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PLASTERING/RENDERING ONTO WOODEN LATHS

Underlying Principles
While there is an element of adhesion of render / plaster to laths, it is the all-important bit of render or plaster between the laths that is essential to it all remaining in place.

This "neck" connects the nib - the lump of render that forms on the back of the laths - with the render face, anchoring the render to the lath. Hence the importance of adequate hair in the mix, as without such reinforcement, the tensile strength of this "neck" is minimal.

(Adequate hair also gives the plaster flexibility to accommodate any movement in the substrate). Equally essential are sufficiently wide spaces - 10mm to 12mm - between the laths to enable render to be pushed between them to form this anchor. A simple gauge when fixing laths is the thickness of the tip of one’s little finger.

The two principle causes of failure of plaster and render applied to wooden laths are insufficient hair in the mix and insufficient gaps between the laths.

We are often surprised that while the other ingredients of a mix are specified in careful proportion, the hair content is frequently only referred to in the vaguest terms, with the specifier clearly relying on the plasterer to judge for himself. All too often the inadequacy of such "specifications" is later exposed, together with bare laths, by a sudden and unexpected collapse of plaster.

More information regarding our wooden laths can be found on our products page.

Haired Chalk Plasters
In restoring timber-framed buildings, a mix of one part lime and 3 parts sand, with the addition of animal hair, is often specified for plaster and renderwork on wooden laths. This can give very satisfactory results - as long as the plasterer is actually aware of how much hair there should be in the mix.

However, such a mix will give a plaster that is the consistency of biscuit, whereas the original is often clearly different and much more flexible material, much softer and paler in colour.

Thirteen years ago, we examined the plasters of various recently repaired ceilings that had suddenly collapsed without warning. In each case the plaster was a brittle, lime and sand mix containing hardly any hair, bearing no resemblance to the original plasters that are widespread in so many old buildings and are still in excellent condition after several hundred years.

Analysis of many old plasters showed them to contain no sand at all. They comprised lime, chalk and a great deal of hair - up to 16kg per cu metre, much more than we were aware of anybody using. Since then we have been replicating those old plasters with our Haired Chalk Mix, and there are now many examples of its very satisfactory use, on both interiors and exteriors.

As well as being soft and extremely flexible, well able to accommodate considerable movement, even on recently repaired timber frames, it is also much lighter in weight that the equivalent lime and sand mix, This considerably reduces the load hanging on the building and makes it a perfect material for ceilings.

Hair Chalk Mix is easy to apply - unconventionally and very satisfactorily, in one application up to 20mm thick. However it is tedious to mix. Anglia Lime Company takes that tedium away with their Readymix Hair Chalk Mix.

'More information regarding our wooden laths can be found on our products page.

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