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I'm looking for help on planning a roof over a deck I plan on adding to my house. The roof will be 18' wide and extend out 12' from the house. I want to have at least a 1' overhang on each side making the roof 12x20.

I plan on using polycarbonate sheets as roofing so it does not need to support much weight. Building in a location that does not experience snow. What size post and beam do I need to use to span the 18' between support posts?

Here is the concept design.

Deck Concept Photo

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  • Welcome. I've attempted to simplify your post to remove non-essential information. Feel free to revise further to clarify anything.
    – isherwood
    Aug 12, 2020 at 15:11
  • There are folks here willing to do this for you (and they'll probably be along shortly), but here are some things that might help you: google.com/…
    – isherwood
    Aug 12, 2020 at 15:26

1 Answer 1

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Truss span is what you look up. A 2" by 10" 16" on center will work for 20 ft 8 inches. With a 3/12 pitch. Less than that changes it to 19 ft 9 inches. If you include a ceiling of some sort then go 12 inches on center. The dimensions I noted are found in many places. But the best location is the International Residential Code for rafter size R802.4.1 R802.4.1(1) through R802.4.1(8). This goes through types of wood and a few other variants. Snow loads in northern areas are in the local codes. Usually decreased spacing. But you noted No Snow so not an issue.

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  • You're saying that the snow load makes no difference to the strength requirements for a roof? Especially a fairly flat (3/12) roof? I'd suggest that you edit your answer to include some code references that will support this, as it goes against most information I've seen and common sense.
    – FreeMan
    May 20 at 11:14
  • Yes I guess I could have been more clear. In retrospect I should have left the wording out on snow. A 3/12 roof is not suggested for areas with snow although done with trusses designed for the area snow loads. Examples are flat roof commercial buildings. The dimensions I noted are found in many places. But the best location is the International Residential Code for rafter size R802.4.1 R802.4.1(1) through R802.4.1(8). This goes through types of wood and a few other variants. Snow loads in northern areas are in the local codes. Usually decreased spacing. May 20 at 13:55
  • Please edit that info into your answer. It helps with discoverability (not everyone reads comments) and comments are ephemeral - they can get deleted.
    – FreeMan
    May 20 at 14:05
  • Done.Thanks for the suggestions. May 20 at 15:32

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