Hi all, starting to finish the 2 car garage for a wood shop in a new to me 1970 home and seeking wisdom on reinforcing the ceiling to support insulated drywall. The garage is approximately 20.5'x21.5'. So far I've learned that it has 2x6 24"OC rafters and 2x6 rafter ties on a ~20.5' span with a 2x4 strongback, and 2 king posts connecting 2 of the rafter ties in the middle of the garage to the rafter above.
No storage in the attic space is planned, so only looking to support insulation (probably R36 fiberglass), drywall, lights, and one run of metal ductwork for dust collection. Snow load here requires minimum 20psf for the roof (not sure how this plays into plans below).
My main question is around what live/dead loads and deflection limits I should have in mind when looking at the IRC R802.5.1 span table or the AWC span calculator. Are the 10psf live load + 5psf dead load calculations appropriate for this case? Similarly not sure about the range of L/180, L/240, or L/360 for deflection limits.
Using the AWC calculator I can see #2 Doug Fir 2x6 12"OC can span 21-3 with L/180. This would involve adding additional 2x6 joists between the existing rafter ties to get 12"OC, connected to the 2x4 double top plate with hurricane ties(?) and potentially sistering the existing 2x6 rafter ties with an additional. In this case I believe 1/2" lightweight sheetrock would work given the 12" spacing?
Otherwise, #2 Doug Fir 12"OC 2x8 spans 25-8 with L/240. I could remove the strong back and add these 2x8s between the existing rafter ties, maybe also sister the existing rafter ties with more 2x8. Not sure how sacrilegious leaving the 2x6 rafter ties mixed with 2x8 joists would be.
#2 Doug Fir 24"OC 2x10 spans 22-9 with L/360, so this seems like an even stronger option that could support 5/8" Type X sheets for peace of mind. Removing the strongback then fitting these onto the top plate to sister to the existing 2x6 seems tricky, but would involve half as many boards since I wouldn't be going to 12"OC. Would adding these as joists between the existing 2x6 rafter ties and only securing drywall to them be weird? It seems like that would put the weight of the ceiling entirely on the walls, assuming that's ideal?
Finally, the existing 24"OC 2x6 (not that I can guarantee they're #2 grade) can span 13-6 with L/360. Since I have 2 garage doors and 3.25" between the studs on that end, could I build up a beam pocket in the walls for a beam spanning the 21.5' to cut the rafter tie span in half? It seems like 3.5" is a common beam width so it couldn't squeeze into that 3.25" space without some other modifications. The LVL beam span tables I can find have much higher live+dead load ratings so I'm not sure how to appropriately size something like a 1.75" beam for this application. I have 9'4" of ceiling height so not too concerned with having the beam hanging down the middle between the doors if I can avoid adding a post.
Photos attached show where it's at right now; the extension cord based electrical is also being remedied
No storage in the attic space is planned, so only looking to support insulation (probably R36 fiberglass), drywall, lights, and one run of metal ductwork for dust collection. Snow load here requires minimum 20psf for the roof (not sure how this plays into plans below).
My main question is around what live/dead loads and deflection limits I should have in mind when looking at the IRC R802.5.1 span table or the AWC span calculator. Are the 10psf live load + 5psf dead load calculations appropriate for this case? Similarly not sure about the range of L/180, L/240, or L/360 for deflection limits.
Using the AWC calculator I can see #2 Doug Fir 2x6 12"OC can span 21-3 with L/180. This would involve adding additional 2x6 joists between the existing rafter ties to get 12"OC, connected to the 2x4 double top plate with hurricane ties(?) and potentially sistering the existing 2x6 rafter ties with an additional. In this case I believe 1/2" lightweight sheetrock would work given the 12" spacing?
Otherwise, #2 Doug Fir 12"OC 2x8 spans 25-8 with L/240. I could remove the strong back and add these 2x8s between the existing rafter ties, maybe also sister the existing rafter ties with more 2x8. Not sure how sacrilegious leaving the 2x6 rafter ties mixed with 2x8 joists would be.
#2 Doug Fir 24"OC 2x10 spans 22-9 with L/360, so this seems like an even stronger option that could support 5/8" Type X sheets for peace of mind. Removing the strongback then fitting these onto the top plate to sister to the existing 2x6 seems tricky, but would involve half as many boards since I wouldn't be going to 12"OC. Would adding these as joists between the existing 2x6 rafter ties and only securing drywall to them be weird? It seems like that would put the weight of the ceiling entirely on the walls, assuming that's ideal?
Finally, the existing 24"OC 2x6 (not that I can guarantee they're #2 grade) can span 13-6 with L/360. Since I have 2 garage doors and 3.25" between the studs on that end, could I build up a beam pocket in the walls for a beam spanning the 21.5' to cut the rafter tie span in half? It seems like 3.5" is a common beam width so it couldn't squeeze into that 3.25" space without some other modifications. The LVL beam span tables I can find have much higher live+dead load ratings so I'm not sure how to appropriately size something like a 1.75" beam for this application. I have 9'4" of ceiling height so not too concerned with having the beam hanging down the middle between the doors if I can avoid adding a post.
Photos attached show where it's at right now; the extension cord based electrical is also being remedied
