Does anyone know what the maximum span is for running 2X6 or 2X8 cedar lumber, as rafters for a carport? What I want to build is not really a carport in the traditional sense. It is more af a trellis/arbor, that blocks sunlight more than rain. I will probably grow vines on it and such. The problem is that I need to go about 20 feet between the posts, and I was told that having a center post between the cars would be a safety issue. Is there any way to make that span without a pitching the trellis rafters?

Here is the side view of what I have built so far, so you can get the idea...

I'm no engineer, but I can tell you this: a 2x anything isn't going to be enough. If you ever find a straight one that long, it won't be straight after a year outdoors. Once it warps out of it's plane, it loses its ability to bear a load, and can buckle. You can reduce this with blocking, but that gets ugly fast.

I would suggest some sort of engineered wood product, like a laminated beam. Maybe a 4x10? It will have to be watherproofed and occasinally re-coated. If you run 3 of those across the large span, you can then run 2x6s between them in the other direction (perpendicular to the big beams). They make brackets to hang the smaller wood between the beams instead of having to go on top and make it even taller. What about steel beams clad in wood?

You definitely want to run this by an engineer.. your side walls may not be strong enough for the load, especially in shear. Attaching it to the house will help, but without any sheathing on top you'll still have problems. You can help that by putting an X of cables across the whole top, but you'll have to make sure the ends are mounted well and that they stay tight.

Here's one I build a few years back. It's all redwood. 6x6 posts, 2x8's making the long span (16 feet) topped by 2x6's about every 8 inches, topped by 2x2's about every 6-8 inches.

Problem making a 20 foot span might be getting wood in more than 20 foot lengths.

Demick

I just measured the width of where the posts would be. 18'-10" from center to center. So, a 20' 2X8 could make the long span, but is that overreaching?

whats going to be on top of the 20' 2X's? I have a handy the handy dandy "US Span Book fo Major Lumber Species" (Canadian and US Species Groups)

Ill look it up

I was going to run 2X4s on top, in the other direction, spaced every 16" or so. Then, I would cover the whole thing with 4X8 sheets of plastic lattice.

According to this guide.. 20' span with a 2X8 is fine.. unfortunatley they dont have specs for Cedar.

I could root around and find the modulus of Cedar..

but this gives you the basic idea.... your local building code will drive this, if you choose to follow them