If we can come up with a final design, I can create a dxf line drawing file that any structural engineer using modern cad based software can import to apply loads to the structure and come up with the lvl (laminated veneer lumber) sizes.
Boise Cascade VERSA-LAM® LVL Beams & Headers
And, I can live with calling the cross things rafters. Or are they Joists? 
In reply to VolvoHeretic :
Thanks!
After killing too much time digging through various websites that are frequently over my head, I think that paired treated wood 2x10s around a perimeter will work. The 25' sides will have to be split into 12' sections with a center support post, and I'm pretty sure I can get away with an 18' span of paired 2x10s on the 18' sides with only corner support posts. If they sag too much or warp I'll add center posts. Basically like your last drawing, but without the center beam.
Menards carries Cedar Tone Pressure Treated lumber in the right sizes and according to their website is it available and not stupidly expensive - $45 for a 2x10 x 20'. Need to finalize the cross beam canvas support "rails", but I think a heavy grade chain link top rail will do it. Need to call Master Halco tomorrow to see if the stuff is available and if they'll sell to me.
Here is a post cap bracket for beam corner installation. I would highly recommend a structural engineer's stamp on anything this large. I wonder how big the four or six posts need to be to hold what is essentially an open carport without a roof up in sheering forces.
Simpson (StrongTie.com) ECCLQ End Column Cap L-Shaped, Quick-Install
In reply to VolvoHeretic :
The problem (which I think you are alluding to) is that the "roofless" system has only 4 connections trying to keep the entire structure together. They need to be STRONG connections.
When a typical roof (or pergola) is built, there are many more connections trying to keep the whole thing together.
Let's say 60 mph wind load hits the exposed face of a beam (trying to push the structure over). The exposed faces are about 35 SF of surface area , with 60 mph pushing on them. That's the equivalent of a full sheet of plywood. (If you tried to hold a sheet of plywood in 60 mph wind you wouldn't be able to.)
As the wind tries to push the structure over, there are 26 different joints and connections in your earlier diagram trying to resist the wind load (excluding the bracing. That would add 32 more). In a structure that only has 4 corner connections, there are only 4 connections holding it together.
Every connection is being subjected to 6X the load when there are only 4 connections. 14X if we count the bracing.
Thats what I meant earlier by "using less wood". It doesn't save money. It makes a weak structure.
...and the canvas increases the wind load area from 35 SF to well over 400 SF.
In reply to SV reX :
Thanks for the explanation. Definitely need an engineer on board.
In reply to VolvoHeretic :
Honestly, this is a super simple structure. There really is no issue that would require an engineer IF STANDARD CONSTRUCTION TECHNIQUES ARE USED.
The design you posted with cross bracing is a VERY typical design with no unusual features that would require engineering. Any weekend warrior DIYer could build it successfully without engineering, and without failure.
The problem with this design is that every time the conversation proposes an idea that may be able to help, the OP uses it to REDUCE the materials in the structure. We are now down to 4 2x10's, and 4 posts.
That's simply not enough. And it is outside the normal parameters of building a structure like this. So, you're right. Engineering would be wise.
...or just add back some typical framing members.
I apologize for sounding like a jerk. It's totally not my intent. I'm just trying to help, but this thing is taking minimalism a bit further than I am comfortable with.
I'll back down, but please consider rethinking this project to be closer to more typical construction methods.
In reply to SV reX :
Actually we're down to 3 posts - on the yard side - but the other side would be connected to the house. The ft2 exposure to the west would be about less than 18ft2 frontal and the same on the back. The canvas is between the beams and doesn't add anything like 400ft2 exposure. I'm guessing the long side exposure to the north doesn't matter, since it will be pushing on the house (or pulling but shielded from the wind, in the case of a southern wind) If fence top rails are used to spread the canvas, this is another qty 10 x 49lbs each holding things down.
SV reX - don't think you're being a jerk at all, I appreciate all the help. My gut tells me this approach will work, but I'm not s structural engineer. It's probably time to find one to get a professional opinion.
In reply to jwagner (Forum Supporter) :
It adds 400 SF in wind LIFT exposure, which the structure has to counteract.
Its not the frontal area. It's the lift.
The wind lift on that structure at 30 mph will be about 1500 lbs of force. At 60 mph it will exceed 5600 lbs of force.
Your fence rails aren't gonna hold it down.
Not completely done yet, but close enough for a few pics.
Thought I screwed up the color in gimp on this one, but the pink house is due to reflection off the cayenne sails.
The spans, etc., were checked with an engineer, and the designer at the shade sail place told me there wouldn't be enough wind lift to be concerned about due to the spacing of the weave.
I'm happy with it - grilling with some friends Sunday to christen it.
BTW, putting dumb questions out on the forum changed my ideas several times before I had the discussion with the engineer. Thanks to all you guys.
In reply to jwagner (Forum Supporter) :
Good job. Looks nice.
Looks great!
I'd love to be able to do something similar on my back patio, but unfortunately it won't work due to the way the roof of the house is framed there.
What's going on back here?
OHSCrifle said:What's going on back here?
The guys I contracted to build it decided the flying end needed support, so it's a brace just sitting on the roof. It's not necessary as the 2x10 could support the beams for that span but they were really set on it.
That's looks great! Glad to see you raised that above the right hand roof line.
Does the fabric stay in place year round, or is it removable? (Cleaning, leaves, snow, etc)
In reply to OHSCrifle :
(Translating to architect-speak...)
I think he is saying it's a brace under the cantilevered 2x10 ledger beam.
In reply to jwagner:
Did they nail that brace through the roofing?
In reply to SV reX :
Thanks - I do see the "flying" ledger now that it's been described.
Now I wonder if the 2nd story vinyl siding (including the corner trim) was cut out when the ledger was screwed in place. Details details.
@SV reX: The fabric will come off in fall. Should be pretty easy - it's three stainless hex bolts per fabric end screwed into inserts. The shades will support "moderate snow load" but I'm in Wisconsin and getting it out of the sun will reduce fade anyway and give a little more light in our bleak northern tundra winter. ;-) The beam support is just sitting on the roof, no nails through. The height was decided by the 2nd story floor height so they had something solid to attach to, and that was over the garage roof line. Worked out pretty well.
@OHSCrifle: The siding was cut out and there's a drip rail on top and undersill trim on the ledger beam.
In reply to jwagner (Forum Supporter) :
Sounds like they did a good job (and looks like you found some 18' material!)
Id pull out that brace so it doesn't wear through the roofing.
Enjoy the heck out of it!
SV reX said:Side note...
I love the look of that woven fabric, but it sure is gonna get loaded with a lot of leaves and crap, and be very difficult to clean.
I was drawn to it as well. My neighbor was suggesting a pergola with solar panels on top for above our parking pad. I think he's on to something
In reply to jwagner (Forum Supporter) :
Those are good answers. Sounds like it was done well.
In reply to Keith Tanner :
Yes but nails are steel rods that are either in sheer or better if they are angled gravity actually pulls the boards together. I mean yes the weight is minimal but it just seems to be the absolute minimum required and that is just not what I like to see.
jwagner (Forum Supporter) said:I did find a pergola maker in Illinois that will do a 18' x 25' aluminum lattice topped pergola for about $7K. (without center support posts) Looks good but I prefer the canvas weave if we can figure out a way to make the span.
Hi @jwagner, I'm actually in the market for this size pergola, but in aluminum. Would you mind sharing the name of that pergola maker you mentioned?
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