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
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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
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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
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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
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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
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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.