Floor joist sizing

[Steve Barker LT]

I'm replacing the lower level floor joists in a 135 yr old farm house. Basically sitting 6" off the ground on a rock foundation. The current joists (what's left of them) are rough oak 2x10 on 24" centers. Length = 15.5'. If I go to 16" centers and support them in the middle, would a 2x8 Douglas Fir be sufficient, or should I stay with the 2x10? Flooring will be underlayment then 1x6 pine planking. (similar to original)

THANKS,

[dpb]

2x8 will hold but probably will be more "bouncy" even on 16" instead of 24" spacing. If it were me, for that application I'd stay w/ the 2x10 and use treated for the extra moisture resistance and still be far less expensive than Doug Fir (unless you're in the PNW where it can still be found--ask the average worker-bee at the local yards here for "fir" and they'll think "fur" and when you tell them what you mean the response will be "Never heard of it!" :( )

[Steve Barker LT]

thanks for your reply and input. Here in the KC area the PT is about a dollar more than the DF in the 2x10x16.

[<aemeijers]

I&#39;m no expert, but that sounds consistent with what I have seen used over the years. While you have the floor open, I&#39;d strongly consider doing some digging and making that glorified sleeper foundation into an actual crawlspace with a vapor barrier, and usable access for wiring and such. If that isn&#39;t practical, I&#39;d make all the new timbers out of treated wood, and use a solid poured concrete pier up the middle for your center support. I suppose house jacking and a proper foundation are out of the question?

aem sends...

[Steve Barker LT]

Raising the whole thing would have been nice, but the "wings" so to speak off the kitchen are on slabs. And it would be cost prohibitive. I&#39;ve already done about 80% of the wiring and have the plumbing planned and none of it goes under. We even had a guy do a superb job of putting in a high efficiency heat pump/furnace/ac unit with full duct work with minimal hassles. I hadn&#39;t even planned for this floor fiasco, but we opened it up to level it and found the joists in a bad way. My preliminary plan for the center support is to dig out the loose dirt and get to some solid ground and lay down a PT 6x6 and block up on top of that. I don&#39;t think pouring concrete is possible. I&#39;m on a bit of a time crunch and winter is moving in here fast. I figger if what was there lasted 135 years, then anything I do will out last me for sure. And we plan for this to be the last stop.

thanks for your reply and input,