Fat lime plaster
Application Guide
Description
Tŷ-Mawr Lime Plasters/Renders are made from a high
calcium lime (also known as a fat /air/putty or nonhydraulic lime) and carefully selected aggregates. Lime
plasters require exposure to Carbon Dioxide in the
presence of moisture to harden. Preparation and
protection are just as important as correct application.
coats totalling about 3mm. Externally, the process is
the same for the first two coats but the Top Coat
Plaster (external) should be applied in one coat of
about 4mm-6mm.
■ each coat should be dampened before applying the
next coat.
Please note
The application of lime plaster is more involved than
using conventional plaster. It is highly recommended
to use a plasterer experienced in the use of lime
plasters or at the very least you should get some
practical experience e.g. on a course, as it is not
possible to cover every point in detail here. However,
if you do experience problems with our products after
following the advice given, please contact our Product
Support Team at tymawr@lime.org.uk
Preparing the mix
■ the plaster needs to be ‘knocked-up’ - a process of
chopping, beating and turning which will release some
water. Only the minimum amount of water should be
added if required to make a workable mix, it should be
used as stiff as possible.
■ a mortar mill or forced action mixer is the ideal way to
‘knock-up’ the lime plaster but this is not always
possible. If a bell mixer is used, then it should be left
turning for long enough to achieve a suitable
consistency without adding water (20-30 minutes).
■ if water is required, it should be added CAUTIOUSLY.
■ if hair is required, gently tease in towards the end of
the ‘knocking-up’ process – avoid hair balls.
Preparing the surface
■ pre-wet absorbent surfaces such as
stones/bricks/laths by spraying (usually 2 or 3 times)
before application. Lime plasters or renders can only
carbonate whilst moist, if they dry out before they
carbonate, they can fail. Please note if you are using
Tŷ-Mawr wood wool boards or woodfibre boards as a
background for your plaster, pre-wetting is not usually
required.
■ dub-out the walls with Lime Hemp or Base Coat
Plaster (haired) where necessary before applying the
first coat of base coat plaster to create an even
surface (each coat of lime plaster can then be applied
at the same even thickness). This should be left to
harden before applying the first base coat.
Nibs forming at the back of
the laths give a ‘finger hold’
Scratch coat being scratched
with a wire comb/lath scratcher!
■ apply the first coat of Lime Base Coat Plaster (scratch
coat) with a steel float evenly. When it has firmed up,
scratch the plaster (to about 3-4mm depth) diagonally
using a wire scratcher. This could be the same day on
brick or stone or, up to a week or more, on lath. This
coat takes up any shrinkage and may crack. You need
only worry if the plaster becomes detached from the
background.
■ apply the next (floating) coat - as above but instead of
scratching, you need to float the plaster whilst it is still
soft enough to take the indent of your thumb. This will
compact the plaster to avoid shrinkage cracks as well
as flattening the wall. It should be carried out using
either a straight grained (for uneven surfaces) or cross
grained (for flat walls and ceilings) wooden float, not a
plastic float. Floating is hard physical work, sometimes
a little water sprayed onto the surface can help the
process.
■ after the floating coat has been floated and the plaster
has firmed up, use a devil float to form the key for the
finish coat, this is done by rubbing the surface of the
float coat in small, circular motions to achieve a depth
of not less than 2mm.
Application of the Plaster/Render
■ onto stone/brick/lath: Internally, it is usual to apply
three coats – our Lime Base Coat Plaster is used for
the scratch (first) and floating (second) coats, they
should both be applied at about 8mm thick, the Top
Coat Plaster (internal) should be applied in two thin
■ the surface of the floating coat should be scraped
down with the side of the trowel and then brushed to
remove loose material whilst still green (holding
moisture).
T ŷ-Mawr manufacturers and suppliers of traditional and ecological building materials
version 10/10
Reedboard
Scratch Coat
Floating Coat
Finish Plaster
Traditional three coat work
■ internally, lay one thin coat of the Internal Top Coat
Plaster at about 2mm onto the surface. Once it has
firmed up, scour the surface with a well-worn wooden
float. Lay on the second coat of Lime Top Coat Plaster.
When this has firmed up, trowel up with a steel trowel,
using a light spray of water to remove the fat – do not
over trowel. If required, wet the surface before working
it with the trowel or sponge floats, by misting with
water, however over-wetting should be avoided
because it will draw too much lime to the surface.
■ externally, one coat at 4-6mm of the External Lime Top
Coat Plaster can be applied.
Protection of the work
■ allow each coat to ‘go off’ before applying the next
one. As a guide, it takes one-two weeks for the first
coat, one week for the second, and a few days for the
finish coat. This will vary considerably depending on
weather conditions and substrate. Each coat should
be hard enough to resist indentation from a knuckle,
but be soft enough to scratch with a fingernail.
