Plaster Work and Renders

Listed Buildings

Conservation in Tendring

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This advice is provided to guide those preparing applicationsfor planning permission and/or Listed Building Consent orconsidering repairs. More general advice on both ListedBuildings and buildings in Conservation Areas can be foundin the two advisory booklets which are related to this seriesof publications.

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Introduction – Plasterwork and Renders

Internal

Plaster is the term usually given to applied internal ceilingand wall finishes. Its finish is smooth and it serves to hidethe structure underneath. It tends to be applied in two orthree coats depending upon the substrate.

When used on ceilings it is applied over wooden laths inthree or more coats. Similarly, wooden laths may be fixed totimber stud partitions. Alternatively the plaster may beapplied direct to the masonry. For the purposes of thisbooklet traditional plaster was usually made from lime, eithernon-hydraulic (the traditional material which sets throughdrying in the air) or hydraulic, (a much later invention whichsets chemically with water).

On internal work ox, goat or horse hair was used forreinforcing or binding the base coats, although grass, waterreed or straw was also sometimes used.

Lime was combined with a filler in the form of well gradedsand or grit to make plasters. Other fillers such as brickdust, chalk or sea shell were also used. Such fillers usuallyoccupied about 70% of the volume of the mix.

Modern gypsum plaster is a common mineral consisting ofhydrated calcium sulphate, and is obtained from naturalsources or as a by-product of certain industrial processes. It is usually applied in two or three coats, with the base coatbeing the coarsest, and the finishing coat the finest. Thismaterial tends to be rather stiff, and waterproof, and it canprevent a wall from breathing. It should generally not beused for historic building repairs.

External

Render usually refers to the hard-wearing lime-based plasterwhich protects external walls from weather, but may alsoact as a decorative covering, to enhance architecturalquality. Sometimes it was applied simply to hide poorly builtwalls. As with plaster, lime was combined with sand or grit.The appearance is less shiny than internal plaster.

Roman cement is a chemically setting lime render which isbreathable and dense but not as inflexible as moderncement.

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18th Century plastered frontage, Great Clacton

15th Century timber framed and plasteredout building, Manningtree

Rendered cottages, Spring Road, St. Osyth

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Roughcast, by definition is a more roughly applied, lumpierrender mix, made up of large stony aggregates, can moresuitably be applied to ragged edges of existing render asthe joint between new and old work is more readilydisguised.

Modern Portland cement is sometimes, inadvisably andinappropriately, used to render old external walls. Somebelieve that because it is waterproof and strong, it will helpto ensure the structure remains dry and in good condition.This is far from true.

Render and Stucco

Render, usually a plain external wall coating has been in usefrom the very earliest times. Renders used for the walls ofold houses range from single low-strength lime coatingsapplied over wattle or lath to high strength cement - basedrenders applied to brickwork.

The term “stucco” encompasses many different applications- from covering exterior walls in imitation stone to fineplaster applied to interior walls or ceilings and the creationof all manner of decorative mouldings. The term is nowoften employed to describe the high-quality renders used to provide a cheap yet convincing substitute for stonework -brick or rubble masonry being coated to look like best-quality ashlar, complete with mouldings, quoins and cornices.

Stucco became highly fashionable in the late eighteenthcentury and was to remain popular throughout the Victorianperiod, being widely used for decorative window and doorsurrounds. Although the illusion was frequently reinforced bypainting to resemble nicely weathered masonry, there waskeen competition to produce stuccos that would drynaturally to match the colours of building stones. The materials used for stucco varied considerably, but inEngland in the early nineteenth century stucco finishes weregenerally lime-based renders applied as either two part orthree part coatings to building facades.

Towards the end of the nineteenth century a very differenttype of render, known as roughcast, became fashionable formock-Tudor houses. The top coat of this coarsely texturedrustic looking render is mixed with gravel. Pebble dash, a

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East Terrace, Walton - Stucco frontage

Rendered frontage with some patterning

John Warner’s House, Walton - part ofthe East Terrace development

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slightly later type is similar in composition but a dryaggregate is thrown onto the wet render so that theaggregate remains exposed on the surface.

