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I want to design a flat roof extension to create a screen patio. I would like to find a reference for maximum span and sizing with standard (such as SY pine) lumber for beams and joists. I'm not going to decide on final dimensions until I know whether I have to change from 2x6 to 2x8, for example, if I want joists to run x number of feet between beams -- and to know how big the beams have to be. Can anyone point to an online site or a reference book for these types of calculations? I'm in Florida USA, so no snow load -- but I will use some kind of steel hurricane straps/joist tie downs.

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To calculate the maximum span of roof joists you first add up the Dead Load (weight of joists, roofing material, etc.) to Live Load (weight of snow) to determine the Total Load.

Your Dead Load is

  1. roofing, which is going to be from 1 lbs. per square foot (psf) up to 3 psf, depending on the type of roofing.
  2. roof sheathing is going to be from 1 1/2 psf up to 2 psf depending on the thickness, which depends on the spacing between the joists.
  3. joists are from 2 psf for 2x6’s 12” on center, or 1 psf for 2x6’s 24” on center up to 3 psf for 2x8’s at 12” on center.
  4. ceiling material, if any.

I’d guess your Total Load is about 7 psf, because you have no Live Load. Then you can use a joist calculator like this one:

https://www.awc.org/codes-standards/calculators-software/spancalc

You’ll notice that the span calculator only goes down to 10 psf Live Load. This is okay for your situation because you may hang lights, hanging flower pots, etc.

So, your total load will be about 17 psf. Then try various sizes and spans until you get one you like.

If you use 16” on center, don’t forget to convert your load from psf to 16” on center by multiplying 1.33 to your psf load. (So, 17 plf becomes about 23 lbs. per linear foot on your 2x6 joist. )

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  • Great info, Lee, thanks! It looked like you were going to start with a list, but then forgot. I do that all the time, so I added that in for ya'. Is there a reference somewhere to get the standard dead loads you referenced, in case someone in the future wants to use a different type of material? – FreeMan Dec 17 '20 at 16:40
  • Excellent information Lee Sam, things I hadn't considered. You laid it all out for me, thank you! – user8356 Dec 18 '20 at 14:56

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