I am finishing the room over my garage. It is a 20 ft span. The floor joists are 2x8's. Spoke with inspector and he told me to use 2x12's 12 o.c. and it will be fine. Would it strengthen the floor if I sister them to the 2x8's or should they stand alone? Also, is it possible that the floor could be to heavy for the garage wall's that it's sitting on?

Thanks in advance,
mt

 

It is easier and better to fasten them together.

 

20 foot span is a huge span and there is no way 2x8's will work (no matter how you rig them). Do what the inspector suggested since he has the final say anyways. You could do 2x12 @ 16" o.c. and double everyother joist. It is the same as 12" o.c. Personally I think 12" o.c. sucks......doesn't leave you room to put duct work or is a PIA to do bridging/blocking in between joists.

 

I agree with all that Timberland said. 2x12's, 16" oc is appropriate, but the inspector said 12" oc so you will need his OK. By the way, 20 ft is the max span for 2x12's so it cant hurt to exceed that requirement by a little. Ask the inspector if you can use 16" oc if you fasten the 2x12's to the 2x8's (assuming that the 2x8's are currently 16" oc). This way you can keep the customary 16" spacing and satisfy the inspector's desire to exceed the minimum support required. While you are talking to the inspector, you can ask if he has a preference for fastening the joists together. This may save you some rework.
Good Luck.

 

You could use engineered lumber, too; 14" I-joists could go 16" OC and would be stiffer than 2x12s.

 

I would say what he said....(but I would of had many misspellings)