Ashlar Effect

On close examination what appears to be ashlar (dressedstone) blocks making up the ground floor of an earlynineteenth century house often proved to be nothing morethan an illusion created by a plasterer drawing a toolthrough wet stucco in an attempt to disguise cheapbrickwork.

The horizontal and vertical joints of closely fitting ashlar blocksare frequently represented in the stucco-work of the period.Alternatively, pronounced grooves are incised - often asbanded rustication, which does not include vertical joints.When executed by a skilled plasterer, it is difficult to distinguishrusticated stucco from genuine stonework unless part of therender has broken away to reveal a telltale brick background.

There various examples to be found in Tendring District.

Pargetting

Although much more widespread in other parts of Essexthere are some local examples of pargetting in Tendring.This specialist decorating is found on timber-framed andplastered buildings of the 16th or 17th Centuries. The decoration takes the form of either incised orraised/moulded patterns. There are recognisable historicand characteristic patterns although the intention is usuallyto make the finished product look “hand made”.

Ashlar effect on Listed Building in Harwich

Clear Ashlar effect seen on front elevationof Wellington House

Simple patterning in plastered frontage

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Brick and rendered frontages -The Green, Mistley

Modern Cement and Gypsum

Cements such as Ordinary Portland which is the mostcommon, are very strong, inflexible and waterproof. Theirapplication to old buildings is inappropriate and coulddamage the substrate, which is usually far softer and moreflexible than cement. This inflexibility does not allow forseasonal movement of the structure, like lime does. Whencement cracks (which in time it usually does), water canreadily penetrate down the back of the crack. Localisedareas of dampness develop, which can lead to rot. It canalso freeze, expand and “blow” the render off the wall -sometimes damaging the structural fabric below.

Unfortunately however, some walls have been fully coated incement. In these cases, it may not be advisable to removethe coating, as this could seriously damage the materialunderneath. A small patch in a discrete part of the wallshould be removed as a test to clarify whether damage iscaused by its removal.

If there are small areas of damage to cement render, thenrepairs should use cement of similar density and strength. In these instances, using lime is not appropriate becausethe different degrees of expansion and flexibility will causefurther cracking, so allowing more water into the crack.

In the case of a listed building, if you wish to remove anentire wall of cement render, listed building consent will berequired.

The use of modern bagged gypsum in plastering, even forsmall areas of patch repair, is not appropriate, as it isincompatible with lime plaster, for several reasons. Mosttypes usually break down in the presence of moisture.However, whilst some bagged gypsum does resist theaction of dampness, it contains a water repellent that sealsthe surface of the wall and prevents it from breathing.Gypsum is also very rigid and inflexible, it sets rapidly andcan cause cracking at the edges where it joins with limeplaster. Its appearance is also different. It is smooth andusually finished mirror-smooth, rather than textured, like lime plaster.

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The Parade, Walton - c1830 Stuccoedfrontage

Rendered historic buildings - South Street,Manningtree

Mix of brick and rendered historic buildings, Manningtree

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External Renders - Repairs and Application of New Render

Extent of repair

Simple patch repairing is the traditional approach tomaintenance and although likely to create an irregularfinish, is the preferred option.

The extent of repair necessary should be carefullyascertained by tapping the surface. Render that hasdetached from the backing will produce a hollow soundand may be noticeably loose. Only the very minimumshould be taken off.Many old buildings show generations of patch repair,which adds positively to the character of the structure.

Applying new render

Plastering bead, fixed guides, metal lath or chicken wiremust not be used to artificially straighten the edges of awall. The render should follow the line of the building.

As a guide, two thin coats, each approximately 6mmthick, should be applied to the wall. If both coats are tobe put on smoothly, then the first coat should be scoredin a diamond pattern (about 75mm wide) to provide agood key for the second coat. Wetting between coats isnecessary. Plaster should feather away at its edges, forexample against existing sections of render.

