Lathing around exposed purlin

[DavidB]

Hi

I have support in principle from CO to replace modern plasterboard ceilings and expose the loft area above bedrooms. Now I'm just getting ideas for the detail of how I actually plaster up between the rafters.

My challenge is what to do with the purlins. They are just above head height, irregular in shape and are not parallel to the floor. There would be a gap between the rear surface off the purlins and the laths and I wouldn't be able to lath right up behind the purlins. The gap would be less than the width of a lath i.e. 2-4 cm... I am thinking of options such as:

1. Box-in the purlins with a thin frame and lath over the top of it - I'm not sure would look right proportionally and would probably look too straight.
2. I could leave the purlins exposed and place a filler piece i.e. piece of wood behind the purlins (top and bottom) to butt up against the laths - not sure how I'd fix these and I'd like to do something with the least additional nails / screws being used on the historic fabric.
3. I could somehow try and plug the gaps with lime - but not sure this will simply break off over time without support of some kind... perhaps I could use a metal lath/mesh but a mesh would be spoiling my proposed traditional approach.
4. I guess another option would be to position small batons attached to the rafters so that they simply angle out toward the purlin to reduce the gap to a minimum - then I could blend that in with the lime plaster.

Does anyone have exposed purlins in their property? How are they finished with plastering behind them? Any ideas or tips would be welcome - preferable a traditional approach.

Thanks.

 

[88v8]

A picture might help.
In the meantime....

Boxing in... yuk.
Yes, with an inch or more gap the lime will fall out.

I think what I would do, without seeing it, is slip a piece of plasterboard into each gap. Screw it onto the back of an adjacent lathe. Pre-plaster it where you won't be able to get. Won't be very visible, will it? Then you can join up with the main plastering. And tiddly little gaps won't matter anyway. If you want to be really perfect, embed a bit of scrim tape halfway onto the plasterboard and then feather the other half into the limework, then it won't crack.
Fiddly....

Yes, I really suggested plasterboard... what's come over me.... but sometimes...

I wouldn't ever use it for general work - flat, characterless, nasty botchers material. But it has its uses.

In general I wouldn't worry too about using anachronistic materials in a concealed context... stainless nails for instance.
After all, your lathe will be anachronistic, unless you're using riven lathe.
And so will your nails. In 1790 they would have been hand-made or hand finished.

A picture might help.

Ivor

 

[Zebra]

I'm agree with 88v8. I wouldn't box in, and I think that the best idea would be to attach some sort of boarding and plaster up to the purlins. However, I wonder if you've considered woodwool or woodfibre boards. Both can be lime plastered directly onto.
If you bought a sheet of say 50mm woodfibre board (various thicknesses are out there, get one which is suitable for the gap you have, and reduce the thickness if necessary - this is quite easy to do), cut it to shape and wodged it up behind the purlin, you could simply lime plaster from the laths up and onto it. It is very easy to shape as required, and is breathable so compatible with traditional materials.
Or, if you used woodwool board, you could attach it to the rafters on some sort of wooden batten arrangement, as per your option 4, and similarly plaster up and onto it with no seam. Use big scrim tape, and even gripfill the edges of the woodwool (I know! Gripfill! Shh!) - even then you will still get cracking, because the timber frame moves. It is unavoidable, but going over the cracks with caulk in a year or two's time and relimewashing will fill them in OK.
As Ivor says, modern materials have their place, and wisely used they are a great assistance.

 

[Flyfisher]

In general I wouldn't worry too about using anachronistic materials in a concealed context... stainless nails for instance.
After all, your lathe will be anachronistic, unless you're using riven lathe.
And so will your nails. In 1790 they would have been hand-made or hand finished.

It's a funny old business isn't it?

I can see nothing inherently superior about hand-made nails as such. A nail is a nail and, as Ivor rightly says, in 1790 hand-made nails would have been the only choice, so the craftsmen of old did not really 'choose' hand-made nails, they simply used the best or most appropriate items available to them. But when it comes to fixing laths there seems little doubt that modern, machine made, stainless steel nails are vastly superior and I'm convinced that they would have been used long ago had they actually been available! Similarly with screws. Slotted screws might be 'traditional', but that's only because they were easier to make than crosshead screws, which as anyone who has used them will know, are vastly superior to slotted screws. Again, had slotted screws actually been available in 1790 I have no doubt they would have been used in preference to slotted screws.

In general, we just cherry pick the 'traditional' methods and materials that are appropriate and convenient for our own preferences. Thus, we'll have 'traditional' and highly insecure locks on our internal doors (fitted with carefully aligned slotted screws of course :wink: ) but will fit modern locks to our external doors where far greater security is desired (and usually demanded by insurance companies!). Similarly, we'll have all the other 'mod cons' such as hot running water, comfortable bathrooms (multiple en-suites even), central heating, not to mention the ultimate modern convenience - electricity, and all the appliances that go with it! None of which would have been available in 1790, but who among us really wants to be traditional enough to live with a single open fire for heating, candles for lighting, a barrel to wash our clothes and an outside privy? :lol:

None of which is a criticism, just an observation that might provide some context when agonising over whether to use a bit of plasterboard or stainless EML to help with a spot of plastering. What would the craftsmen of old have used, had such things been available to them?