Guidesheets:    Plastering onto Lath

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Types of Lath:
Traditional timber laths were commonly riven oak or chestnut. These are laths that have been split along the grain of the wood by hand. They are generally irregular in shape, width and thickness with a coarse surface that provides extra key. Laths varied between 1 1/4" to 1 1/2" (31 - 37mm) in width and were around 1/4 " thick.

The main key is formed by the lime plaster being squeezed between the lath by the trowelling action. Backing coats of lime plaster were typically haired to help the plaster keys stay in place whilst curing occurs.

By the end of the 19th century sawn lath produced by machinery was also much in evidence. This is much more uniform in nature and has a smoother surface giving less key to the mortar. Hence the key formed by the plaster squeezed between the lath is of even greater importance. Sawn laths are generally a little narrower at around an inch (25mm). Timber laths were generally spaced out by around 1/4" to 3/8", and a lath on its edge was used to set the spacing.

During the 20th century, expanded metal lath (EML) began to supercede timber lath both in new work and often in renovation work as well, being cheaper to buy and quicker to fix. Lime plasters stick less easily to EML and there was also a move towards using harder cementitious plasters and gypsums. Many of these developments were out of keeping with the properties for which they were specified but also introduced their own problems due to their relative lack of breathability.

Preparation:
It is important to control suction from dry timber lath by lightly spraying with water 30 minutes before the first coat. There must never be any moisture on the surface of the lath.

Premixing:
All lime mortars and plasters benefit from being pre-mixed for a week or two in advance and then "knocked up" again prior to use to plasticise them - this reduces shrinkage in the plaster.

Example Specification:

Apply a first scratch coat of 3/1 haired lime mortar through the lath, leaving around 1/3" (8mm) on top of the lath itself. Do not over trowel this coat otherwise too much plaster may be lost through the lath. Do not trowel this coat too smooth but instead leave an open textured surface for extra key for the next coat of plaster.

• lightly scratch this coat with a lath or comb scratcher and leave to dry and cure until green hard. A lime mortar or plaster is green hard when it can only be marked with a metal tool. It is dry enough for any shrinkage to have taken place without having to be completely dry.
• apply one float coat of 3/1 haired or unhaired lime mortar to straighten the surface as required. This coat may be 1/3" - 1/2 " ( 8 - 12mm) thick. Float this coat with a wooden devil float to provide a suitable surface for the final skim coat and leave to dry and cure until green hard.
• trowel on a double top coat of our lime-rich plaster, 3/2 unhaired, based on a very fine sand and lime putty. This can be in a single coat or two very thin coats for finer work. If any shrinkage cracks appear, lightly spray the plaster with water and trowel or sponge in the cracks.

Gauging:
Where it is a ceiling that is being plastered and there is a floor above that will be walked on, sufficient time MUST be left for the plaster coats to carbonate to gain sufficient strength before using the room above. This time will depend on circumstances such as time of year, ventilation etc but may be a minimum of 6 months. This is especially true if there is any play in the joists that cannot be eradicated. If it's essential to access the room above sooner, then the first scratch coat may be double haired and also gauged with 10%-15% pozzolan to get an earlier set. Even so, subsequent fitting of floorboards should be screwed rather than nailed down. A pozzolan can also be added to the top coat plaster to ensure extra durability where knocks are expected.

Quantities:
one scratch coat 3/1 haired lime putty mortar, 30kg per m2 (15mm)
one float coat 3/1 unhaired lime putty mortar, 20kg per m2 (10mm)
two top coats of 3/2 lime putty plaster, totalling 6kg m2 (3mm)

For the very finest finish, instead of two coats of 3/2:
One coate of 3/2 lime putty plaster 4kg per m2 (2mm)
One coat of Regency lime plaster 2kg per m2 (1mm)

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 is important that users satisfy themselves that they have chosen an appropriate product and have a suitably skilled workforce.

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