View Full Version : Span tables- 2x10x20
May 10th, 2005, 07:59 AM
I am planning on building a sun shade over my Southern facing stamped concrete patio slab. The patio extends 18 feet out from the house. I plan on installing a ledger board on the house under an existing eave, 16 feet wide. From that I will run 2x10x20's out over the patio, extending 2 feet beyond the slab. I will pour footers and have two 2x4x10's as columns to support a double 2x10x16. The rafters will run from the ledger board 18 feet out to the double beam. The $64,000 question is will the 2x10x20's sag under their own weight? This is not a weight bearing structure, but I will run 2x2's across the rafters for added shade. My buddy sent me span tables, but I can't read them. Am I okay running the 18 feet? I could certainly shorten the span by placing the columns on the pad, but my concrete guy cautioned against that, with the other side tied to the house. Plus, the job foreman (the wife) doesn't want them on the pad. Any help deciphering span tables? Do I even need to worry about a 2x10x20 spanning 18 feet with no load? Help! :eek:
May 10th, 2005, 11:32 AM
Based on my calculations, I come up with about a ?? midspan deflection with 2x10?s at 24? o.c. with 2x2 slats (with a 1 ?? space between the slats) using wet (i.e. heavy) lumber. I think the 2x10?s would be fine under a no load condition, but I would be a little concerned if you got two guys up on the structure to nail the slats. Also, I would be concerned if you got any kind of snow accumulation on it. Hope this helps.
May 10th, 2005, 12:49 PM
Thanks for the information. I am going to use douglas fir, rather than pressure treated lumber. I plan on sealing the fir, priming two coats, and two coats of top coat of the best quality sealer, primer and paint available.
I'll definately hang it from a ladder, rather than from on top. I plan on using joist hangers on the ledger and the appropriate Simpson tie over the double beam.
Snow is definately a concern (we have our reputation to uphold here in Buffalo, NY!), so I'm going to space the 2x2's accordingly. I may even use a composite lattice material, which I could theoretically remove over the winter months. Any further thoughts you might have would be greatly appreciated, and thanks again for the reply!
May 10th, 2005, 06:21 PM
New York should be rated with 30 pound snow load. With that chart douglas fir 2x10 repetitive 24" is allowed to span maximum of 15 feet. You are spanning 18 feet. The chart for 30 pound snow load for rafters not with ceiling attached calls for a 2 x 12 repetitive 24 on center.
If you placed those rafters with douglas fir 16" on center you would be fine with chart allowing 18' 5" douglas fir # 2.
Your posts should not be smaller than 4" square. I suggest you beaf up to three 2x4. Remember these should be resistant to rot.
Your headers spanning 16' is off the charts calling for a glue lamb engineered beam. 2 - 2x10s are only allowed to serve as a header supporting a roof 18' long no further 8' 5". Call your lumber company for an engineered beam in place to serve as your headers for this roof load.
May 10th, 2005, 06:32 PM
New York should be rated with 30 pound snow load.
Why would you design for the 30 pound snow load on a structure that is being constructed specially to not accumulate snow? It doesn't sound like a roof, per the IRC definition. More like a decorative wood assembly.
May 10th, 2005, 07:03 PM
It all depends on how far apart he is installing his 2x2 lathing. If these slats are less than about a foot apart the snow will still bridge by icing creating the snow load.
If the slats are spread enough to ensure no snow accumilation then there will be limited shade.
If the owner then plants climbing growth to cover that roof for shading again the snow will build to the snow load of that area.
No load is a strong statement. Too many variables involved when that statement is made. If truly designed as a no load so snow can not accumilate the Code does not address that type alternative design requiring an engineer to back the alternative design. It would be cheaper and easier to use the code eliminating the special designing of an engineer.
Just being practical and safe
May 11th, 2005, 09:16 AM
Great information, guys, and I appreciate the input. If I use 2x12 rafters 24" o.c., or 2x10's 16" o.c. I'm within code for snow load, if I read correctly. They will span the 18'.
The double header, while actually measuring 20', will only need to span a distance of 12'. I'd bring the posts in 4 feet from each edge. The rafters cover 16 feet, and each end has a two foot cantilever, for decorative purposes. If I use a double 2x12, will that span 12', WG?
Basically, I'm building a 16 foot box, 20' rafters going ledger to header, 2 foot cantilever. The 20' header spans 2 4x4 pressure treated columns, spaced 12 feet apart, four feet extending beyond, 2 of those feet a cantilever for decoration. I could stick a third column in, one two foot in from the edge, one in the middle (span of 8') and the third 2 foot from the other edge. That limits the maximum span to 8'.
Any additional comments or suggestions, fellas? I'd really like for the thing to stay put and not to crash under the weight of snow or ice. Thanks! :confused:
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