[Cob] foundation rock type

[billc_lists at greenbuilder.com]

At 1:01 PM -0400 6/25/04, 211212 wrote:
>Hi-dee-ho everyone.
>
>We are busy over here preparing land and house site... questions:
>
>1. Our "soil" is almost entirely clay - on all 10 acres. How does 
>this affect (if at all) the foundation, since clay expands and 
>shrinks with the rain.

As others have mentioned, it could affect things a lot.

The rubble trench foundation can be a good idea, *if you have a 
slope*. As Amanda mentioned in passing, you have to be able to run 
your drain (downhill, of course) to daylight. If you're level or 
nearly level you won't be able to do that.  I have the same problem, 
and won't be doing RTF as a result - though it would otherwise make 
the most sense.

Whatever foundation you use, you want to be sure that it's well 
stabilized and will move as a unit if it moves at all, or you'll get 
cracking in your walls.

>
>2. What is the best way to make drainage take the water away from 
>the house, as the land has very little slope to it. I did read the 
>books, but still not sure.

Build up.

Typically, when you excavate for your footer and stem wall (or 
whatever) you put the excavated dirt beside the house and slope it 
away.  Build your floor a good foot or more above the highest point 
on the perimeter.

Depending upon where in Texas you are (I'm in Austin), standard road 
base can make a very nice earthen floor. After you have it roughed 
in, sift the base and add it in lifts, chunkier stuff first.  Wet it, 
compact it, repeat. I don't recall what size screens are typically 
used for the various layers, but you could shoot an email to 
frank at thangmaker.com and ask.  He's done this a number of times. 
Someone else on this list (I think) recently did an earthen floor 
too, so maybe they can give you their input.

>
>3. There are many rocks, of various sizes a fist size to the size of 
>a bed pillow. They are called honeycomb rocks and look like swiss 
>cheese. Can these be used for the foundation? Do I need to fill in 
>the holes with cement or something? Suggestions?

Jeez, sounds like you're right in our back yard.

That rock may not be the best for foundation work, depending upon how 
big the holes are, how brittle the rock is, etc etc. Actually, I've 
been told that limestone is generally not the best for a rubble 
trench, as it has a tendency to  dissolve in water (hence the 
honeycomb) and recalcify elsewhere, often where you don't want it to.

In our area, there's also a fair bit of flint/chert - many are right 
about fist sized - which was left on or near the surface when the 
limestone dissolved away.   That would be a better choice - it's 
already proven it won't dissolve on you.

>
>4. Our home site is in Texas - hot during the summer. How does this 
>affect the southern facing windows? Do I just rely on the roof 
>overhangs? Or a porch or trellis overhead? Or do I angle it so I am 
>not dead on facing south?

In the summer, the afternoon sun is pert' near straight up at noon. 
You can definitely handle your southern exposure with overhangs, 
though the porch, trellis, awnings, etc are also very nice, as they 
can create a cooler buffer area on that side of the house and keep 
out some of the reflected heat.

You actually want your long axis to run east-west.  Your home will 
take the worst solar pounding on the east and west ends, not the 
south.  (not counting the roof, of course, which really gets pounded, 
and therefore should be light colored or reflective if you can manage 
it.  And well insulated.)  See the Passive Solar section of the 
Sustainable Building Sourcebook at 
http://greenbuilder.com/sourcebook/ for more details.

Hope that helps.
-- 
Bill Christensen
http://greenbuilder.com/contact/

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