On 26 Mar, 17:27, Joe wrote:
I wouldn't be concerned with any sagging. I would be concerned of the
liability risk of having a load bearing overhead structure who's sole
purpose is to have people under it.

Get a steel I-beam and box it with a thin facia to make a "fake" beam.
You can make this look slightly curved (the facia exterior) or make a
much larger beam than the steel inside. Many cosmetic possibilities. It
will never sag and that span in steel is trivial.

Or

Purchase an exterior grade glulam. (usually cedar)

Steel I-beams in my area are cheaper than the correct wood would cost. (a
redwood 2x10 should span 16' 8" (5.03m) for a rafter with minimal snow
load)http://www.awc.org/calculators/span/...rcalcstyle.asp

Don't worry about sagging, worry about hurting a friend's child who's
over for a barbeque when the structure fails. Even though you may have
seen poorly built structures that survived a long time, remember that
much of the wood today is nowhere near the quality of the old growth wood
of the past.

Thanks all for your answers. Not exactly what I was hoping for. More
what I was fearing....
I'll see if I can get a thicker / longer piece. I guess I knew all
along it was not right.

John - ease up on the paranoia. I might just line it with lead. That
way, the people sitting under it will be safe from radiation as well.
If I for one second think that it's dodgy - It'll come down and I'll
be here asking for help. To make a pergola from two steel I beams -
practical yes. Easy on the eye? Hardly...


Thanks again everyone