The thickness and mix will vary, but a traditionalcombination of coats comprise the following;

Render coat (9mm) 2 lime putty 5 sandFloat coat (9mm) 2 lime putty 5 sandFinish coat (6mm) 1 lime putty 3 sand

When applying lime render, weather conditions must betaken into account. Where there is a risk of hot sun ordrying winds, some form of protection should be erectedin order to prevent rapid drying, which would cause therender to drop off. Render should not be applied whenthere is any risk of frost. In this instance, the render willlose adhesion before setting has occurred and again, indue course, will fall off.

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External Renders - Repairs and Application of New Render

Extent of repair

Simple patch repairing is the traditional approach tomaintenance and although likely to create an irregularfinish, is the preferred option.

The extent of repair necessary should be carefullyascertained by tapping the surface. Render that hasdetached from the backing will produce a hollow soundand may be noticeably loose. Only the very minimumshould be taken off.Many old buildings show generations of patch repair,which adds positively to the character of the structure.

Applying new render

Plastering bead, fixed guides, metal lath or chicken wiremust not be used to artificially straighten the edges of awall. The render should follow the line of the building.

As a guide, two thin coats, each approximately 6mmthick, should be applied to the wall. If both coats are tobe put on smoothly, then the first coat should be scoredin a diamond pattern (about 75mm wide) to provide agood key for the second coat. Wetting between coats isnecessary. Plaster should feather away at its edges, forexample against existing sections of render.

The thickness and mix will vary, but a traditionalcombination of coats comprise the following;

Render coat (9mm) 2 lime putty 5 sandFloat coat (9mm) 2 lime putty 5 sandFinish coat (6mm) 1 lime putty 3 sand

When applying lime render, weather conditions must betaken into account. Where there is a risk of hot sun ordrying winds, some form of protection should be erectedin order to prevent rapid drying, which would cause therender to drop off. Render should not be applied whenthere is any risk of frost. In this instance, the render willlose adhesion before setting has occurred and again, indue course, will fall off.

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Repair of Stucco

The formality of stucco means that more effort needs to betaken to disguise repairs than in the case of humbler,vernacular buildings.

When wet, new stucco is usually “lined out” to imitate largesmooth faced stone blocks, which are known as ashlar. If asection of this material requires replacement, then it mustbe carefully cut out to follow the joint lines incised in therender-even if this means removing a little of the soundstucco in the process. Unlined stucco should be cut torectangular profiles, if necessary extending the repair asbefore, where possible using details of the building todisguise any joints in the new material.

Historic Cements

Roman cements tend to be applied smoothly to walls. Anymissing areas should be repaired by cutting out neatrectangular sections and reapplying as above. Traditionallythe mix was 1 part hydraulic lime, 1 part gritty sand.

Early Portland and Improved Portland cements were alsousually applied smoothly. Traditionally, the mix for thismaterial was generally between 1 part cement and 5.5 to 8parts gritty sand.

Plaster and Render Materials

In all cases of repair, the new materials need to becompatible with the existing in terms of size of aggregate,consistency, texture and degree of adhesion.

For plaster, well graded means that aggregates shouldrange in size from extremely fine up to about 0.6mmdiameter and in shape from rounded to angular, in order tocreate a strong matrix for sound bonding. The air trappedwithin this sort of mix also helps setting.

Hair in plaster should be approximately 1” long, and course.Goat hair is ideal. As is hair from the body of a horse. Bothmane and tail hair is too shiny, as is human hair. This should

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Ashlar effect on rendered historic building, Harwich

Shop units in Market Street, Harwich

Detail of plasterwork / roof junction

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Church Square, St. Osyth - Roughrendered timber framed cottages c1500

Part of Orwell Terrace, Dovercourt -Roman cement render

Ramsey village

be added to the plaster at the time of mixing andimmediately before application, because the alkalinity of wetlime attacks the protein in the hair and over a period ofweeks and months may break it down.

Clumps of hair create weak spots in the finished work andcan be as detrimental to the integrity of the plaster as usingno hair at all.

For external renders the sand should also range fromextremely fine out to about 1.2mm in diameter.

