rendering on wooden lath using St. Astier products.
Timber framed buildings, historic or new construction have a tendency to greater
movement than solid masonry construction, particularly new green timber construction.
To achieve the maximum degree of tolerance to movement in a render on wooden lath,
a low strength mortar of exceptional modulus of elasticity should be used. Plasticity,
mainly depending on the free lime content in the binder, is also essential.
The St. Astier range offers 2 products highly suitable for wooden lath work: NHL
2 and EcoMortar.
NHL2 : a binder to be used with
well graded sharp sands, coarse or fine depending on the use (see: Sands
for NHL mortars). Its elasticity moduli is around 1,306 PSI at 28 days
and the free lime content is over 50%.
EcoMortar is a pre-mixed Natural
Hydraulic Lime Mortar that has a modulus of elasticity of 4000 to 5000
Mpa and it is based on NHL 3.5 (type G) or NHL 2 (type F) with free lime
content in the region of 25% and over 50% respectively.
Lath should be break and but jointed, clean and sufficiently strong to adequately
span between framing members.
On refurbishment and repair
work, laths must be thoroughly cleaned to remove old mortar and all dust and should
be dampened to avoid moisture absorption from the mortar.
When working with NHL 2 the mortar should have a hair or fibre reinforcing
well distributed throughout the mix. In the case of animal hair at least
2.5 lbs. of long strong hair per m3 of mortar
will be required, more in some cases. All coats will require hair or fibre
reinforcing with the exception of the finishing coat, which should not
be greater than 3/16".
Mortar should be applied by a laying on trowel to a properly prepared background,
ensuring a good penetration between the laths and covering the outer face of the
laths by at least 8-10mm.
Control of drying conditions and
protection from rain, wind and frost are essential to ensure a good slow rate
This will vary due to atmospheric conditions and temperature but
a minimum curing period of 4 -7 days between coats is recommended. Good keying
between coats is essential as well as dampening.
thickness of the render coat will be determined by several factors, such as detailing
and existing conditions, but in all cases exposure will be the primary consideration.
Thicker render coats will be required in areas of high rainfall.
Coats should not be stronger or thicker than the preceding coats.
rendering on lath is a specialised skill and should only be carried out by experienced
For internal work see Plastering with NHL.
above information is given as general indication only. Dosages and application
may vary in accordance to materials and aggregates used and site conditions. Please
liaise with your St. Astier distributor to finalise mortars mixes. Good working
practices in mortar mixing, protection and curing are assumed at all times.|