Top Beams + Waterproof Or Not? (2nd attempt)

[Shannon C. Dealy]

On Mon, 7 Oct 1996, M J Epko wrote:

[SNIP]
> 
> >The bond beam, I heard one person mentioning a cement bond beam.
> >regardless of cement or wood....is this the same thing as a top plate in
> >straw bale????
> 
No bond beams in cob, not needed.

> 
>         What about attaching the roof? Here in deep-snow country, a fair
> pitch is generally desirable; we need to attach the rafters somehow. Would a
> wood beam (perhaps nothing more than 2-by dimension lumber, say 2x10 or
> 2x12?) be attached to the top of the wall using the porcupine method? That
> would seem insufficient to withstand uplift, especially if generous
> overhangs are part of the plan. Perhaps foundation-type anchor bolts are
> built into the top layers of the wall & the beam/plate is simply bolted on?

Actually you just embed your attachment in the cob, the specifics depend
on the desired attachment technique, but you could for example wrap short
sections of steel cable around small chunks of fire wood and embed the
chunks of wood in the wall a foot or so below the top of the wall with the
cables extending up through the top to attach to.  Alternatively you could
make an inverted wooden 'T' and embed it in the wall with the base of the
'T' sticking up out of the top of the wall.

> 
>         Same concern for concrete. And what, besides being easier to level
> than cob, would be the advantage of a concrete running bond beam? Couldn't a
> person use an easier-to-handle-than-concrete wood beam and just stuff cob
> under it into any unlevel parts once it's bolted down? Am I missing
> something obvious again?

NOTE: You can't really 'bolt' anything into cob, you must embed something
in the cob to attach to.

> 
>         How about using peeled poles viga-style (protecting the exposed ends
> from the copious precipitation with the pitched roof), which would be
> incorporated *into* the wall & to which a wood top beam (perhaps flattened
> logs) could be bolted. Since wood's R-value is roughly comparable to cob, it
> wouldn't be much of a thermal break (if any.) Perhaps horizontal holes could
> be bored through these vigas where they cross the center of the wall and
> rebar could be threaded through for additional uplift resistance. Oh, I'm
> just swimmin' with unusual and unneccesary ideas.

Viga's are one method commonly used.

[SNIP]
>         Here's what we're thinking, and I welcome all comments and
> suggestions: phase one will be a somewhat spiral-like (like a top view of a
> snail shell) cob structure on a shallow (non-frost-protected) rubble trench,
> about 12' inner-diameter minimum at the small part. This will be the bigass
> bathroom (our current one has 3-1/2'x4' of open floor space not occupied by
> permanent fixtures, and we're fed up with that), and will vent directly
> outside through the roof. It will be fully enclosed by a circular or
> octangular strawbale structure 10 to 12 feet larger all around than the cob
> part. The cob wall will be the bearing system for the interior of the radial
> joists. The chimney for the wood-heat system will also be incorporated into
> this wall, and it will provide more-than-adequate thermal mass
> fully-protected from exterior temperature swings.

I am not sure I understand this correctly, is the cob spiraling up so
that is forms an interior roof as well as support structure?  If so, this
would make me nervous, but I have a general philosophy of never putting
anything over my head that I wouldn't want to fall on it.

> 
>         The question: should the bathroom side of the cob wall be
> waterproofed, or do we suspect that the direct ventilation will suffice? The
> source of my wedded bliss wants a sauna in there - this changes the picture
> some, eh?

If you are using it for a sauna, then you won't be providing any direct
ventilation while it's performing it's sauna function.  I would say
waterproof the heck out of it for sauna use.  For plain old bathroom use,
adequate direct ventilation, and splash protection at the base of the
walls should be sufficient, though you might have more of a problem
preventing and removing mold & mildew on the rougher surface you get with
a wall that isn't sealed.

> 
>         The other question: will this work? Will it be far more difficult
> than I'm anticipating?

Don't know, I'm having a little trouble visualizing it, though maybe it's
just to late at night.


Shannon Dealy
dealy at deatech.com