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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.
A
typical render specification:
1. Ensure that appropriate scaffolding is in place and the worksite safe for workers
and public.
2. Take off the existing render, except any existing sound
lime mortars, taking care not to damage the structure. Look out for very thick
patches of render that are effectively load bearing. It may be preferable to render
on top rather than risk rebuilding an area.
3. Dub out any deep holes
in the wall with a haired lime putty mortar, rebuilding defects with cob blocks,
bricks or stone as appropriate.
4. Treat wooden lintels with preservative
and counter batten with oak lath if rendering over them.
5. Apply one
hand harled coat of lime putty
mortar to provide a key to the wall. This is usual with cob but depends on the
state and size of stone. This coat should include a gauge of Pozzolan
to strengthen the mortar. If Pozzolans are used in subsequent coats, the gauge
for the scat should be greater.
6. Apply sufficient coats of haired lime
putty mortar to smooth the contours of the wall. With a suitable animal hair in
the mortar coats can be applied up to 20mm thick rather than the 8–10mm thickness
of unhaired mortar. The hair reduces any slumping whilst applying and shrinkage
cracking whilst curing. Each backing is keyed with a scratch comb.
7.
Apply a top coat of floated or hand harled lime putty mortar as desired. Damping
& curing:
It
is very important to control suction from the wall by light spraying with water
half an hour before applying each coat (especially cob and porous brick) and in
warm weather it will be necessary to spray each coat afterwards. Whatever the
season, protect each render coat during the curing process from all the elements
such as hot drying wind, strong sun, rain and don’t apply in temperatures below
5 degrees centigrade or if there’s a risk of frost. A heavy cloth such as hessian
sheeting will provide a suitable physical barrier and should be left in place
as long as required. Materials:
Lime putty mortars gain
added strength by carbonating over many months with atmospheric carbon dioxide.
Whilst pure lime putty mortars are suitable inside or for sheltered locations,
it’s recommended that for exposed elevations each coat of lime mortar has a pozzolan
added. These are burnt clays that react with the lime to give harder more frost
resistant renders and historically ranged from volcanic ash, crushed bricks and
other forms of burnt clay. Curing:
All coats need to be given a few days to harden before subsequent coats are applied.
To test whether a coat is ‘green hard’ the surface should be resistant to a fingernail.
Many factors will influence the timing such as the season, exposure of wall and
the thickness of the coat but it’s normal to expect a couple of days for the harled
coat to harden and perhaps 4 - 6 days for each of the thicker coats. Painting:
It is important
that lime renders are not totally sealed with an inappropriate paint. Limewash
is the most cost effective paint to apply, offers the most traditional finish
and will aid rainwater shedding . A minimum of four coats of limewash incorporating
a little linseed oil should be painted on the final coat of render. Where a mixture
of differing wall surfaces is to be painted it's possible that a silicate masonry
paint will also be suitable. Conclusions:
Lime mortars are
easy to use and can readily be applied by the enthusiastic amateur given a little
tuition and guidance. Although, the work needs to be carried out correctly, the
beauty of the traditional building is that it rarely looks right when everything
is straight and perfect, so the DIY owner can begin work comfortable in the knowledge
that a certain amount of ‘character’ would not look out of place.
Quantities;
for
the Example Specification per square metre one
3mm scat coat of of
3.5/1 unhaired
mortar ~ 6kg/m2
one 15mm scratch
coat of 3.5/1
haired mortar ~ 30kg/m2
one 10mm floated top coat of 3.5/1
unhaired mortar ~ 20kg/m2 or repeat the first scat coat for a rough cast finish. 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. BACK
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