Lime Plaster onto Laths

Lath and lime plaster is a building technique used mainly for interior walls in period buildings. A lath is a narrow strip of wood approximately 2 inches wide which is nailed horizontally to each stud in the frame. Each lath is spaced approximately 1/4inch (thickness of the average little finger) away from the next leaving enough space for the mortar to push through and hook over the lath to form a key.

Preparation

The LATHS must be free from dirt, grease and vegetation. These elements should be removed before damaged laths are repaired. Riven laths compared to sawn laths are the strongest and should always be used on ceilings. Stainless steel ring shank nails should be used to fix the laths. Spray a clean cold water mist on to the laths prior to applying the scratch coat.

First Coat

Our hot mixed non hydraulic COARSE STUFF with fibres or hair is ideal for the first coat onto laths. Hot lime mixed plasters are more flexible and therefore more suitable for lath work. However if an NHL is specified then NHL2 should be used. If the plaster is made on site, the HORSE HAIR needs to be evenly distributed throughout the mortar.  Working to a layer of 10mm thick, push the plaster into and across the pre-wet laths at a 45 degree angle to the laths.The plaster should be left to steady up before being scratched in a diamond pattern ensuring the scratches are not running along the laths. The plaster should then be left until set hard but protected from heat and draughts. Any cracks that appear in the first coat should not be rubbbed out as this may damage the nibs of the plaster behind the laths. 

Second Coat

The second coat also known as the intermediate or float coat will be applied similar to the first coat.  This  should be with MEDIUM STUFF but may or may not have hair added. You must ensure you wet down the first coat with a fine mist before spreading a 7mm coat. This may be lightly scratched with a devil float before leaving in preparation of the finish coat.

Finish Coat

For the final coat use FINE STUFF - an internal top coat plaster. As before, emember to wet down the surface before application with a fine mist spray. Allow time for the water to absorb into the plaster then apply a coat of approximately 2mm. When this has slighly stiffened up apply a further 2mm coat and trowel to a smooth finish with a steel trowel or a wooden float will give a more open texture.

Coverage

Laths - Bundle of 50 of 4ft laths will cover approx 2.08m2.

NAILS - 1kg of stainless steel ringshank nails will cover 7m2.

Medium Stuff with hair - 25kg will cover 1m2 for the first coat and 1.2m2 for the second coat.

Fine Stuff - 25kg will cover 4m2 at a thickness of approximately 3mm.

Health and Safety

Lime mortar by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to eye and skin protection. EYEWASH should always be on hand. GLOVES should be worn when working with any mortars or plasters.