufactured plaster.
• Because of its minimal processing, site-sourced clay binders can vary greatly. Experimentation along with experience of locally obtained material is usually required.
• Clays vary in colour, particle size, shrinkage/expansion and binding strength.
• Commercial clay plasters are characterised by predictable qualities.

Characteristics

• Clay plaster is ‘hydrophilic’ ie it attracts water
• Clay expands when wet, the wetter it becomes, the more it resists moisture. This makes it an ideal plaster to use in contact with wood or straw
• Clay plaster can be wetted and re-worked indefinitely
• Weather erosion is a characteristic of clay render. The rate of erosion is controlled by the exposure to rain (particularly driving rain) and structural protection such as deep overhangs and rain screens.
• Erosion can be further inhibited by the inclusion of fibres such as chopped straw.
• Applying several coats of limewash can also increase the mechanical strength of clay after wetting ie, they will increase the resistance to rain erosion
• Sometimes a top coat of lime render can be used to protect the clay – though care should be taken over the bonding between dissimilar materials.
• Clay plaster regulates temperature and air-moisture.
• Dyed commercial plasters, plaster skim and plaster finishes usually contain natural pigments..
• Clay plaster is not as resistant as other plasters, but damage can be repaired relatively easily.
• Adding linseed oil to a clay plaster mix is ineffective as a water repellent. However heavy applications of the oil to the surface of finished clay plaster will reduce water absorption to almost zero, but will markedly decrease vapour permeance.

Environmental considerations:

Render and plaster products:

• L551 Plaster & Renders

 

Further information

• Lime mortar, render and plaster
• Technical glossary & terms for plastering, rendering & repair trades
(www.refina.co.uk/)

Movies

 

Plaster and Render products on GreenSpec
 

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