RENDERS

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RENDERS

• TYPES
• AND
• MATERIALS

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RENDER

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

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TYPES OF FINISH

• 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.

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ASHLAR

• 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.

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PEBBLE DASHING

• 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.

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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.

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ALPINE FINISH

• 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.

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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.

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TYPES OF SAND

• 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.

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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.

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Well graded sand

• 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.

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CLAY AND FINES

• 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.

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TOO MUCH CLAY

• 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.

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CEMENT

• 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.

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TYPES AND USES

• Extra Portland has the same setting times as OPC
  but sets harder.
• High Alumina use in the 50s and 60s 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.

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SAFETY ISSUES

• 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.

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LIME

• 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.

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HYDRAULIC LIME

• 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.

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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.

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Slaking Lime

• http//www.youtube.com/watch?veSKX0gzESwI

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Slaking Lime

• 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.

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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.

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Mixing

• 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.

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Poor mixing

• 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.

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BEADS

• 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.

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BELL CAST BEAD

• 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

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MOVEMENT BEADS

• 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.

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BACKGROUND PREPARATION

• 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.

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SLURRY / SPATTERDASH

• 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.

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S.B.R

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

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Expanded Metal Lathing ( EML)

• 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.

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SCAFFOLD

• 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.

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Independent Scaffold

• 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.

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Tower Scaffold

• 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

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RE CAP

• 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.

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RE CAP

• 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.

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RE CAP

• Fat lime is a Non Hydraulic lime with a ..
  workability.
• Information about gauge / mix ratios 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 .

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RE CAP

• 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.