■ protect new work as conditions dictate eg for hot, dry
or windy weather, damp hessian and plastic sheeting
may be required to prevent rapid drying. Gentle
spraying may be necessary if areas of plaster or render
are drying too quickly. Note often the top needs to be
treated differently to the bottom of the plaster/render
as moisture moves down and different elevations may
need different levels of protection.
■ do not use dehumidifiers and heaters to speed up the
set. Good, even ventilation is key, accelerated drying
will prevent carbonation and may cause the
plaster/render to fail. Gentle heat may be used
cautiously in cold, damp buildings.
■ protect new lime plaster or render from frost - external
work, wherever possible should not be carried out
when temperatures are likely to fall below 5°C. before
carbonation has taken place. If work must continue
externally, although not advisable, the scaffold should
be fully enclosed and well-circulated heating supplied.
Fat lime plaster
Application Guide
Plastering with fat lime plaster
After care
Your finished lime plaster will care for your building for
years to come as well as helping to control the humidity
of the internal environment. It gives a beautiful finish that
no modern plaster can replicate. We highly recommend
that you finish it with a ‘breathable’ and preferably a
natural paint, your choice will depend on the level of
durability, required vapour control and desired aesthetic.
Call 01874 611350 for advice or visit www.lime.org.uk.
Additional Information
Further instructions on plastering and rendering with fat
lime plaster onto different substrates are available from
your local supplier. Courses are run regularly for
contractors, home owners and specifiers which can save
much time on-site as well as ensuring a successful
project. Please see www.lime.org.uk.
Approximate coverage rates
Plaster
1 x 25kg bag covers 1sqm @ 7-8mm thick
(scratch/floating coats)
1 x 25kg bag covers 3sqm @ 3mm thick
(finish coat)
Hair
/2-3/4 of a bundle (50-75mm (2”-3”) in
length) per bell mixer load
1
1.5kg per tonne of mix
New quantity calculators are now available on
www.lime.org.uk to help you calculate how much
plaster you might need for each coat.
Health and Safety Information
WARNING
Skin Irritation 2 H315 Causes skin
irritation.
STOT SE 3 H335 May cause respiratory
problems.
DANGER
■ protect from rain - heavy rain can wash the lime out, or
at least, draw it to the surface before the render has
carbonated. A new render should be protected until
surface carbonation has taken place.
Storage
■ store airtight and frost-free.
■ plaster with natural hair must be used within four
weeks otherwise more hair will need to be added.
■ premixed plasters/renders will become hard over time,
although still usable, they will take considerable effort
to ‘knock up’. For easier results use as soon as
possible.
Eye Damage 1 H318 Causes serious eye
damage.
Precautionary Statements
P102 Keep out of reach of children.
P280 Wear protective gloves, eye
protection/face mask.
P305 + P351 + P310 If in eyes rinse
cautiously with water for several minutes
and immediately get medical assistance.
P352 + P352 If on skin, wash affected
parts immediately with plenty of soap and
water.
For further information about the whole subject and illustrated diagrams of plastering and pointing techniques,
The Lime Handbook is available at a reduced price of £12.50 to T ŷ-Mawr customers, quote IS09 when ordering.
© Copyright - please do not infringe our copyright by copying or reproducing any part of this document.
Disclaimer - no responsibility can be accepted for any errors or omissions or any loss or damage as a result of using the information provided in these guidlines.
Distribution warehouse: Unit 12, Brecon Enterprise Park,
Brecon, Powys LD3 8BT Tel: 01874 611350 Fax: 01874 658502
Email: tymawr@lime.org.uk www.lime.org.uk
Tŷ-Mawr
Lime hemp
plaster
Application Guide
Description
Preparing the surface
Tŷ-Mawr Lime Hemp Plasters are made from a high
calcium lime (also known as a fat/air/putty or
■ there is usually no need for pre-wetting backgrounds
(but please assess first) before applying lime hemp
plasters as they hold more water than standard lime
plasters. The suction created by differences in
background moisture will help bond the lime hemp
plaster to the background.
non-hydraulic lime) blended with the hemp fibres and a
pozzolan to aid the set.
The Tŷ-Mawr product is entirely made from British
materials. It requires exposure to Carbon Dioxide in the
presence of moisture to harden however, it can be
applied in thicker coats than conventional lime plasters.
It has good flexural strength and is breathable, which
makes it suitable for use in the restoration of old 'solid
wall' construction buildings as well as ecological new
builds, it also provides additional insulation, is easier to
use and is ideal for patching and improving air tightness.
■ if Tŷ-Mawr wood wool boards or woodfibre boards are
being used as the background, they do not require
damping down. N.B. do not put scrim or mesh directly
onto the boards.
Please note
The application of lime plaster is more involved than
using conventional plaster. It is highly recommended
to use a plasterer experienced in the use of lime
plasters or at the very least you should get some
practical experience e.g. on a course, as it is not
possible to cover every point in detail here. However,
if you do experience problems with our products after
following the advice given, please contact our Product
Support Team at tymawr@lime.org.uk
Preparing the mix
■ the plaster needs to be ‘knocked-up’ - a process of
chopping, beating and turning which will release some
water. Water should be added cautiously to make a
workable mix.
