External solid plastering
chapter
8
OVERVIEW
Render provides a weatherproof surface that prevents the penetration of moisture into the outside of
the building, and forms a decorative finish that enhances the building’s appearance.
Traditional render is normally applied by hand, but modern renders can also be applied by machine.
Rendering materials have changed dramatically over the years. In the mid 18th century, rendering
was carried out using a mortar of hydraulic lime and sharp sand. This provided a breathable render,
but it took a long time to set. It was later replaced by mortars of cement and hydrated lime, which
had a quicker setting time and so speeded up production – essential in the housebuilding boom
after the Second World War. Today, new renders are being developed to replace traditional renders
with a better quality product to meet today’s needs. In order to speed up productivity these new
renders are premixed so that they are uniform in colour and have a consistent strength.
Traditional lime renders are still used however, especially for restoration work on chapels, churches
and listed buildings.
Key issues when choosing the type of render are: planning regulations and conservation
requirements; fire and insulation properties to comply with building regulations; the background to
which the render is to be applied; climatic exposure; the client’s requirements; and, of course, cost.
This chapter will cover the following:
• Materials used for rendering
• Additives used for rendering
• Preparing backgrounds to receive rendering
• Types of bonding adhesive
• Mixing and applying render
• Plain face render finish
• Other types of render finish
• Modern render
• Metal trims.
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Chapter 8 External solid plastering
Plastering NVQ and Technical Certificate Level 2
Materials used for external rendering
Safety tip
It is important to choose the correct materials to produce render. This will produce high-quality
render mixes that will not develop faults such as cracking and blowing of the surface, which
affect the appearance and allow moisture to penetrate.
Method
Step 1 Place 25 mm of water into the jar, add 1 teaspoon
of salt and gradually add the sand until the level of the top
of the sand reaches 50 mm.
Step 2 Shake the jar for 1 minute.
Photo to be supplied
Some rendering products
can be harmful. Always read
the manufacturers’ data
sheets, and follow their
instructions carefully
Sand
Sand makes up the bulk of the render mix. For traditional external rendering mixes the
following types of sand are used:
• pit sand – quarried inland, usually red in colour
• dredged sand from the river or sea bed, usually a dark yellow colour
Step 3 Leave to settle for 3 hours. Measure the height of
the aggregate and the thickness of the silt layer.
To work out the percentage of silt in the aggregate,
calculate the following sum:
Thickness of silt
100
×
Total height of aggregate and silt
1
• artificially created sand, available in different colours.
Step 1 Ingredients being added to water
Modern render mixes are made with artificially created sand. This is formed by crushing stone
or gravel to the required grade. It is available in different colours, such as white, red or black.
Remember
Protect sand when it is
not in use, to help prevent
contamination. Use a
weighted down tarpaulin
or similar
When mixed with cement and lime, the sand will form the bulk of the mix; the sand is known
as the aggregate.
Poor quality sand will cause the render to crack and become loose and hollow, or it may
eventually crumble. In good quality sand, the particles are irregular in shape (not rounded),
and well graded. This means that they range from small to large grains, so that they fill all the
voids in the mix. The largest grains should not be more than 5 mm. This is assessed using a
sieve test to ensure no grains over 5mm are present in the sand.
Photo to be supplied
Photo to be supplied
The sand should have been washed to remove impurities. Clay and silt particles in the mix
prevent the cement from bonding to the aggregate, so each load delivered to the site should
be checked for ‘cleanliness’. The amount of silt must not be more than 10% of the volume of
aggregate. The silt test is used to check this.
Silt test
Materials and equipment
• sample of sand
Jar after shaking
Jar after 3 hours
For example, if we measure the sand to be 45 mm and the silt thickness is 5 mm:
5
100
×
45 + 5
1
• water
• salt
5 100
×
50
1
• glass jar or measuring cylinder
• tape measure.
0.1 × 100
= 10
Conclusion – this sample would be suitable, but only just.
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Storage of sand
Sand should be stored in a clean bay, and covered with
a tarpaulin to reduce contamination from animals,
leaves and other aggregates.
Cement
Portland cement
Portland cement is the basic active ingredient of render
mixes. It is a closely controlled chemical combination
of calcium, silicon, aluminium and iron, plus small
amounts of other ingredients. Gypsum is added in the
final grinding process to regulate or retard the setting
of the cement.
Definition
Stored cement
Cement is used to bind and harden the mix for rendering. Therefore it is important to gauge
the mix correctly. Mixes that are weak (not enough cement) will crumble. Mixes that are
strong (too much cement) may create stresses that could cause the render to blow from the
background.
Safety tip
Several different manufacturers make cement, but the product must always be manufactured
in accordance with British Standard EN 197-1 Cement: Composition, specifications and
conformity criteria for common cements.
