Lath and plaster system

Hi everyone.

I'm pondering a lath & plaster (lime) ceiling for my soon to be built house.
I could go for the "regular" drywall which would make my life simpler in the very short term (installation wise).
But drywall has some problems in being sealed to reach air barrier status.
Plus, if somehow it gets wet, goodbye.

So, putting lime plaster thru lath seems like a lot of work but has other advantages (like it's more healthy).
What i'm having difficulties is in realizing how it's done.
The good craftsmen that knew how to do it right are gone (either in time or space) and my questions don't have clear answers.

Traditionally, lath was made of wood (rive in the beginnings and sawn later on).
But what kind of wood ?
I have access mostly to conifer wood (spruce, fir, pine) which comes in good prices.

And the wood laths, should they be green or dried ?
Needless to say, most wood available is green, which could be good or bad (but with our low knowledge of how wood works, it's mostly bad).

And the nails ?
Or maybe screws, or staples (galvanized presumably) ?

Or maybe something else to replace the wooden lath, maybe metallic or even resin / plastic fibre / netting ?

I need info on these subjects, pros / cons, anything.

PS
Where's a certain light colored cloud when you need one ?

hey... in old buildings here you find that they nailed reet-mats on wooden ceilings (with staples) and then plastered it. i think, it s lime plaster what they used.

maybe drywall boards from clay could be an option, if they re available in your region

Hi Tobias.

I'm after the lime plaster.
Gypsum could be used but it's a moisture trap, not happy in my application.

I have access to some kinds of drywall, some types of metal meshes (expanded for example) and other industrialized materials i'm not keen on using.
I've seen too many modern buildings with major health issues (moisture - mold) which people just treated with chlorine.
But that's just treating the symptoms not the cause.
Cause was too much humidity from everywhere combined with impermeable walls and finishes (latex paint anyone?) and atop of that, little or no ventilation.

Never seen mold on lime plaster since nothing can grow on it.
I did see mold on limewash over earthen walls tho.

All in all, i prefer tried and tested methods that work, maybe with some later improvements (like metal mesh instead of wooden lath) - maybe.
Also, maybe some fibers in the plaster for extra reinforcement.

But the details can ruin a job, so knowledge and attention is key.

I got this dvd which I found very informative: The Art and Science of Natural Plaster

Cedar is used as lath because color won't leach into the plaster, also it is easy to split out thin slats for the lath. When applying the plaster, first wet the lathe generously so that the plaster has time to set rather than dry which causes cracks.

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.

Thanks Tom.

Thanks for all the details.
I really don't have access to riven oak or chestnut (mostly due to jaw dropping prices for the wood itself, not to mention the splitting).
Whatever is available is sawn wood, mostly fir or spruce, maybe some pine.
I know historically people used small split green branches of chestnut but i don't have any source of it.

I could compensate by using some mesh, for example the ubiquitous fiberglass mesh used to reinforce plaster over insulation (EPS).
I could densely staple that to the underside of the lath so there is a double key.
The traditional key is formed in the plane formed by the top face of the lath, by the plaster oozing inbetween the lath and forming "mushrooms".
The second key is obtained in the plane formed by the bottom lath faces where the plaster oozes thru the mesh into the lath spaces.

An alternative to fiberglass mesh is galvanized steel mesh which is much stronger but i'm not sure i want to use it?
Some people have used expanded mesh but that tends to be pretty expensive around here.
I know in the US people used sometimes this expanded mesh alone as the lath, without any wooden lath.

My local church has been painted this summer in a traditional fresco (mineral paints directly over fresh lime plaster) and they reinforced the plaster with hemp fiber (the kind used by plumbers in pipe joints), basically a hair like thing.
This fiver is ready available and quite cheap and i think i can use that to great success.

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.

Hey Tom, great info.

I know people who still do church plastering and fresco paintings per the old traditions use hemp "hair" similar to the image below (maybe the hairs are a little thicker).
They chop it in smaller parts and mix that with old slaked lime and sand to make the plaster.
It must be well behaved since it's an old recipe and still used successfully today.

Regarding fiberglass mesh, the only available is 4-5mm eye openings.
I guess my best bet would be to use a layer above (between lath and joists) and one below the lath, just to be sure.
Both fixed with galvanized staples.

Regarding the very well dried wood, most of the times it's really not so but more on the green.
But i think i have time to buy it sooner and let it dry until i get to actually use it.