Decorating New Lime Plaster and Render

New lime plaster should be decorated with limewash. Thiswater-thin coating usually consists of 1 part lime, 2 partswater. If however, following mixing but prior to application,the limewash in the bucket is found to have a sludge at thebottom, then a little more water is required to mix theconstituent parts together.

The first coat of wash should be painted onto a damp wall,from the top down, to avoid streaking. A whole area of wallshould be painted in one go, as join marks will show, whendry. Each coat must be dry before applying the next.Leaving the surface to dry overnight is usually sufficient.

Six coats or more should be applied to create a relativelysmooth, visually uniform surface.

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Mix of brick frontage and renderedupper storeys, Church Street, Harwich

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On external render, lime wash used to be mixed with tallow,a type of animal fat. This produced a water droplet resistantbut vapour permeable surface, which aid weathering.Nowadays, casein is often used, which aids vapourpermeability, but has the added advantage of reducingdusting of the surface.

If a colour is required to finish the surface, pigmentedlimewash can be applied on the last 2 coats. Alternatively, inorder to achieve a uniformly flat, deep colour, all 6 coatscould be applied with pigment.

Alternatively, for internal plaster distemper can be used. This is a water-based dispersion of inorganic pigments in aglue or casein and linseed binder. Limewash and distempercan be purchased from suppliers ready-mixed in a range of colours.

Modern paints such as emulsions do not adhere well to limeplaster and can peel soon after application. They havereduced vapour permeability and huge expansion co-efficients, which means they expand and crack, allowingrainwater to percolate behind. This is then trapped andresults in peeling.

Listed Buildings - Applications for Consent

Applications for Listed Building Consent do not attract a feewhich is a recognition that there is a cost incurred in preparingthe necessary quality of Plans required to accompany suchapplications. The Council strongly advises the owners ofListed Buildings to engage properly qualified and experiencedprofessionals to prepare such applications.

Such professionals can also provide advice on methods ofrepair. Wherever possible the Council expects owners ofListed Buildings to explore repair of historic fabric rather thanseek to replace it with modern materials.

This advice in this series of booklets draws on a variety ofspecialist publications prepared by other organisationsincluding Essex County Council, English Heritage, S.P.A.B.Links to these publications and/or the organisations’websites are given at the end of each booklet.

11Rusticated rendered ground floor withcream gault bricks above c1820, Harwich

Rendered rear elevations of ListedBuildings in Harwich

Further information

Planning Policy Guidance Note 15 Annex C

English Heritage: Conservation Principles Policies andGuidance 2008

Essex County Council Leaflets:

Conservation in Practice: Plastering & LimewashAdvice on the maintenance, repair and conservation ofplastered and limewashed surfaces on traditional housesand service buildings.1999

Conservation Practice: Plastering/Lime wash 1986

Traditional Building Materials in Essex No.1 Pargetting 1982

The Georgian Group Guide No.5 Render, Stucco & Plaster

Other usefulsources ofinformation?There are various nationalorganisations providing designadvice and guidance in relationto Plaster Work and Renders.These include the following:

English Heritagewww.english-heritage.org.uk

Historic Environment LocalManagement - HELMwww.helm.org.uk

Society for the Protection ofAncient Buildings - SPABwww.spab.org.uk

The Victorian Societywww.victorian-society.org.uk

The Georgian Groupwww.georgiangroup.org.uk

The Twentieth Century Societywww.c20society.demon.co.uk

Institute of Historic Building Conservationwww.ihbc.org.uk

Historic Towns Forumwww.historictownsforum.org.uk

Essex County Councilwww.essex.gov.uk

Communities and Local Governmentwww.communities.gov.uk

Note; Tendring District Council is not responsible for the content ofany external websites.

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The Lion Pub - TheStreet, Ardleigh

Rendered ground and first floorelevations Listed Buildings in ChurchStreet, Harwich

Early C19th Rendered / Plasteredterrace, Harwich

Plaster Work and RendersFor more information write to:

Planning ServicesTendring District CouncilCouncil OfficesWeeleyClacton-On-SeaEssex CO16 9AJ