RENDERS

RENDERS

TYPES AND MATERIALS

RENDER 

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RENDER PROVIDES A WEATHERPROOF SURFACE. IT PREVENTS THE PENETRATION OF MOISTURE INTO THE FABRIC OF THE BUILDING. IT FORMS A DECORATIVE FINISH.

TYPES OF FINISH  

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PLAIN FACE RENDER Plain face render is a flat finish. The surface is consolidated with a float to leave a sandstone effect. Can be painted for added weather protection.

ASHLAR 

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Plain face render marked out in blocks to look like masonry blocks. Chosen block size marked out using a gauge rod while render is still soft. Joints are marked using a nail pressed into surface and drawn along a straight edge.

PEBBLE DASHING 

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Produced by throwing pebbles or spar onto the final coat. Pebble dashing is a two person job. One applies the ‘butter coat’ the other dashes the surface. Dashing cannot be patched, any walls should be completed in one go.

TYROLEAN    

Applied to a plain face finish that has set. Cement based product. Comes in a range of colours. Tyrolean machine is a box with spring steel strips which flick the material onto the wall.

ALPINE FINISH 

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Pre mixed cement based consisting of white Portland cement with a graded aggregate. The finish is obtained by dragging a float across the face of the work.

SAND     

Sand for rendering needs to be well graded. A well graded sand means it has grains of varying sizes. A well graded sand should not have grains larger than 5mm. A well graded sand ensures there are no voids in the finished render coat. This will give a strong, workable mix.

TYPES OF SAND 

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Bricklayers Soft sand :this has fine grains of sand. Too soft and clayey for render. Can have 10% or more clay content. Silver Sand: has very low clay and silt content – virtually nil. Used in the manufacture of glass – can be used with lime putty as a finishing plaster. Sharp Sand: contains coarse grains, used for screeding or concrete.

PLASTERING SAND     

Plastering sand should conform to British Standard 1199. The best source for plastering sand is quarried. Sea sand is not good as it contains salt which comes out of the finished surface as ‘efflorescence’. The grains of river sand have no sharp angles due to erosion by the moving water of the river. This sand cannot ‘hold’ the water in the mix and will ‘work short’.

Well graded sand 

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Well graded sand has medium sized grains to fill in the large voids, and small grains to fill in the smaller voids. The sand grains should have sharp angles otherwise the material ‘works short’.

CLAY AND FINES   

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Rendering sand needs a little clay to help work the material. Rendering sand should have no more than 10% clay and silt. The sand can be tested for this by doing a silt test. Silt is very fine grains of sand. It helps to plasticise the mix.

TOO MUCH CLAY

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If sand has too much clay, it can cause faults in using the material and in the finished work The cement takes longer to set. Retarders in Class B plasters are attacked by the clay and their set is speeded up. More water is needed to improve the spreading of the mix. When this water evaporates the material will shrink and crack. The shrinkage of the clay itself will cause cracking of the finished work.

CEMENT 

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The cement used in plastering and rendering is Ordinary Portland Cement (OPC). The main raw material in cement is Limestone with clay as a second. It is crushed and heated to 1500oC. Chemical reactions take place to form cement clinker. 3-5% gypsum is added to regulate its set and then ground to a powder.

TYPES AND USE’S 

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Extra Portland: has the same setting times as OPC but sets harder. High Alumina : use in the 50’s and 60’s in pre cast concrete beams for building : not used much anymore due to collapse issues. White Portland : made from pure limestone and white china clay : used to produce bright colours with pigments.

SAFETY ISSUES 

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When cement is mixed with water it produces a highly alkaline solution. Cement can cause serious burns if you have prolonged contact with the skin.

LIME  

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Lime acts as a plasticiser in the mix. There are two types of lime used in construction : Hydraulic and Non Hydraulic. A Hydraulic Lime has the ability to set under water. A Non Hydraulic Lime sets by exposure to the air.

HYDRAULIC LIME 

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A Hydraulic Lime is made by heating a Limestone that contains clay and other impurities. The Calcium in the Lime reacts with the clay to form silicates that let the lime set without it being exposed to the air. A lime sand mix lets a wall ‘breathe’. Cement is too dense for this.

NON HYDRAULIC LIME       

Non Hydraulic Limes are made from Limestone with a high calcium content. A Non Hydraulic Lime sets by exposure to the air. A process called Carbonation. The lime reacts with atmospheric carbon dioxide to produce Calcium Carbonate, which is the raw material we started with. This limestone is relatively free from impurities. Non hydraulic limes used in plastering are Hydrated Lime and Fat Lime / Lime putty. Fat lime has a high workability.

