Tom Balch

Hi Ionel,

I've plastered with hemp and lime and it goes on really well. The hemp I've used was chopped hemp 'shiv' rather than fibres and creates a porridge like consistency which has a small thermal benefit. It would need to be applied very thickly to really act in an insulative capacity.

I would think hemp fibres could work as a replacement for hair providing they don't biodegrade over time and weaken the plaster.

I've seen redwood/pine laths and would think they would be fine if well dried.  You can plaster directly onto sawn laths as long as you get good nibs through.

In terms of mesh I'd be reluctant to use metal. Fibre glass works well with lime and Mike Wye near to me supplies mesh made by Secil by the reel.

I've used reed sheets stapled directly to joists and plastered directly on to it with a haired lime mix, if you're going to need mesh this maybe a less labour intensive option to laths.  If you use reed don't wet it! It repels the water and the plaster will never stick! Again you need to apply plenty of pressure to force the plaster between the reeds.

Hi Ionel,

In England lath is traditionally riven oak or chestnut, riven wood being thought to be stronger and more flexible than sawn. The riven wood also gives a better key for the plaster.

Laths are fixed to the joists with reasonably large headed galvanised nails and spaced about a little fingers width apart. Mild steel may corrode in the caustic environment of the lime plaster and cause the laths to fail prematurely.

The lath should be dampened prior to working to reduce shrinkage cracks but it should be allowed to soak in the moisture. If the surface is running wet the plaster will slump away (often on to your head!).

The plaster should be a mix of lime putty and sharp sand for the base or scratch coat in a ratio of about 3/1 with the important addition of fibres. Traditionally this would be animal hair although you can use modern alternatives. The fibres give the plaster its strength, reduce cracking and strengthen the 'nibs' or the plaster pushed through the gaps in the laths.

Try to work in thin coats, trowling diagonally across the laths and applying quite a lot of pressure to force the plaster between the laths and creating nibs above to hold it. It can help to use a small narrow based trowel to get equal pressure and reduce wastage of the back edge (again often on to your head!).

Once the first coat has stiffened up you should scratch it in a diamond pattern to provide a key for later coats.

Later coats maybe a 3/2 mix with finer sands or even pure lime or lime with chalk or dust for the finish.  Most high quality work was in 3 coats - scratch coat, floating or levelling coat and finish coat. The finish coat being very thin and often applied in two passes.

The work can be done in two coats but you won't get such a smooth finish. Also don't apply your coats too thick or the weight will pull the plaster down before it dries.

Hope that helps.