[Cob] Sill plates and tractor cob

[roemiller at infostations.net]

Spreading the load and securing the framing and roof are both good reasons
to use 
sill plates for a second story.  The cob buildings we're currently working
on or that are 
being built in our community in the Sierra Foothills with moderate seismic
risk, have 
all thread ties  secured to the steel in the foundation, and running up to
the top plate.  
This method of tying the top plate to the foundation provides integrity to
the structure 
as well as a means of securing the roof to the top plate, through the cob
to the sill 
plate, and finally to the foundation.  We use a mechanical tie for the roof
rafters to top 
plate connection, such as hurricane clips or plumber's tape with the same
nailing 
pattern as would be required in conventional framing.  It's probably
overkill when the 
roof is a living one (because the weight alone would hold it in place - no
hurricanes 
here), but that's what we're comfortable with in our seismic activity.  We
want 
everything to be tied together and work as a unit which will translate
forces 
throughout the structure rather than allowing them to demolish a weak point
and 
thereby compromise the structure.

As for tractor cob, with proper layering and mixing, we find it superior to
foot cob, and 
much faster.  We can mix drier which means with simple plywood forms we can
go 
higher in one day's working without the spluging associated with wetter
mixes.  With  
help we can put up four feet by the length of the building in one day.  A
locally 
devised slipform board approach will probably be detailed in the next
California 
Straw Building Assn. newsletter (CASBA).  You can subscribe at 
www.strawbuilding.org/  The technique means that cob is approaching stick
framing 
in terms of time when you account for providing thermal mass and a wall
that's ready 
to plaster.

Barbara

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