Cob: cubic feet per day and drying times

[Shannon C. Dealy]

On Sat, 9 Aug 2003, Kim West wrote:

> Thanks for writing Shannon. I am wondering--if it is possible to mix and
> apply this amount of cob per day, does that mean that it can be done each
> and every day until finished, or does one have to wait periods of time for
> each day's work to dry?  Will a home containing 60 yards of cob be built in
> 60 days or will it take many, many months longer due to drying time?

Yes it is possible to build 60 yards in 60 days.  There are a few
limiting factors like temperature, humidity and water content of the mix
(usually the more clay in the mix, the higher the water content needs to
be).  Providing you build at a reasonable time of year (which I never
seem to do), and your average day time temperatures are 70+ deg. F. and
your humidity isn't excessively high, the only limiting factor would be
the approximately one foot per day of wall height that you can build
before the weight of any additional cob will cause the walls to spluge
out.  You can get a foot a day in lower temperatures, but it gets more
difficult and is more dependent on the amount of sun and wind you get at
the building site.  In other words, if you have sufficient labor on hand,
build in summer, and your house needs walls all the way around that are
eight feet tall, in theory you could complete the walls in eight days
regardless of how many yards of cob are required.

> It seems from reading about Oregon cob that work can generally proceed
> onward day to day--but when reading about ancient cob building, weeks
> between lifts must be allowed for drying time. Is this due to the difference
> in moisture content along with the larger proportion of sand and straw in
> Oregon cob? Or am I misunderstanding something in thinking that Oregon cob
> does not have to have weeks of drying time in between lifts?

I can't speak to what the old techniques required for drying time (it's
been probably seven years since I last read anything on them), but
increasing the sand content and straw content in a cob mixture both
increase the rate at which you can build (providing you still have enough
clay to hold it all together).  Increasing the sand content reduces the
amount of water required for the mix (since sand doesn't absorb water)
which reduces drying time.  Increasing sand also increases the
compressive strength even when the cob is still wet, which helps it to
support more weight thereby reducing the "spluge" factor.  Increasing the
straw content helps cob to dry faster because the straw acts as a wick,
helping to move the moisture from the interior of the wall to the surface
where it can evaporate more quickly.  The increased straw also increases
the tensile strength of the mixture which helps hold it together also
reducing the "spluge" factor.

[snip]
> PS: How is the $0 per square foot house coming along? It was by a
> happenstance stumbling upon your site of it being built that I was first
> introduced to cob in the first place.
[snip]

Well, I built it primarily as a tool shed, and once the walls reached a
certain height and a tarp went over it, it was serving the purpose for
which it was intended, so the project got put on the back burner.  I
expect that in the next year I will get the project done, now that I am
living on site.  I don't remember if I have mentioned to the list or not,
but due to a change in personal circumstances, I built a cob house over
last winter (15 feet away from the $0 dollar building) and moved in at the
end of May.  To avoid the permit issue (for now), it is done within the
120 square foot footprint and 10 feet above grade "utility building"
limits (think interior cubic footage of a 20 foot motor home).

One side effect of all this is that the server computer that hosts
the coblist now resides in a cob house and is completely solar powered!

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com