Guidesheets:    Rendering

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Lime putty mortars offer advantages over cement based mortars for external rendering, especially when decorated with a traditional limewash:

As with all lime based materials the best outcome requires patience and careful control of drying and suction, the reward being a good looking and long lasting render.

Preparation;

Any existing render must be removed, except any sound lime mortars. Care must obviously be taken to ensure that the structure isn’t damaged. Look out for very thick patches of render that are effectively load bearing. It may be necessary to render on top rather than risk rebuilding an area.

Damping;

It is very important to control suction from the background material (substrate) by wetting before applying each coat, especially cob and porous brick. Do not saturate the substrate as suction is still required. In warm weather it will be necessary to spray each coat with water afterwards and cover with hessian to slow down drying.

Premixing:

All lime mortars and plasters benefit from being pre-mixed for a minimum of a couple of weeks and stored in a sealed container. The mortar is then “knocked up” again prior to use to plasticise them – this reduces shrinkage in the plaster. Keep agitating the mortar until it sticks to your hawk when held upside down.

Example Specification;

Scat on to cob	Scratched coat

a. dub out any deep holes in the wall with a haired lime putty mortar, rebuilding defects with cob blocks, cob bricks or stone.
b. treat wooden lintels with preservative and counter batten with oak lath.
c. apply one hand harled coat of lime putty mortar to provide a key to cob or on to bricks, with stone this is a matter of preference depending on the friability and size of stone/joint.
d. apply sufficient coats of haired lime putty mortar to smooth the contours of the wall.
e. apply a top coat of floated or hand harled lime putty mortar as desired.
f. protect all coats from the weather during and after application with hessian sheets to improve curing. As with any external render don’t apply in temperatures below 5oC or if there’s a risk of frost.
N.B Leave each coat to harden until you cannot mark it with your thumb nail before applying next coat.

Gauging;

We suggest a 3.5/1 mix of coarse sharp well graded sand where a pozzolan is to be added. Lime renders gain strength by carbonation with carbon dioxide from the air. In damp, frost prone or exposed situations it may be appropriate to add an extra ingredient to a lime render to increase its compressive strength and frost resistance. Traditionally volcanic ash or brick dust were added, these are forms of burnt clay called pozzolans after the Italian town of Pozzuoli where volcanic ash was used by the Romans. We use a calcined clay from Cornwall called Metastar at a volume gauge of 10 -25% depending on degree of exposure. It’s only added when knocking up the mortar just prior to use. It won’t give an overnight set but will slowly begin to add a little extra compressive strength to the mortar after a couple of weeks.

Quantities;

for the Example Specification per square metre

one scat coat of 0.3mm - 0.3 tubs of 3.5/1 unhaired mortar ~ 21kg size
one scratch coat of 15mm - 1.5 tubs of 3.5/1 haired mortar ~ 21kg size
one floated top coat of 10mm - 1.0 tubs of 3.5/1 unhaired mortar ~ 21kg size

Training;

We offer a one day practical course in using lime in renovation and we also sell a training DVD based on the course.

Safety: Limes are caustic.Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels. Our advice and information are given in good faith. It’s important that users satisfy themselves that they’ve chosen an appropriate product and have a suitably skilled workforce.

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