How to Lath and Plaster

Lath and Plaster

Lath and plaster work traditionally used riven oak or riven chestnut lath. 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 vary between 1¼” to 1½” (31 – 37 mm) in width and are around ¼” thick.

Forming the keys is achieved by squeezing the lime plaster between the lath when trowelling. Backing coats of lime plaster should contain animal hair 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 (25 mm).
Space gaps between lath by approximately ¼” (use a lath on its edge to set the spacing).

During the 20th century, expanded metal lath (EML) began to supersede timber lath in new work and often in renovation work as well, being cheaper and quicker to fix. Lime plasters stick less easily to EML and there was also a move towards using harder cement-based plasters and gypsum.
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 (in the case of cement).

(Mike Wye supplies both sawn and riven laths.)

Application

Fixing Lath

It is important you stagger the lath on installation so that the ends of the laths are not all in a straight line. This will avoid introducing a weak stress line in the finished lath and plaster.

Soak new lath before fixing to ensure that they tighten when they dry.

Preparation

Control the 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.

Example Lath and Plaster Specification

• Apply scratch coat of Haired Lime Mortar through the lath (15 mm for material consumption), leaving around 1/3″ (8 mm) on the surface. Do not over trowel this coat as too much plaster may be lost through the lath.
  Lightly scratch this coat with a lath or scratch comb and leave to dry and cure until green hard. A lime mortar or plaster is green hard when it can only just be scratched. If you can make an indentation with your thumb it is not ready.
• Next, apply a 8-10 mm float coat of Unhaired Lime Mortar to straighten the surface as required. Float with a wooden float followed by a devil float to provide a light key for the final lime skim. Leave to dry and cure until green hard.
• Apply Heritage Lime Plaster in two passes totalling 3 mm for the top coat skim. If shrinkage cracks appear, lightly spray the plaster with water and trowel or sponge them out.

Gauging

Leave sufficient time for plaster coats to gain strength when there is a floor in use above, leave sufficient time for the plaster coats to gain sufficient strength before significant use (e.g. moving furniture, fixing floor boards etc.). This is especially true if there is any play in the joists that cannot be eradicated.
Use Extra Haired Mortar for the scratch coat and consider a gauge of 10% NHL5 for an earlier set. 

Coverage

• 15 mm scratch coat Haired Lime Mortar, 30 kg per m²

• 10mm float coat Unhaired Lime Mortar, 20 kg per m²

• 3 mm skim Heritage Lime Plaster, 6 kg m²

• For the very finest top coat skim, use an additional 1 mm coat of Regency Lime Plaster 2 kg per m²

Safety

Limes are caustic. Always wear eye protection and protective gloves and clothing and follow the safety instructions on the labels.

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