Traditional Lath and Plaster
Timber laths used for lath and plaster 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 – 37 mm) 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 typically contained 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 (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 supersede 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 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).
(We supply both sawn and riven laths.)
Fixing Lath
It is important to 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 a first scratch coat of Haired Lime Mortar through the lath, leaving around 1/3″ (8 mm) on top of the lath itself. Do not over trowel this coat otherwise 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 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 Unhaired Lime Mortar to straighten the surface as required. This coat may be 1/3″ – 1/2 ” ( 8 – 12 mm) thick. Float this coat with a wooden float followed by 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 two thin coats of Heritage Lime Plaster, based on a very fine sand and lime putty. If any shrinkage cracks appear, lightly spray the plaster with water and trowel or sponge in the cracks.
Gauging
Where it is a lath and plaster ceiling that is being plastered and there is a floor above that will be walked on, leave sufficient time for the plaster coats to carbonate to gain sufficient strength before significant use of the room above (e.g. moving furniture, fixing floor boards etc.). 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. The first scratch coat should have extra hair added and could also be gauged with 10% NHL5 to get an earlier set. Even so, subsequent fitting of floorboards above should be screwed rather than nailed down.
Coverage
- One scratch coat Haired Lime Mortar, 30 kg per m² (15 mm)
- One float coat Unhaired Lime Mortar 20 kg per m² (10 mm)
- Two top coats of Heritage Lime Plaster, totalling 6 kg m² (combined thickness of 3 mm for the two coats)
For the very finest skim finish use:
One coat of Heritage Lime Plaster 4 kg per m² (2 mm)
One coat of Regency Lime Plaster 2 kg per m² (1 mm)
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.