joist spans

[me59]

Hi I brought a cabin at a auction so I no nothing about it but what I see the floor jois
t are 2 X 6 spanning 10' to the beam ( double 2 x 6) at 16" on center with no footer setting on cinder blocks with a total span of 20' so here is my question would I be better off coming in and put the 2 x 6 on 8" centers or sistering the 2 x 6 before trying to put a foundation under it?

[Don_P]

Now there is an interesting question, I don't know. My gut reaction is that if they are well connected to one another there will be better sharing of load across defects in any one of the 2 connected joists... you have already achieved the only allowable increase of 15% by having joists <24 on center so I'm just giving my opinion. I think it'll be easier to get stuff around in the bays with them sistered. Here they often ran overspanned 2x8's on 24's in old work, I go back at 12" on center because it reinforces the single board flooring above it better. 2 rows of piers and girders underfoot would make it rock solid on 16" centers if framing above allows.

[ChugiakTinkerer]

Welcome to the forum!

Is this a portable building, that you want to put on a permanent foundation?  Just trying to understand what your situation is.

If you got the International Residential Code which is available online at http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%205.html you will find all the prescriptive requirements for a wood-framed floor.  Scroll down there to Table 502.3.1(2) which is the joist span table for living spaces.  For 2x6 on 16" centers nearly all wood types come up just short of 10'.  Bear in mind though that the span is the unsupported portion of the joist; you subtract out the parts where it is supported on a beam, girder, or even a joist hanger.  Depending on the grade of wood used there's a possibility the unsupported span is just within the minimum allowed by the table.

Assuming it isn't and you need to stiffen the floor, either method you suggest will increase the load-carrying ability of the joists.  From the joists standpoint sistering will be better than putting a joist on 8" center, assuming you use enough nails attaching the sister to the original joist so that they form a 3x5.5 beam.  If however your sub-floor is marginal and there there is any give between the joists, putting a joist in at 8" will take the flex out of the sub-floor better than sistering will.  If the floor is otherwise sound I would do whichever is easiest to install.