Slaking Lime 

http://www.youtube.com/watch?v=eSKX0gzE SwI

Slaking Lime   

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The limestone is heated to about 500oC The heat drives off carbon dioxide and water in the rock. Calcium carbonate and magnesium carbonate in the rock are converted to calcium oxide and magnesium oxide. This is known as Quicklime.

Quicklime It is called Quicklime because it reacts very quickly when added to water. When Quicklime is soaked in water, it is known as ‘slaking’. The Quicklime hydrates and the oxides react with the water. The oxides become Calcium Hydroxide and Magnesium Hydroxide. This creates Lime Putty.

Mixing 

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Information about gauge / mix ratios can be found in a specification. The most accurate way to gauge proportions is to use buckets. Sand is the aggregate in the mix. Cement is the binder. Lime is the plasticiser.

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Each progressive of coat of render should be weaker than the previous one. A strong mix has a lower ratio of sand eg 3 : 1 Applying strong renders onto weaker backgrounds causes stress which can result in the render falling off.

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Most cement based renders should be left at least 24 hours before the next coat is applied. The longer a scratch coat is left for the better. This gives as much time for all the shrinkage and cracking to take place. Lime based renders can take weeks to cure.

Poor mixing 

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An over sanded mix will give poor workability and results in a weak mix. An under sanded mix will be fatty, quick drying and lead to increased cracking. A strong mix on a weak background can lead to cracking, water ingress behind the render and failure of the bond between coats.

BEADS 

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Angle beads for external work should be stainless steel or plastic. Galvanised beads will rust if used outside. Any beads should be fixed with sand and cement. Gypsum plasters should not be used outside.

BELL CAST BEAD 

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Bell cast beads are used to form a weathering at the base of the wall. Used above windows and door heads. Never place a bell bead below the physical D.P.C

MOVEMENT BEADS 

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Should be placed wherever there is a movement joint in the background. Flexible joint allows movement without cracking. Back to back stop beads can be used with a mastic filler.

BACKGROUND PREPARATION 

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Preparation is key to a good finish and long lasting render. New blockwork will require little preparation normally only damping the blocks with water. Older buildings will require a method of controlling or improving adhesion.

SLURRY / SPATTERDASH 

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A very wet mix of sand cement spattered on the wall surface to form a key. Background should be clean and grease free. Any loose paint etc should be cleaned off.

S.B.R  

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Styrene butadiene copolymer rubber. PVA cannot be used outside as it’s adhesive properties are affected by water. SBR is a latex and not affected by water. Use as a primer before rendering.

Expanded Metal Lathing ( EML)  

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Use on backgrounds that are worn or friable. Use over paint. Use over dissimilar backgrounds to prevent cracking from thermal movement. Fixed by drilling and plugging to the background surface.

SCAFFOLD 

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Scaffold use for rendering should provide easy and clear access to any walls. The scaffold should not touch the wall to prevent any patching work after the scaffold is removed. Some renders cannot be patched up eg. Alpine finish, K rend and Tyrolean.

Independent Scaffold 

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Carries its own weight and the vertical weight of all loads placed upon it down to the ground. Independent of the building or structure. Scaffold will be tied into the building for stability eg. through window opening.

Tower Scaffold   

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Light portable system. Easy to erect. Needs outriggers for stability and to increase working height. Max height outdoors is no more than 3x the size of the smallest base size. Indoors 2.5x the smallest base size. Eg outdoors a 2m x 3m base = max height of 6m

RE CAP   

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Sand used in the mix is called the ………………… Sand used for rendering needs to be ………. graded. A …………………… sand will give a strong, workable mix. If the sand used cannot hold the water the mix, it will …………….. short.

RE CAP    

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Rendering sand should have no more than ……………. Clay and silt. Sand can be tested by using a ………… test. Cement used in plastering and rendering is ………….......... Cement. Lime acts as a ………………. In the mix. A …………………… lime sets by exposure to the air.

RE CAP   

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Fat lime is a Non Hydraulic lime with a …….. workability. Information about gauge / mix ratio’s can be found in the ……………………… Every progressive coat of render should be …………… than the previous one. Any beads used fro external rendering should be ………….. Steel or …………….

RE CAP 

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A bell cast bead forms a …………….. at the base of the wall and above windows. A slurry spatterdash coat is a ……………… mix of sand / cement. PVA …………….. be used outside as an adhesive.