Non hydraulic lime is supplied as lime putty which is mixed with sand -1 part lime putty : 2 parts clean washed well graded sand ( for timber laths hair is added at the rate of 10kg / m3 ), Non hydraulic limes set from carbonification from the atmosphere which is a long process and can take up to 2 weeks for a coat to be strong enough to take the next coat.
Hydraulic limes have a chemical set similar to cements and can be feebly hydraulic, moderately hydraulic or eminently hydraulic (which is also called natural cement), the material is best mixed in a forced action pan mixer such as a ‘ SoRoTo‘ which we use and which gives good reliable consistency and disperses the hair (if added) evenly and generally applied to masonry backgrounds or timber lath backgrounds but can also be used on metal lathing usually applied in 3 coats.
First coat scratch coat (on timber lath or metal lath called the pricking up coat) 10mm- 12mm thick lime and sand plaster (on lath haired lime and sand).
Second coat or floating coat 12mm – 15mm thick in lime & sand plaster (or haired lime and sand on lath) If there are any severe irregularities they can be ’dubbed out‘ between the first and second coats.
Finish coat lime putty – fine washed sand – 1:1 usually 3-4mm thick applied in a min of two coats. This finish coat is not ‘polished’ with the trowel as gypsum plasters.
Timber laths for this work can be sawn or riven depending on the age of the building being restored.
Riven laths are expensive and only available in short lengths – 1.2m and can be of Oak, Chestnut or Scotch pine and these can vary considerably in width, thickness and shape but are usually around 25mm -30mm wide and 4mm – 8mm thick.
Sawn laths are usually supplied in 2.4m -3.00m lengths and can be of Oak, Chestnut, Pine, Deal or Larch and are of a much more uniform shape and thickness usually 25mm wide x 6mm thick. The laths are fixed to timber joists using stainless steel or galvanised ring shank lath nails 30mm – 40mm long and the joints staggered every 8-10 laths.