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Re: GSBN:Plaster on Framed Walls
After watching a whole bunch of bale wall panels get tested (ie, squashed)
at the lab at Queen's University, it's clear to me that it's the places
where the plaster doesn't bond to the straw that are the weak points in
the wall. Not that walls with some hollows in them are in danger of
crumbling to dust, but when they fail they fail first at those points
where the plaster was just sitting on the mesh or not making much if any
contact with the bales.
Interestingly, in plastering some of those test panels I noticed something
that I think is common on site built bale walls: the use of mesh makes it
much easier for plasterers to make plaster stay on the wall even if it's
making zero contact with the straw. Unmeshed walls force the plasterers to
get the plaster to stick to the wall itself to a much greater degree.
For those parts of a wall system that are plastered over wood or other
backings with the use of mesh, a couple things seem to happen. First, the
mesh is often stapled tightly to the wooden backing, which means the
plaster can't surround the mesh and embed it, it just sits in and on the
mesh. Where that mesh is held to the backing with only 3/8 or 1/2 inch
staples, those parts of the plaster system fail first under compression
I'm not saying all of this to advocate meshless walls, as I know that the
mesh serves some useful functions too. But meshed walls do tend to have
way more hollows than unmeshed walls, in my experience.
The walls with the least hollows? Those that have been "French dipped"
which is why I like that method of building so much.
Another note about hollows: When we have explored the damp patches that
have occured on some walls up here in the winter/spring transition, the
majority of those patches occur where these hollows between bales and
plaster exist. That air space creates conditions different from where
plaster and bales make good contact. I don't think the damp patches are
causing major degradation of the walls (some we've looked at have been 10
years old and still the straw is okay), but it's another reason to stick
that stuff to the wall!
> If a contractor were to ever do that again I may
> have a different response based on the experience explained above, and
> was wondering if anyone else has had such an experience?
> I've never made a study of it, but in the many plaster walls I've torn
> I always assumed that, to some degree or other, the plaster sheet was just
> hanging there on the lath, and, when tapping on a wall with a hammer, I've
> always heard plenty of things that sounded like hollow spots....
> John "Hola-Hollow" Swearingen
> On 10/8/07, Jeff Ruppert jeff@... wrote:
>> I have to agree with everything you have said. In particular, we had
>> the experience of delamination of the lime color coat from repair
>> patches in the earth base coats. Since then, I have cautioned clients
>> in applying lime over earth for some of the very reasons you have
>> To explain, we had applied earth plaster base coats, which included a
>> small amount of lime, over plywood sheathing covered with building paper
>> and mesh. There were some spots on one wall that were not applied
>> against the wall well enough, leaving a hollow space behind the base
>> coats (this was the final wall on the project and was done in the
>> dark). After drying, the contractor went around poking holes in the
>> wall where this was the case (I am not totally convinced every spot he
>> poked was going to be an issue, but that is for another discussion). We
>> proceeded to patch those areas with fresh earth plaster and let it dry
>> for a few days. Everything appeared to dry out after a short period,
>> but what had actually happened is that the plaster had froze, holding
>> the moisture in (we were working in October with cold nights and this
>> was a north facing wall). We then put the lime color coat on the entire
>> building. Some warmer weather arrived shortly thereafter as it can here
>> in the mountains (Indian summer...).
>> After a week or two, on that one wall where we had patched, the color
>> coat was left hanging and could be scraped off with no effort on every
>> patched spot. What you described as the earth base coats shrinking
>> during drying makes sense. I have to admit that it took me awhile to
>> figure this all out myself and you confirmed my thoughts. We have used
>> lime in freezing conditions with very successful results given the harsh
>> conditions, so to me it was an issue with the bond and the differences
>> in movement between the materials.
>> It was a good lesson to learn, and one that brought up another question
>> that has been in the back of my mind since. How many small air pockets
>> end up behind plaster applied to framed buildings with building paper
>> and mesh on an average job? I can't believe that /every /successful
>> plaster job on framed walls out there is 100% sucked to the wall.
>> Granted this wall was done in the dark and it was ultimately our
>> responsibility. It was sprayed on as we have done many other times.
>> But no one has ever gone back to poke at the walls in the way this
>> contractor did, creating areas for us to patch. It seems suction from
>> the trowel could easily, and unknowingly, pull the plaster away from the
>> wall in small spots no matter who is doing the work. I have never
>> witnessed plaster failure due to this, so I cannot speak to it as a
>> failure mechanism. If a contractor were to ever do that again I may
>> have a different response based on the experience explained above, and
>> was wondering if anyone else has had such an experience?
>> If anyone responds to this second topic specifically, maybe a new topic
>> is in order so as not to combine the two.
>> For instructions on joining, leaving, or otherwise using the GSBN list,
>> send email to GSBN@...HELP in the SUBJECT line.
> John Swearingen
> Skillful Means, Inc.
> Design and Construction
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