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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.
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 10 –12mm 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.
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
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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