I have a detached garage that I'd like to finish the attic space to make into a office like workspace.

The garage ceiling joists are 2x8x14' (unnotched) with a 13'6" span on 16". The joist appear to be SPF but are unmarked. I suspect #2 or #3, but if #3 then I should probably increase the strength. Any good way to identify the grade?

If #3 then AWG.org reports that the L/180, w/25 psf max span is 13'6". With #2 >40psf for the span is allowed.

The attic floor would be 5/8 OSB and the garage ceiling either 1/2 drywall or OSB sheeting.

I'll post some photos shortly.


Code requires a Live Load of 40 lbs. per square foot plus a Dead Load (flooring, joists, insulation, etc.) of 15 psf (probably) for a total load of 55 psf.

#3 spruce/ fir (fb = 1200 psi & fv = 95 psi) spanning 14’ will support about 40 psf at 16” oc. #2 (fb = 1750 psi & fv = 120 psi) will support about 60 psf. The main difference between #2 and #3 are knotholes. If you have loose or missing knotholes, then it’s probably #3. If the knotholes are less than 2” in diameter and “tight”, then it’s probably #2 and better.

Code requires 1-hr. fire protection between a garage and “habitable space”. Therefore, you’ll want 1/2” Type C or 5/8” Type X gypsum board on the bottom of the floor joists, wall by stairs, etc. for protection.

The door leading to the upstairs needs to be rated: 1) solid core, and 2) frame needs to be solid material (no applied stop), 3) weatherstripping around on 3 sides... but consider threshold too due to fumes.

If you use L/180 the gypsum board will crack under deflection. I’d use L/360...then, no cracks.

  • Your PSF numbers are a bit different than AWC.org - but w/my numbers then even #2 requires an upgrade, with yours #2 is good enough. My next question will be how to upgrade the joist strength 'easily' - adding joists is problematic. Re code - humm - the structure is actually an IBC shed - but its size is that of a 2 story, 1 bay garage. Use wise, its presently storage (aka shed), but soon will be storing my computer on a desk.in the attic.
    – pathfinder
    Apr 8 '18 at 16:52
  • Code allows higher values for “repetitive” installations. To increase “strength”, you can: 1) increase number of joists, 2) increase joist strength, or 3) decrease span. Items 1) and 2) sounds tough, so providing a shorter span by installing a wall to decrease the span or provide angle braces on one or both sides might work. If you don’t take the angled braces to the slab, be careful putting “bending stress” into the existing wall.
    – Lee Sam
    Apr 8 '18 at 17:42
  • Any thoughts in the difference in allowable spans between AWC.org and your numbers?
    – pathfinder
    Apr 8 '18 at 20:38
  • I’m sure it’s in the “allowable bending stress” values used. I see AWC uses Fb=1350 for Hem-Fir (South) and 1380 for Southern Pine and 1210 for spruce/pine/fir...while I used 1750...which is more like #1: 1520, 1725, 1210 respectively or 2095, 2645, and 1725 respectively for Select Structural. I’m unable to determine if they increased the allowable stress values for repetitive use. So, obtaining the correct species and grade is critical. Can you take pictures and send to a local grader? Or, can we logically determine ourselves? First, size: is it exactly 1 1/2” x 7 1/2”? Second, are the
    – Lee Sam
    Apr 8 '18 at 23:56
  • Second, are the edges eased? What is the largest size knothole, how many and spacing/grouping? (Sounds weird, but grading rules are available on line.) Third, can you look in your house attic and determine a species and grade? Fourth, can you compare to neighbors houses and climb into their attics? Lastly, can you determine if your detached garage was constructed at the same time as house and was there a Building Permit/plans?
    – Lee Sam
    Apr 9 '18 at 0:02

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