Will This Sag?

[thiggy]

A little old lady wants me to build a cover over her 2nd story deck, approx. 16’ long by 10’ deep. She drew up the plans supporting the two outside corners with 4 X 4s and the rear rim joist bolted to the house under the eaves. She visualized using 2 x 6 x 10’ for the rafters, attached to a doubled 2 x 6 x 16’ with a steel flitch plate bolted between them for rigidity. The roofing material is to be a PVC sheet product she described as Palruf PVC. It sounds to be a lightweight transluscent panel. She says it is not corrugated. She is spacing the rafters 16” OC using saddle hangars. I told her that the flitch plate will probably be rather expensive and I don’t know for sure where to purchase it. She would prefer not to have a support post in the middle of the front rim joist. Could just a doubled 2 x 6 span 16’ in this installation without sagging?

[jabe]

Doubled 2 x 6 will sag on a 16' span. Usually the distance of the span will dictate the width of the beam therefore a 16' span will require a 16" wide beam or a 4x16x 16' or 4x 18x 16' beam. Now-a-days the lumber/beams are cut from young trees and do not have the strength like old growth lumber/beams. I have seen 4x20 beams sag on a 20' span, they don't have the carrying power like before. Or you can go with an engineered beam/truss for the span and weight it will support. I'm not a structural engineer, just my opionion/experience building homes. I would have the lady consult an structural engineer for this before taking the job and risk having a recall.

[91FE]

I take it you guys don't need building permits in Alabama?

I would have the lady consult an structural engineer for this before taking the job and risk having a recall

sounds like a fine idea.

[Russianwolf]

my old 2 story deck is made of either 2x10 or 2x12 framing spaced 24" on center. The largest unsupported span is 12foot and it doesn't sag with with 5/4 decking on top of it. This deck is nearly 20 years old.

I think for what she is trying to do (minimal weight load, no snow) 2x10's would work. 2x6's might sag a bit.

[Stytooner]

@x6 will indeed sag. I would go with a minimum doubled 2x8 with good crown. 2x10's are actually more expensive than 2x12's most places. 2x12's are very heavy and may sag some too. I think the best bet, since the roofing is so light, is to go with doubled 2x8. I have a doubled 16' 2x8 span accross my living room that I built. No sagging after 10 years so far.
You do need a permit for some things. This would be one of those things, unless there was an existing roof.
Getting a permit is just a matter of submitting a drawing of your intentions in this case. Then pay for it. All done in about 30 minutes where I live.