[GSBN] Lime plaster and expansion joints

[Laura Bartels]

Thanks for everyone who has chimed in on this expansion joint with  
lime question. The points made about isolating framing (particularly  
steel) from the plaster is something that has always made sense as of  
course keeping any steel, if used, to the interior. Even placed on the  
interior, I still would rather isolate movement of one vs. the other.

Who else remembers hearing Andy DeGruchy talk at the ISBBC in Ontario  
about the flexibility of lime vs. cement? I'll have to dig up my notes  
(or refer to that very comprehensive CD that was compiled) to see what  
the details were. Going by memory (often a dangerous thing for me), it  
leaves me thinking that by eliminating issues such as differential  
expansion rates as John points out, and by proper technique in  
finishing the lime as RT points out, that not using expansion joints  
could be the safer thing to do given that expansion joints can be a  
moisture risk and lime, besides its flexibility, is easier to repair  
than cement stucco, where this whole expansion joint thing got started.

Andre, I have not before come across information on mixing gypsum and  
lime historically. Thanks for raising this. Of course, I too, have no  
idea what kind of gypsum in the states would be comparable. Anyone  
know any more about this?

And on Andre's last note, below, I am surprised. I have always thought  
of rough, sanded textures as being more water resistant. You have  
brought the most durable part of the mix to the surface to collect  
water and allow it todrip off, severely lessening the sheeting action  
that occurs on smoother surfaces such as glass and smooth troweled  
plasters. Hence harled lime on the lower part of exterior walls in  
England. This is something that Barbara reinforced when we were at the  
ISBBC. On the other hand, it does make sense to me that scratched  
plaster would be less likely to crack. Thoughts on this?

And let me add my huge congratulations to Barbara and all those  
awesome women over there. I am especially excited as a woman to see  
this kind of recognition being bestowed on you, Barbara. It's  
wonderful. (It also brings hilarious memories of our women straw bale  
olympics team!)

Laura


> I also heared a couple of days ago (at the St Astier hydraulic lime  
> plant) that scratched lime plaster is much less likely to crack than  
> smooth (troweled) plaster but that the latter is more water  
> repelling. They also claimed that in the old days plaster was almost  
> always allways troweled/smooth.
>