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I'm looking into building a covered patio and am having a bit of difficulty finding the proper beam size required for the spans between posts.

The cover will consist of either a ledger board attached to the exterior wall (pending structural review) OR beams on the new patio slab leading to a 15' span away from the house to 6x6 posts on galvanized anchor brackets at the other end of the slab. The patio will be 30ft. wide and 15ft. deep, but this can be adjusted if it significantly cuts down on lumber to do 14ft spans or so. So, either 6 posts in a 3x2 grid or 3 posts and a ledger board.

I live in North Texas where snow is extremely rare (maybe once a year with a couple inches). Will match roof pitch to my home (looks like a 3:12 or 4:12). I can add knee braces. Post height will be 8ft.

Wood species has not been chosen, but pressure-treated pine would be nice as I can easily source posts with that too.

This link seems to indicate I would need 3.5x11.25 #2 PT beams. I'm not sure if I'm using the right tables or not, hence my question here. It seems to me that two 2x12s with a 1/2 plywood along the grain sandwiched in-between would be more than suitable judging by some of the new construction I've seen around here. That sound right?

I would really like to figure out what would be required now so that I can figure out a rough estimate of the project cost and decide if I want to proceed with it. Otherwise, I would have just hired a structural engineer to define them for me already.

Here's an example of what I'm looking at creating. Only the ledger would attach to a two-story wall. I know a 4x6 beam likely wouldn't be sufficient and I'll have more stout 2x10 joists at the recommended spacing for the span (24" on center).

Covered patio isometric view

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    Beams are horizontal, posts are vertical. "6x6 beams on galvanized anchor brackets" sure sounds like you are using "beam" to mean post, which confuses your question. A sketch would be helpful. I'm unclear whether you are asking about joists, or a beam to support joists, or what.
    – Ecnerwal
    Dec 15, 2016 at 13:42
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    Note: a beam supports joists. a beam transfers load weight to a post. Dec 15, 2016 at 14:07
  • Whoops, you're right. Meant to say post in that one instance. I am asking about beams covering a span between posts. Dec 16, 2016 at 4:55
  • How much weight is this supporting? Is it a full on roof with trusses, decking, and roofing? Or is it a layer of corrugated tin?
    – wallyk
    Dec 16, 2016 at 7:33
  • @wallyk Yes, a full roof. No gables or anything, just straight trusses (2x10 @ 24" OC) forming a single plane with a 4:12 or so pitch. Decking, felt, flashing, shingles. Likely a basic T1-11 sheathing or Hardie panel ceiling, a couple fans, and a few recessed lights. Dec 16, 2016 at 17:09

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Late to the party, and I hope your patio turned out well.

For anyone finding this question in the future, the answer could be a 4x10 beam with 14' spacing and a patio span of 15' 2x8 rafters.

https://www.escondido.org/Data/Sites/1/media/pdfs/Building/InfoGuideline8b.pdf

A lot of cities or counties will have standard drawings for simple structures, and you can check building departments for things like live load and dead load requirements (snow and wind). This one seems to be a good example that includes span tables and connection details.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming! May 8, 2019 at 23:55
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In building wood beams for houses and garage headers we commonly sandwiched 5/8 CDX plywood as a flitch plate between two 2x12s, nails spaced no more than at 8-12". This brought us to the 3.5" we needed and (probably minimally)reduced deflection in the lumber beam.

If we were building to permit requirements, the beam size and type was clearly specified in the stamped drawings. If not for an inspected job then the only table we used was in the building code.

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  • Yeah that's what I'm looking into right now, although I would probably use constructive adhesive to make a sort of glulam beam, just for appearance sake (these will be visible). Dec 16, 2016 at 5:00

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