[GSBN] Lime plaster and expansion joints

[John Swearingen]

I would definitely say that any steel, either posts or bracing, behind the
plaster needs to be isolated and allowed to expand and contract.  Putting
posts deeper into the wall works, and also may be problematic in structural
detailing.  We've been isolating steel by liberally wrapping steel the foam
that plumbers use bringing pipes out of slabs, as well as putting paper
between the foam and plaster, to make a slip joint.

Expansion joints are a crack in the plaster, and so you must have something
behind the joint to deal with the water that will come in.  In most of the
SB that we do, this isn't possible or desirable.  We aim to reduce cracking
by making the substrate uniform so that the plaster will have a uniform
thickness.  We fill gaps and depressions with straw/clay, and isolate wood
and steel from the plaster.



On Sun, Feb 8, 2009 at 9:09 AM, Jeff Ruppert <jeff at odiseanet.com> wrote:

> Laura,
>
> The horse arena that we built near you has had more cracking on the
> exterior walls than I expected.  We placed regular expansion joints on every
> post, which were roughly 20 feet apart.  The frame was steel.
> In looking at the cracks it seems that the expansion of the frame in a
> vertical direction places the panel in tension and creates the cracks.  The
> vertical expansion joints don't do anything for this action.
> The interior plaster on the same walls do not show many cracks.  The frame
> is against the exterior face of the wall and the mesh was attached to the
> frame.
> The interior bale walls on the same project which are not exposed to the
> extremes of the weather have not cracked.
> In doing it over I would have found a way to not attach the plaster to the
> frame.  I am not sure how this could have been done easily, but steel frames
> expand and contract with temperature so much that attaching a rigid material
> like plaster to it can be problematic.  Maybe horizontal joints would have
> helped a little, but I doubt it.  From the appearance of things the
> magnitude of expansion and contraction was just too much for any type of
> plaster to deal with.  I would expect the same to happen with both earth and
> lime, unless you make them elastic or tell the to stretch when it gets hot.
> If your frame is wood these issues are going to be easier to deal with.
>
> Jeff Ruppert
>
>
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-- 
John Swearingen

Skillful Means
www.skillful-means.com
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