I'm looking to build a patio roof (shed style) and need to span 13ft. I've found several span tables online, some of them with different maximum distances between the rafters.

Is it ok to use 2x8s with 24" OC? - I've used this span calculator with the below variables to get a maximum span of 13'-10":

  • wood: SPF (I'm using the usual pressure treated lumber)
  • deflection limit: L/240
  • spacing: 24in
  • snow load: 30psf
  • dead load: 10psf

It will look something like this: enter image description here

  • 1
    Why would you spend the extra money on pressure treated wood for rafters? The point of a roof is that what is underneath it doesn't get wet.
    – Comintern
    Commented Jun 1, 2014 at 18:50
  • Comintern, this is a good point, thanks for bringing it up! I thought that you need it, if the roof is not fully enclosed. I'm building something like this. Do I need treated wood on the outside skirting then, or not even there?
    – funtastic
    Commented Jun 1, 2014 at 20:11
  • 1
    If the roof material is waterproof and the overhangs are properly configured so water drips off rather than running onto the framing, there's no need for pressure-treated material here. Be sure the flash the top connection to the wall properly as well.
    – Ecnerwal
    Commented Jun 2, 2014 at 1:44
  • Of course you want pressure treated wood. The variation of temperature, fog, rain and other climate phenomenon will make it wet or moist even under roof, and the treatment will also prevent fungus etc. Outside = always treated wood ! Enjoy the pergola!
    – Lars
    Commented Oct 4, 2018 at 22:39

1 Answer 1


A couple problems with the span calculation, IMHO (I am not a Licensed Professional Engineer):

Select structural (the lumber grade) is frequently impossible to come by at an ordinary lumberyard, will cost the earth if it can be had, and is even less available (AFAIK) in pressure treated (though you should not need that, as per comments.)

Number 2 is generally the grade you should design for unless you are sure you have a source for a better grade and can stand the price of it... You can get the same loading from 2x8 number 2 on 19.2" centers (5 per 8 feet - there's a symbol on most modern tape measures for that spacing) as your select structural on 24". Or you can get significantly more load (or span) from 2x10 number 2 on 24" centers

The position of this roof on the sidewall of a house (if it's "like the picture") may cause the buildup of a snowdrift on the roof - that needs to be allowed for if the 30 lb snow load is anything other than mythical (ie, I think that snow load is the code standard minimum, so it is often in code where snow is never expected....)

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