Dashing Information – Render Mixes

The most important rule of rendering is that each coat must be weaker (i.e. less cement) and less thick than that to which it is applied. Not following this rule can lead to shear problems.

There are many variations on mix proportions, if hydrated lime is to be used in mixes the cement/lime to sand ratio should be kept to 1 to 3. In other words lime is considered to be part of the cement content of the mix and by adjusting the proportion of cement to lime the strength can be modified without impairing surface hardness and strength.

A guideline to on-site mixes (as opposed to factory batched mixes) would be as follows:

Rendering onto masonry substraights

Scatch coat (first coat)

1 part Ordinary Portland (OP) Cement : 4 parts sand mixed with water and plasticiser & waterproofer as recommended.

Or

1 part OP Cement : 0.5 part hydrated lime : 4.5 parts sand mixed with water and plasticiser and waterproofer as recommended.

Top Coat (for floating or dashing finish) 1 part OP Cement : 5 or 6 parts sand mixed with water and plasticiser and waterproofer as recommended.

Or

1 part OP Cement : 1 part hydrated lime : 5 or 6 parts sand mixed with water and plasticiser and waterproofer as recommended.Use white cement to make white mortar for top coat if required and always follow manufacturer’s instructions for use of plasticisers and waterproofers.

These mixes are for guidance only and should not be assumed to represent the extent of variations available.

Pre Mixed Dash Receiver

Pre-mixed dash receiver products are available that are pre coloured and require mixing with clean water on site with a paddle mixer. These products allow uniformed colours and a mix consistency that cannot be guaranteed using the above cement and sand mixes.

Base coats are also available to suit. The products often contains silicone and or polymer additives to give a stronger bond that is less liable to cracking than a sand and cement mix. This type of render would be manufactured as a type 11 material under BS 5262:1991. This type of finish is preferable to a sand and cement mix made up on site. The added polymers bond the finish creating a stronger render.

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