||02-17-2008 08:18 AM
As you seem aware of, the correct way to get maximum benefit from sistering the joist is to go from plate to plate. Sistering them as you suggest will most likely help to remove some of the bounce in mid span as a real world situation, but may be a pretty hard sell to the building inspector. Even if not inspected, and this is seen by a home inspector at some time in the future if you sell the home, it will be made into an issue.
I am assumming that you have pipes that are bored through the existing joists, near the ends, if you can get a 10 footer in place. What I have done a few times similar to this is get the heaviest STRUCTURAL gauge metal studs available ((look in phone book for large drywall suppliers who deal with commercial work). Measure the clearance between the top of the offending pipe and the bottom of the sub floor above to see what is the deepest stud that will fit between them, and get enough 12 footers to put one on each side of the existing floor joist. Slip them in place above the pipe, inserting the end above the pipe first, apply a liberal bead of sub floor adhesive to the top flange and side of the metal jamb it tightly to the sub floor and hold it in place with a few self drilling metal screws available from the supplier (short enough so that they do not go completely through the wood floor joist). Do the same on the opposite side of the joist, then drill through the assembly, and install some through bolts every 2 feet or so, at random heights so that they are not all in a straight row. Now cut a squash block to fit between the bottom of the new metal and the plate or beam at the ends and fasten it in place. This will give you plenty extra stiffness at a reasonable cost, as the metal is not that expensive.