External solid plastering

• Task • Look at the picture and identify what has gone wrong

• Poor batch mixing

• Incorrect gauging

• Different types of sand

• Different manufacturers of cement

Mixing and applying renderTake Note

It is essential to gauge all mix materials correctly with a bucket measure.

This will ensure that mix will be of correct

strength and colour throughout

work. Proportioning - 3 buckets of sand to 1 bucket of cement

External render

• Take Notes

• External render consists of three main components

• Sand? The aggregate

• Lime? The Plasticiser

• Ordinary Portland Cement ? The Binder

Materials used for external rendering

Sand

• Sand is used as an aggregate.

• The types of sand used are:– pit sand (quarried inland; red in colour)– dredged sand (from river or sea beds; dark yellow)– artificially created sand (different colours).

• Sand should be clean and contain not more than 10% of silt.

Silt test

To conduct a silt test you need:

• a sand sample• water• salt• a glass jar or

measuring cylinder• a tape measure.

Silt test

• Materials and equipment• sample of sand• water• salt• glass jar or measuring

cylinder• tape measure.

• 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.

• 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

• Silt divided by aggregate 50 , time 100

• Less than 10% is acceptable

Sand• Take Notes

• Poor quality sand will cause external renders to crack and become loose and hollow.

• The external render may even crumble when attacked by weather, especially in winter.

Task Look at the picture and identify what has gone wrong

Oil pollution staining the face of the render

Sand The Aggregate Take Notes

Sand should be:• stored in a clean bay

covered with tarpaulin to reduce contamination from animals, leaves and other aggregates.

• Setting time for cement• Take Notes

• 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.

Cement The Binder

• The part of the mix that sets and binds the aggregate together

Cement The Binder• Sulphate-resisting Portland

cement (SRPC) • Is used in situations where

sulphates are present in concentrations that would damage normal Portland cement mortar.

Cement The Binder• This type of damage

results in white salt stains appearing on the face of backgrounds that could penetrate through the render.

• Take Notes • two special types of

cement.• Sulphate-resisting

Portland cement (SRPC),

• Rapid-hardening Portland cement (RHPC

external render

Take Notes

The type of external render that should be applied to insulation boards would be a specialist lightweight render because sand and cement-based materials would be too heavy.

Additives• Commonly used in cement-

based render mixes.

• They are liquid or powder ingredients.

• They should be carefully measured before adding to the mix.

Additive for external cement-based renderTake Notes

• Plasticiser – improves workability and ease of application.

• Waterproofer – improves workability, forms waterproof barrier and equalises suction.

• Retarder – added in warm conditions to slow down setting.

• Accelerator – added in cold conditions to speed up setting.

Task draw this chart the preparation of

the background

Type of background

Hack off old work

Rake out joints

Brush and dampen

Apply slurry adhesive

New brickwork

Old brickwork

New blockwork

Old stonework

Concrete surface

Bonding adhesives for sand, cement and lime

mixes• Take Notes• Slurry coat or

spatterdash coat

• Spatterdash is a coat applied externally?

• It is used to produce a good quality key for the external render to bond onto.

Application of a slurry coat

• Bonding adhesives are designed for use on poorly keyed dense surfaces;

• They improve adhesion between the background and the first render/scratch coat.

• They come in either powder or liquid form.

• Some require mixing, while others are pre-mixed.

• There are many bonding products on the market.

• One example is SBR.

• This is latex-based. • It is used in areas subject

to humidity, dampness or continuous water contact

• To improve the water resistance of cement mixtures.

• Take Notes

• SBR should be applied externally?

Lime the Plasticiser • Take Notes• Lime is used as a plasticiser

in external rendering.

• Helps to reduce cracking, which also reduces loss of adhesion.

• Gives a slight suction which aids the application of further coats

Ratios of the render mix: why use them?

• Ratios are designed to help the plasterer know the right quantity of each material when mixing.

• Mixing the render too strong may cause subsequent stress resulting in cracking and blowing.

• Mixing the render too weak may cause no initial set and subsequent crumble.

Ratios for dubbing-out mixes• 3 sand, 1 cement,

Ratios for scratch coat• 4 sand, 1 cement, waterproof additive

Ratio for topcoats• 5 or 6 sand, 1 cement, 1 hydrated lime, plasticiser

additive

Take Notes

Scratch coatTake NotesUsed to straighten out

background and is backing coat for render finish.

• Includes a waterproofer.• Thickness is 9–12 mm.

• Should be ruled out as flat as possible.

• Keyed with a comb scratcher.

Two-coat work

Two-coat work comprises:• the scratch coat• the topcoat.• remember never put a

strong mix on top of a weak mix

Topcoat This is applied to scratch coat to form

finished render.

But first, the reveals must be completed.

1.Run trowel over scratched surface to remove any hard sand or cement.

2. Fix/hold timber rule on main face of wall, plumbing or lining to window or doorframe to 10–12 mm.

3.Apply topcoat to reveals.4.When it begins to set, consolidate

surface with plastic or wooden float.5.Sponge or scour over surface to close

face, if necessary.

Forming an external angle to a render coat

MethodHold a timber rule plumb on one side of the wall to a thickness of 10–12 mm.

Apply render up to the timber rule.

• Slide the timber rule upwards and away from the angle.

• Hold the timber rule on previously laid render to a thickness of approximately 10–12 mm.

• Apply render up to the timber rule.

• Slide the timber rule upwards and away from the angle.

Dry piecing ?• Take notes• Large areas where

the work has to be left for more then 24 hours. Another term for this is dog toothing

Plain face/scoured finish• Take Notes

• Thickness of topcoat should be as tight as possible, usually 10–12 mm.

• Rule out without screeds to a flat smooth finish.

• Consolidate the face of the render coat.

• Finish it by the use of a float and also a sponge float.

A plain faced finish