lime render - close up
LIME
Mixing Lime. We hunted for lime we could slake straight from
a Quarry, but had little success. We resorted to Limil. We sourced lime that had
been bagged recently or fresh, which allows for consistency in the ratio of
water to lime. Older lime may become lumpy when slaked with drier clumps
difficult to blend effectively which can spoil the appearance of the finished
render .
The ratio of lime to water is not definite and will have an
impact on strength when blended. An expert eye is helpful here unless you’re
willing to experiment. The ratio we used was 1 x 20kg bags of lime to 30 litres
of water. This blends into a thick creamy lime liquid. Lime is aggressive. Full
body protection, gloves, safety glasses, ear protection and a proper tightly
fitting face mask are a must. No exposed skin is a good philosophy. When this sits, excess water will separate and
sit on the surface. It is important to have tightly sealed drums to reduce
evaporation.
The lime is left for
a week in barrels before water levels
and consistency are checked before remixing, blending the mixes if required.
Thinner mixes were thickened with more viscous mixes, and excess moisture from
thinner mixes used to thin the viscous. A level of expertise and know how is
essential here. Gary Nicholl from Clunes, who has a long history with lime
render that started as an apprentice to his father, has mixed and blended the
lime to a beautiful creamy consistency.
lime barrels
I love lime. Initially it is kilned at 1000 degrees , but then,
when it is slaked and used as a render or mortar, it cures back into a calcium
carbonate, and fixes up to 90% of it’s carbon pollution. Lime maintains it’s flexibility to a degree, as it expands and contracts as it wets and
dries. Concentrated water flow across lime should be avoided with careful
detailing, as it will erode a lime mortar or render. To date we have had no problem with it
on horizontal surfaces even with parapet
walls exposed to full weather . For
example parapet flashing must fall away from the face of walls back onto a roof surface. The use of
lime and it’s suitability needs to be
assessed case to case . Typically we use
a single 10mm lime render over blockwork, when it has dried enough to resist light finger
pressure, it is sponged with a very slightly damp sponge (no fluid). The render
is not struck off before this, so the trowel marks are vaguely visible. The
beauty of this process, is that the single coat of lime render almost
completely conceals the blockwork, something you would need 2 coats of a cement
render to achieve. The soft sandy appearance of the sponged finish is not a highly
reflective white, but a more luminous, softly glowing surface, in contrast a
cement or acrylic render can appear flat or dead.
Historically, lime would have been used for render, washes,
paint and mortar before cement became
commonly available.
Lime Render. The Mix
The mix is where the success of your lime putty becomes
evident. There are bags of slaked lime putty available for those less inclined
to experiment. A carefully measured ratio of 1 lime to 3 sand is blended in a
mixer. Lime is placed in first, then half the sand, a handful of micro fibres
(which need the second ½ of the sand to blend), a generous cup of diluted (1:8)
bondcrete and water. The mix until the mortar folds just so. The type of sand
used may dictate the use of a plasticiser, be sure that the technical data
specifies compatibility with lime use.
‘Economono’ Micro Fibres
Bond Crete diluted with water (1:8) is used as a preparatory
coat, a moist surface helps ensure what
ever you’re rendering does not suck moisture from the mix inconsistently. IE a
highly porous area next to a not so porous area gives rise to difficulty trowel
and finishing. Bondcrete also assists with adhesion. It’s important the surface
is still moist at the time of render application.
The render is then trowelled on, left to dry (for a very
carefully measured time), and sponged off with a barely damp sponge. The sponge
creates a sandy texture which softens trowel marks and edges. A 10mm thick lime
render that has been sponged (no screed required), virtually completely
conceals our blockwork. The same effect using cement based render would require
2 x 10mm coats. The irregularity of the surface remains, and is obvious with angled
light. To me, it is a beautiful, soft surface that has many moods.
We use a ER11 powder coated stopping bead to exterior
junctions of ‘other’ materials, and a Rondo
RELA30 pvc external angle is used to create clean, durable joints to corners.
Make sure the edge of the external angle has a minimal radius or the pvc will
show, the render will not adhere well to
plastic. There are a large range of external angles available. Fixing methods
vary, but care should be taken to ensure the fixes will not rust and spoil the finished
surface of render. When we don’t use a
concrete nail gun, silicone and clouts
in mortar joints can be used to secure a
render bead. When fixing with a nail gun
into block, it’s best if the block is core filled, fixes into the central rib
of block, or the fixer has at least 40mm of block from top or bottom to avoid
chipping and splitting which can be tricky to achieve.
We use an older traditional lime and cement mortar mix for our
blockwork mortar and reinforce with mesh every third course to add strength to create
a more flexible structure. Some cement mortars are so rigid, if there is
movement within a structure, the blockwork
will split in a vertical line, rather than redistributing the stresses
through the mortar joints. It is
debatable that modern brickwork and blockwork structures with more rigid mortars that require regular expansion joints to reduce the concentration
of stress which creates an intrinsic
weakness in a building.
Lime can react with some plasticisers and liquid water
proofers. The manufacturer of these products should be contacted to ensure
their products are compatible with lime.
Before the render is sealed, it is important to protect the
structure from any rainwater that may
splash mud or dirt against the walls. We endeavour to place a 1 meter wide
perimeter of crushed rock around the pre rendered structure to help minimize
splash.
rendered