Always wear gloves when
handling cement or lime to
protect against skin burns
Two types of cement are commonly used with external renders. Ordinary Portland cement
(OPC) consists essentially of 75% limestone and 25% clay. It is grey in colour. White Portland
cement contains 75% limestone and 25% white china clay. It is white in colour, and is used
when a whiter, lighter appearance is desirable in the render finish.
The manufacture of cement
Setting time for cement
Each step in the manufacture of Portland cement is checked by frequent chemical and physical
tests in plant laboratories. The finished product is also analysed and tested to ensure that it
complies with all specifications.
The setting of cement is a chemical reaction, and so it is affected by temperature. In warmer
climates cement will set more quickly than in colder areas. The initial set of cement is 45
minutes, but the final set is not more than 10 hours.
Gauging – measuring
the correct proportions of
materials in a render mix
to achieve full strength
when set
Remember
Always use bagged material
before its sell-by date. All
bagged materials such as
sand, cement and lime and
premixed modern renders
should be rotated – this
is where older stock is
brought to the front so that
it used before any newly
delivered stock
Cement should be stacked off the ground in a container or shed to keep dry.
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Special-purpose cements
Safety tip
When cement is mixed with
water an alkaline solution is
produced. Take precautions
to avoid skin contact with
wet cement, fresh concrete
or mortar, and prevent dry
cement from entering the
eyes, mouth or nose. Wear
suitable protective clothing,
gloves and eye/face
protection
• It improves adhesion to the background.
Sulfate-resisting Portland cement (SRPC) is used in situations where sulfates are present in
concentrations that would damage normal Portland cement mortar. This type of damage
results in white salt stains appearing on the face of backgrounds that could penetrate through
the render.
Rapid-hardening Portland cement (RHPC) is finer than OPC, and is used in concrete to ensure a
higher rate of early-age strength development.
face render.
• It adds suction in the background to prevent sagging when applying Tyrolean finish to a
plain face background.
Definition
suction – this is when
a background absorbs
moisture from the
render mix
Hydraulic lime
Hydraulic lime is produced by slaking lime made from limestone that contains clay and other
impurities. Unlike hydrated lime, which has no set, hydraulic lime sets like cement once it is
mixed with water, but far more slowly.
Lime
Lime is manufactured by crushing
limestone, which is mainly calcium
carbonate, and heating it in a kiln to
drive out the carbon dioxide content
to produce calcium oxide. This results
in the material known as burnt lime,
lump lime, quicklime or just lime.
• It prevents a dark sanding appearance being drawn to the surface when consolidating plain
Hydraulic lime is preferred for use on older buildings that were built using lime mortar. It allows
the building to ‘breathe’; cement mixes are too dense and brittle for this type of background.
Photo to be supplied
Two types of lime are used in
rendering – hydrated lime and
hydraulic lime.
Stored lime
FAQ
What types of lime would you use when rendering,
and when?
Hydraulic lime is used mainly for listed building work, whereas
hydrated lime is used on modern buildings.
Hydrated lime
Definition
slaking – the process
of mixing calcium oxide
(lime) with water
Hydrated lime is
produced by slaking
calcium oxide
(lime) with water
to produce calcium
hydroxide. It is used
in sand and cement
rendering mixes for
application on all
types of background.
Additives
Additives are commonly used in cement-based render mixes, whether for the dubbingout coat, scratchcoat or topcoat. Additives are liquid or powder ingredients, and should be
carefully measured in accordance with the manufacturer’s instructions before adding them to
the mix.
There are several types of additive that can be added to cement-based render:
• Plasticiser – improves the workability of the mix and its ease of application
• Waterproofer – improves workability, and forms a waterproof barrier in the render
The process of slaking lime
Hydrated lime offers the following advantages:
• It prevents shrinkage cracks in the render mix.
• It improves the workability/spreading of the render mix on the background.
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• Retarder – this can be added to the mix in warm conditions to slow down the setting
process
• Accelerator – this can be added in cold conditions to speed up the setting process.
Some modern premixed renders have additives included during manufacture. This will
eliminate problems caused by adding too little or too much when mixing by hand.
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On the job: Using additives
Peter the labourer mixed my render, but used too much plasticiser.
This resulted in the mix not achieving its correct strength, and later
it crumbled. What should Peter do to prevent this happening in future?
Preparation of backgrounds
Remember
Most modern houses and other buildings are built from blocks, which require little preparation.
Older buildings will require some type of preparation before rendering. This is essential to
ensure a good bond between the render and the background.
Some backgrounds may
still have old render. This
will need to be prepared
by hacking off
New blockwork only requires damping down of the surface before rendering, whereas all other
backgrounds require preparation.
Table 8.1 identifies the procedure for preparing backgrounds prior to rendering.
Table 8.1 Preparation of backgrounds
Type of
background
Hack off old
work
Rake out
joints
New
brickwork
Old
brickwork
L
L
New
blockwork
Old
stonework
Concrete
surface
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Brush and
dampen
Apply slurry
adhesive
L
L
L
L
L
L
L
L
L
L
L