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Re: GSBN:fire test -- your opinions?

I'm following up with additional comments, after reading Bob Theis and
Andrew Webb's replies.

It's true that fire-rated walls are more likely to be used in urban
contexts.  Especially 2-hour walls.  But I wouldn't assume that clay based
plasters are less likely to be used in an urban context than a sub-urban or
rural context.  This is probably true for a building's exterior surface, but
it might be less true for interior surfaces.

Also, it is not uncommon (at least in the US) for 1-hours walls to be
required for a residence or accessory building in a suburban context because
of proximity to a property line.  Plus there is the routinely required
residence/garage 1-hour separation (garage side only) (in the US).

Then there are occupancy separations and area separations for larger
buildings, which would be interior walls, where I could see a preference for
a fire-rated wall with a clay based plaster.

Regarding Andrew's suggestion that the walls be tested with lime plaster, it
seems that lime plaster, cement-lime plaster, and cement plaster would
perform more or less the same, and if one succeeded, you could reasonably
infer that the others could be substituted to yield an equally rated wall.
I say this being aware of the extremely rigid adherence to tested assemblies
which is usually enforced in the US.  But if you accept the idea that you
pick one of those plasters to represent the performance of all three, I
would go with cement-lime for the obvious reason that it is a hybrid of the
other two.  Although maybe I would make it 1 part cement to 2 parts lime in
order for its material characteristics to be halfway between the other the

More grist for the mill.

Martin Hammer

Your point that rated walls are more likely necessary in
commercial/institutional situations is true, but I can see clay plasters
being used there, plus there are residential situations that demand a 1-hour