Cob: RE: Re: foundations, tree roots

[billc_lists at greenbuilder.com]

>  > Thanks, Rick, for sharing your experience.  You said the
>"circumference"
>>  is 10-12 feet?  Did you mean the diameter because if the circumference
>>  was 12 feet, then the diameter would be about 4ft or about 48in.
>
>I meant circumference. Perhaps I should have paid more attention to the
>rule. (Doh!) Even so, if we're talking 1" to every 1' in diameter that
>would put the recommendation the builder keep the foundation 48' away
>from the tree. If you want to use the trees as partial shade for the
>house, you would give up a great deal of that benefit by moving the
>house so far away I would surmise.
>
>Also, I think that perhaps the roots *are* underneath this house but
>this particular building is pier and beam (an addition to the old rock
>portion of the house) and rubble trench for the rock portion of the
>house. Thus, perhaps the roots are going to cause minimal interference
>since there isn't a great deal of area for them to disrupt.
>
>Rick

One of the beauties of rubble trench foundations is that they can 
accommodate movment below them better than a poured concrete beam or 
slab.  That's why they're favored in climates with a lot of frost 
heave - a little pressure here and there from roots or freezing soil 
will move a bit of the rubble near the edge, but the majority of it 
will stay where it is.

With a continuous, relatively unbending beam (concrete, wood, 
whatever), if you apply pressure at one point you will affect the 
whole length of the beam.

We've all seen what happens when you pour a concrete sidewalk over a 
large tree's roots - eventually, the growing roots push the sidewalk 
up.   You definitely don't want that happening on a house - 
therefore, to protect both the house and the tree, the recommendation 
to stay outside the dripline is generally pretty wise.  I hadn't 
heard the 1" to 1' rule before, and suspect that it'll tend to break 
down outside of a certain range of tree types and sizes.  Your oaks 
may be exceptions to that rule.

Your shallow soils and limestone bedrock, along with the growth 
patterns of live oaks in general, means a fair bit of shallow root 
system.  If you were building there today you would be very well 
advised to stay out of the dripline with all construction equipment 
and activity in order to keep from compacting the soil and causing 
the phenomenon known around here as "Builder's Blight", where trees 
that appeared perfectly healthy at move-in "inexplicably" die a few 
years later.  Most trees can tolerate a little bit of root pruning, 
but extensive soil compaction around shallow-rooted trees, though 
less visible, will do a lot more damage.

I suspect that in 1890 the heaviest equipment they used was probably 
a horse and wagon, as opposed to today's multi-ton construction 
vehicles and equipment.

It is also true that building outside the dripline often means losing 
out on shading, especially with relatively short trees such as live 
oaks.  Given a choice between a living 500 year old live oak and the 
shade from a dead one, the choice is pretty clear - use some other 
means of reducing solar gain.
-- 
Bill Christensen
billc at greenbuilder.com

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