Hi Larry! Welcome to ePlaya! Naked Larry is a great supporter of Retrofrolic and it is good to see him on here!
As-far-as "conduit" shade structure goes, I use the 1 3/8" top rail for chain link fence which is purchased in 126" lengths. The last 3" are tapered to join another length. In 2011, I had 10'X15' spreads on my 30'X60' roof and did fine. For 2012, I pushed it to 15'X15' spreads and ended up bringing home some bent poles.
So to answer one of the questions:
For top rail, 10'X15' seems to be the maximum spreads. Mind you, I also had diagonal braces on the corners and welded fittings for all the joints from http://www.canopiesandtarps.com/
I'm planning to slope the other side to the ground next year so it will have more of a "barn roof" shape to it. The posts are nailed to the playa with 12" spikes and the bottom of the slope is a pipe "stapled" to the playa with a 3-foot, U-shaped rebar every 5'.
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