[GSBN] Plaster detail at Bale/Plywood Interface

douglas nichols douglasnichols at hotmail.com
Mon Nov 17 21:31:34 CST 2014

Hi Ian
I've come upon this similar tie in when attaching gable trusses with a tall energy heel.  Sheeting over wood bond beam up onto the  truss heel with continuous ply makes for a very strong connection (sheer and uplift) and I have not had to do any special crack protection.  That being said, proper straw bale to wood plaster techniques are crucial.  My process is:  fill any gap between wood bond beam and straw bales with straw clay,  next 2 ply stucco paper over the sheeting (slip plane is a must I feel in these large exterior sheeting / plaster scenarios), 6 inch lap of lathing onto straw (if your not lathing entire building),  and continuous slip coat from straw to lathing.  I use wet clay slip pushed into the bales and then add straw (lots of it)  to the clay to go over the prepped area this gives a very similar monolithic substrate (with high tensile strength from all the straw) for the next plaster layers.  You can always bed some big pieces of fiberglass mesh (or other type) in the brown or finish coat (I've started doing this at all corners of openings to reduce cracking).  Out of curiosity how will deal with overhang?  Will the top cord of the truss run long? 

Doug NicholsNatural Building InstructorCommunity RebuildsMoab Utahwww.communtityrebuilds.org
Date: Mon, 17 Nov 2014 15:26:10 -0700
From: ian at lopezsmolensengineers.com
To: gsbn at sustainablesources.com
Subject: [GSBN] Plaster detail at Bale/Plywood Interface

Hi everyone,

I'm currently helping with the design of a one-story SB house with a rectangular footprint and a simple shed roof.  The SB walls will be constructed with a wood bond beam that's in the horizontal plane around the whole perimeter of the house, supported on wood posts.  Prefabricated wood trusses will then bear on top of these beams.  The trusses will have a "right triangle" shape to create the shed roof and a flat ceiling on the interior.

On the higher side walls of the trusses, the owner would like to have plaster continue all the way up to the underside of the roof (actually, to the underside of the roof sheathing), with minimal trim/flashing, etc. at the interface/transition between the straw bales and the plywood.

I've already mentioned to him that there's the possibility that that plaster will crack at this interface (due to the differences in substrate material, plaster thickness, etc), and I've suggested that there be a control joint in the plaster, or a piece of trim with flashing or something.

I'm just wondering how the rest of you prefer to deal with this detail...

Thanks very much,
Ian Smith, P.E.
Boulder, CO, USA

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