[Cob] Cob structures and "basements" or sunken floors

[otherfish]

JOANNE/DAVID,

I did a small cob greenhouse, 100 s.f. or so, in Pacifica, Ca.  This was
done without a permit under the 120 s.f. no permit required exclusion
allowed by the UBC.  The building is on a sloping site, and was sunken in
the ground an average of 2'.  The 12" wide base wall was built of mortared
3-4" thick urbanite crerating a perimiter foundation in the hole & built up
to appx 4-6" above the ground surface.  The base wall was fnnished of with a
vertical course of urbanite mortared onto the top at the exterior of the
wall - this was to provide rain splash back protection at the base of the
cob.  Every 2' or so, a piece of urbanite as long as the base wall width was
mortared vertically & perpendicular to the exterior - this was to add
hoizontal support to the vertical urbanite at the exterior, and also to give
a keying in structure between the cob and the base wall top.  The 12" thick
cob wall was built on top of this.  There is no drainage pipe at the
exterior of the foundation / basewall.  The floor is gravel.

The building has performed quite well as a greenhouse, but is subject to
flooding of the interior during heavy rains.  No problem for a greenhouse,
but would be a serious defficiency for a dwelling.

In your case, first off make sure your site is away fro sources of natural
ground water flow, i.e. dont' build in a normally damp location or at the
bottom of a sloping area.  Then, to protect your building from any frmaining
ground water, extend the footing at least 12" below where you want your
interior floor.  Providing a drainage and moisture infiltration system at
the exterior of the basewall / footing is absolutly essential.  Use a
perforated pipe at the bottom exterior of the basewall & make sure it can
drain at least level ( slightly sloping is better ) to either daylight or a
remote huge gravel filled sump.  Embed the perf pipe in 12" x 12" of 3/4"
min. gravel & contain this all in filter fabric.  Combine this with a
waterproof membrane ( use a good quality here as this is your forever
protection & you don't want it to fail after a few years ..... a major
bummer it that happens ) agains the basewall & extending from under & below
the drainage system & up to the top of the ground.  Finish this with the
finish graded ground sloping away from your building at ALL sides.  Slope
this finish grade at least 1"in 12" to a surface swale min. of 3' from your
building exterior.  Make sure the swale also drains completely away from the
building.  Do all this & you should be well protected from intrusion of
ground water into your sunken structure.

Oh yea, don't even think about using cob below the surface of the ground.
Cob is cob only if it stays dry.  If it gets wet or even damp, it turns back
into mud & this is not what is needed for your building to suceeed.

Cob on!
john fordice - cob architect & builder

on 3/4/06 2:21 AM, David Boyer at HasteinD at earthlink.net wrote:

> Fellow Cobians and wannabes,
> 
> We have recently been informed by a (Linn) County, Oregon building inspector
> that we can build all the cob cottages we want without a building permit
> providing they are separated by at least 3 feet; that the highest point of the
> roof is not more than 10 feet above ground and that they are not larger than
> 200 square feet. (We want to get this in writing on County
> stationary/correspondence before we begin construction)   A 200 square ft cob
> structure would fit our needs if it had 2 stories (400 sq. ft.). A way to get
> around the 10 foot roof height limit for a 2 story structure would be to have
> the bottom floor be below ground level at least 2 feet (3 feet would be
> better).  
> No cob construction literature I have read covers the subject matter of below
> ground living space. No Cob workshop I have seen offered has this topic on the
> agenda.
> Why?
> Does any one on the cob list have useful suggestions or practical experience
> building the first floor below ground level using natural building techniques?
> 
> Peace,
> Joanne and David Boyer
> HasteinD at earthlink.net
> Why Wait? Move to EarthLink.
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