■ a bell mixer is the ideal way to ‘knock-up’ the lime
hemp plaster. It should be left turning for long enough
to achieve a suitable consistency (20-30 minutes).
Smaller quantities can be ‘knocked-up’ with a whisk
and bucket if no mixer is available.
Application of lime hemp plaster/render
■ lime hemp plaster is suitable for base coats, textured
top coats and for dubbing out.
■ unlike standard lime plasters, lime hemp plasters can
be applied in single coats 10-50mm thick (thickness
will depend on the substrate and application, further
advice should be sought from the manufacturer).
■ lay on and trowel up with a steel trowel, wet the
surface of the plaster before working it over with
trowels or floats, by splashing with water. Over-wetting
should be avoided.
■ the lime hemp plaster can be left without a top coat if
a textured finish is required. If a finer finish is required,
feather edge and soft brush the base coat to give a
key then apply a top coat of fine lime hemp plaster at
5mm or 3mm of standard lime top coat plaster.
Following good practice, this should be applied before
the base coat is completely hard.
■ finish with a limewash or breathable paint if required.
Assessing the workability of the lime hemp plaster and deciding whether
Please note for ceilings, onto Tŷ-Mawr wood wool
boards, it is best to apply a 3mm coat of 50% fine lime
hemp plaster and 50% standard finish plaster first, then a
coat (max 25mm) of coarse lime hemp plaster
immediately afterwards. The purpose of this initial fine
coat is to provide adequate suction for holding up the
coarse plaster whilst it is wet.
or not to add water
T ŷ-Mawr manufacturers and suppliers of traditional and ecological building materials
version 10/10
Lime hemp plaster
Application Guide
Application on ceilings
Storage
■ for ceilings (heavy stress), onto Tŷ-Mawr wood wool
boards, it is best to apply a 6mm coat of medium
grade lime hemp plaster, then the render mesh can be
trowelled in, followed by a 6mm top coat of the same
medium grade lime hemp plaster. If thicker coats are
applied, the mesh would not be required.
■ store airtight, dry and frost-free.
■ for ceilings (light stress) onto Tŷ-Mawr wood wool
boards, a single 15mm coat (with no mesh) can be
applied of the medium grade lime hemp plaster.
Your finished plaster will care for your building for years
to come as well as helping to control the humidity of the
internal environment. It will give a beautiful finish that no
‘conventional’ plaster can replicate. We highly
recommend that if you paint it, that you should finish it
with a ‘breathable’ and preferably a natural paint, your
choice will depend on the level of durability, required
vapour control and the desired aesthetic.
Protection of the work
■ lime hemp plasters hold more moisture than
sand/glass lime plasters and therefore do not require
as much tending.
■ it can take anything between 1 week to a month or
more to completely dry depending upon the substrate,
thickness of application, wetness of material and
environmental conditions.
■ monitor new work - as much care needs to be
provided to encourage drying as to protection against
rapid drying.
■ to encourage drying - good ventilation is vital and
possibly the gentle use of dehumidifiers, blowers
and/or gentle heat (please consult manufacturer before
using). Care should be taken in winter months as
drying is slower.
■ to slow drying down - gentle spraying may be
required.
■ premixed lime hemp plaster will become hard over
time and require more ‘knocking-up’, therefore, use as
soon as possible after purchase.
After care
Please note
Refer to specific manufacturer’s instructions for
detailed notes on plastering and rendering with lime
hemp plaster onto different substrates.
Health and Safety Information
WARNING
Skin Irritation 2 H315 Causes skin
irritation.
STOT SE 3 H335 May cause respiratory
problems.
DANGER
Eye Damage 1 H318 Causes serious eye
damage.
Please note
Precautionary Statements
Failure to provide adequate ventilation can lead to
temporary mould growth whilst carbonation is taking
place. Immediate action should be taken to remedy
this situation. Call 01874 611350.
P102 Keep out of reach of children.
P280 Wear protective gloves, eye
protection/face mask.
P305 + P351 + P310 If in eyes rinse
cautiously with water for several minutes
and immediately get medical assistance.
P352 + P352 If on skin, wash affected
parts immediately with plenty of soap and
water.
For further information about the whole subject and
illustrated diagrams of plastering and pointing
techniques, The Lime Handbook is available at a
reduced price of £12.50 to T ŷ-Mawr customers, quote
IS09 when ordering.
Tŷ-Mawr Training Centre - plastered internally with lime hemp plaster.
© Copyright - please do not infringe our copyright by copying or reproducing any part of this document.
Disclaimer - no responsibility can be accepted for any errors or omissions or any loss or damage as a result of using the information provided in these guidlines.
Distribution warehouse: Unit 12, Brecon Enterprise Park,
Brecon, Powys LD3 8BT Tel: 01874 611350 Fax: 01874 658502
Email: tymawr@lime.org.uk www.lime.org.uk
Tŷ-Mawr