[GSBN] Control joints (was Re: Lime plaster and expansion joints)

Wed Feb 11 21:23:51 UTC 2009

Hi All, Great discussion,

I have been waiting to here back from the project we consulted on that had
more than 800 linear feet of lime walls, many of them 28 feet tall, without
any control joints but wanted to chime in before we got too far along. The
last time I saw or heard of the building, it was doing just fine.

We don't use or recommend control joints for earth or lime (quicklime)
plasters and definitely not over the bales, for many of the same reasons
that John S. mentioned. We have used control joints where we had to start
and stop work over a period of years but only over wood (large posts or
plywood) which can be more conventionally flashed with the plaster absorbing
the water that gets in the "crack by design" and releasing it overtime
before it can work it's way through the flashing or horizontally over to the
bales. At least that's the thought that lets me sleep at night. Come to
think of it each of these locations has plenty of overhang too. I think that
the concept of control joints really doesn't translate well to straw bale in
that in conventionally plastered buildings the plaster acts more like a
sheet hung off of but otherwise isolated from the structure, and in
"plastered straw bale construction" as Bruce likes to remind us, the
plasters are an integrated part of the structure. Therefore I think the key
lies in the application, curing, and detailing where loads might concentrate
in order to avoid control joint type cracking. I think the John Glassfords
mesh between 2nd and third coats, or the Steen's approach of get as much
fiber as you can stand approach are the Best practices. We've experimented
with the bent metal fibers, hemp, hair and sisal fibers, and even synthetics
in lime.

We plan for and prepare our clients for a modest amount of micro cracking
that is part of the aesthetic overtime and never seams to be through more
than the top coat. I suppose that we are counting on the ongoing flexibility
of the plasters to not concentrate any of the expansion and contraction in
one place and mesh of various types where that could become an issue. Where
we have seen control joint type cracking is where we hadn't had enough
overlap in our mesh (prior to the testing program).

If I were to have a client (read Architect) that for some reason loved the
idea of control joint patterning I would hazard a guess that 300 Sq ft not
to exceed 20ft in any one direction would be safe. but again I wouldn't have
the joints occur over straw bales.

We recently lime plastered a three story round tower designed by Skillful
Means and we added a drip band of tile sloped out below each floors flashing
which effectively act as an edge every 10 -12 ft to expand or shrink behind
or from.  No additional crack prevention was added except at doors and
windows on the 80 ft round exterior.

It's Possible that the lime is going to harden over time that it might
develop more control joint type cracking at these long runs but I have a
hunch that that is part of what brought on the limewashing maintenance
regimes of historic buildings. keeping our eye on the whole building as it
evolves.

I guess all of this rambling is just my way of saying: Tracy Vogel and I
have been discussing at various times how great it would be to have a
professional natural plasterers gathering where we can geek out on the
details of what and how we are all doing what we do. As to Laura's comment
about posting it. Maybe Bruce's model of having every member who is invited
to attend submit a document to be included as a published proceedings and
then charge someone, or a team of someones, with coalescing the best of what
comes out of the gathering, shared best practices, new lessons learned, and
further research and testing needed. I really think that these buildings
need to be conceived as more or less structural,insulative, or massive
substrates for plaster. And there definately isn't enough published about
these topics in one place.

OK, that's more than two cents worth so I'm going to have to go borrow some
time from others.

Enjoy Being,

Tim

On Wed, Feb 11, 2009 at 11:51 AM, Athena & Bill Steen <absteen at dakotacom.net
> wrote:

> John,
> Just an idea that might result in a little work for you.  How about you put
> together a couple of images in an email and send it out to those on this
> iist that are interested and request a copy?  That way we can either admire
> your creativity and intelligent design or really take it to you
> for......well you know what. Put me on the list.
>
> Bill
> On Feb 10, 2009, at 6:51 PM, John Glassford wrote:
>
>  G ' day
>>
>> Just an idea.  What we are doing is this; with houses anyhow.  We are
>> using a hybrid method combining the bales and the window and door frames to
>> support the roof.  The frames usually 250mm x 50mm go the full height of the
>> wall and after compression; done by all thread in pairs every 1.8 metres;
>> these frames are then locked into the top plate.
>>
>> Thus we have a perfect expansion joint wherever windows and doors are
>> located.  We then render to the frames and cover the edge with skirts or
>> architraves.  The wall is now protected for water penetration even if the
>> render has a crack where it joins the timber.  We find that using this
>> method we have not experienced any cracking on the vertical.
>>
>> With walls with no windows and doors over 3 metres long you would expect
>> cracks in cement renders and our experience is such.  We do not get any
>> cracking in earthen renders with plenty of straw or chaff.  However these
>> earthen renders are not easy to apply with a pump.  So for commercial
>> buildings and the like we use lime and sand which we pump on.  We then apply
>> a fibre mesh to the second coat and to date zero cracking even in walls 24
>> metres long and 5.4 m high.
>>
>> I will post some photos to the web site below one day soon.
>>
>> Kind regards
>> The Straw Wolf
>> Huff 'n' Puff Constructions
>> http://www.glassford.com.au
>> 61 2 6927 6027
>> In Australia (02) 6927 6027
>>
>> The Orphan Rescue Kit Africa
>> http://theork.com
>>
>> Mountains of the Moon 2010
>> http://www.mountainsofthemoon.org
>> _______________________________________________
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>>
>
> Athena & Bill Steen
> The Canelo Project
> HC1 Box 324
> Canelo/Elgin, AZ 85611
> absteen at dakotacom.net
> www.caneloproject.com
>
>
>
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