Will This Sag?

[Stan Thigpen]

I'm not sure if this is the best location for my question, but here it is. 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?

[Rob Will]

Not a chance with the posts at the outside corners.
It will sag under it's own weight in due time.
If you want to keep the beam down to a trim size, put the steel to it.\


rob

[Jim Becker]

I'd also be concerned about wind lifting without at least two supports in the middle of that 16' span. But I'm speculating...

[Ed Nelson978]

Check with someone at TrusJoist on a Lam. The engineered products are more than sufficient to support that span and TrussJoist will send someone out to spec it for you.

[Stan Thigpen]

Thanks for the suggestion Ed. I had already thought about an engineered beam for this application, but my concern is that there is a good bit of weather exposure. I am sure that on occasion some rain may blow enough to get on the beam. How much water can these products tolerate without deterioration? Was your suggestion for a Glue Lam solid beam, or to consider the 'I' beam products?

[Cliff Rohrabacher]

Could just a doubled 2 x 6 span 16’ in this installation without sagging?

I think it will sag and quickly. Two 2x12" might do it but 16 feet is a long way.

How about moving the poles at the corners in a little and cantelevering the roof out pas them a bit to reduce the load between the posts.

I agree with the suggestion you consider a steel beam.

[Julio Navarro]

That little old lady was either an engineer or has one heck of a vision.

You could make a flitch beam with the double 2x6 and a 1/4" or 3/8" steel plate the length of the 2x6's sandwiched between them.

[Stan Thigpen]

She is 73 years old and a former nurse by profession. She helped her husband when they bought a house about 20 years ago and had it moved to their land. She took an active part in the completion of that house. She is a do-it-yourselfer and not afraid to try much of anything.

[Ben Grunow]

You were planning on trimming the 2x/flitch setup right so why not use an LVL and trim it with 1x pine or cedar? Jim is right about uplift but some straps (Simpson makes hurricane straps that are perforated, galvanized and 1 1/2" wide in long lengths) from the footings or deck to the posts and from post to beam and some tie downs for the rafters are easy and eliminate the concern. BTW, I would use 2x8 for the rafters if snow is ever to rest up there (alabama snow?).

[Charlie Plesums]

I am not sure what to suggest for support, but if you are going to use a steel plate, it should go on the bottom, not between the wood beams.

The rule of thumb is that you should plan on something "rigid" sagging 1/360 of the length, so in this case, roughly a half inch in the middle (even concrete bridges sag that much). As it sags, the bottom stretches and the top compresses. Steel is VERY strong in stretch-mode, so get the steel on the bottom where it can do it's thing. (That is why the rebar in a driveway or sidewalk is placed near the bottom rather than the top).

[Ben Grunow]

Charlie (hi again)- a steel flitch plate is typically laminated between two framing members and stands on edge to take advantage of the depth of the section to resist bending. Typically a 2x8 (or 2x10 or 12) is set on either side of a plate of steel measuring about 7" (to allow for future shrinkage of the lumber) and carriage bolts are passed through to tie the pieces together. The bolts are installed at 16" to 24" intervals and the wood and bolts really just serve to stabilize the flexible steel over such a long length.

These are very strong but take a long time to make versus engineered lumber if possible. For a porch that will have a fiberglass roof I think another post is in order.

[jud dinsmore]

2x10's on 16" o.c. can span just under 16' when used as second floor joists (a little different but somewhat similar). i wish i knew the exact amount but we haven't used sawn lumber in our flooring systems (just about everything is going tji's) for some time. i would recommend consulting your lumber yard supplier (i don't think they can do this at home depot or lowes). they should have someone on staff qualified to calculate loads. i don't know the answer to your question because all of the load calculations for our home plans (i'm a home builder by day) are done by our lumber supplier. unfortunately, i recently met this person as we had a grossely undersized beam issue at a job they figured. it was sagging 1 3/8" over 21'. the cat walk above it was an inch out of level.

i would suggest 6x6 posts instead of 4x4's so you can notch the post so the double beams land in the middle (like a bridal joint) providing more nailing surface and avoiding any simpson connectors there (they only look good when they aren't seen). i would also recommend gluing (subfloor adheasive) the double beams together. hurricane clips will provide insurance against uplift but metal L-brackets (or smalling nailing straps bent to form an L) would look better since they would connect to the top, not the side, of the double beam. hangers on the ledger against the house are a must. good luck.

jud

[Stan Thigpen]

Thanks for all your suggestion and comments!!