This is a quick aside. A customer had a ceiling he needed me to plaster, he had already fitted the laths, sawn oak, he had fitted them really well with very good spacing, the job was simple the ceiling only 15 square metres – easy.
I got the first coat on on a Friday and on the Monday got a call that quite a lot had fallen off.
As the picture shows it hadn’t all fallen off but even so there were a few patches and it wasn’t immediately clear why. What was plain to see was that some of the laths had bent forcing the plaster off.
In the end we settled upon the likely cause – the laths had been tightly screwed rather than nailed and they were too tightly fixed to allow any stretching as they absorbed moisture from the lime. and so they had to bend.
If they had been nailed they would have split and and although adequately fixed would have been able to move just that little bit and wouldn’t have needed to bend.
If you think about it, each individual lath needn’t be fixed very securely or need be very strong as one lath isn’t doing much work and neither is one nail or screw in this case.
Thanks for that post. We did screw our laths when we did a large sloped ceiling and everything went ok. Do you always recommend nailing the laths or do you think screwing them would be OK if you didn’t go to mad? Maybe we were just lucky?
I’m enjoying reading your blog. I’m just learning lime plastering myself and loving it.
Thanks for your interest.It wasn’t a problem we had ever encountered before and I suppose it may have been something to do with the way the laths were stored or even something to do with the wood itself. On the next job we soaked the laths in water for a while and fixed them whilst a bit damp, screwing them up shouldn’t be a problem, unless perhaps if the screws are really tight (and the customer pre-drilled them all as well – rather him than me!) I generally use a paslode stapler and 20mm staples, much quicker.
Hello, just a thought – riven laths follow the grain of the wood, so the movement would be a lot different to sawn, where lots of the ends of the fibres are exposed. This might mean the sawn laths take up more water from the plaster. I wonder if fixing the laths when green, or soaking them beforehand would help counter this? Also, nails being more malleable than screws, would be able to move more with the wood – this is apparently why medieval furniture has lasted so long.
Very informative blog by the way – I am looking to start up in conservation masonry work and need all the help I can get!
HEllo Benn, Subsequently we soaked the laths before fitting and did not suffer the same expansion problem after plastering. The thing with nails is that they split the wood and this I think is a good thing because it allows the lath to expand a little and slip along the nail, whereas the screwed laths, which had been pre drilled – where fixed very tightly and could not slip.
I wish you well in the world of conservation.