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Old 11-01-2017, 01:49 PM   #14
wwhitney
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Join Date: Dec 2015
Location: Berkeley, CA
Posts: 761
Regarding adding blocking, having I joists rather than solid sawn joists makes it more complicated, but I'm not convinced that necessarily makes it harder than interleaving new joists. Seems like you have about 10 joist bays, and if you add 2 blocks per joist bay to get 24" o.c. support, we are talking about 20 blocks or so.

The gold standard for doing blocking would be to determine the joist manufacturer and contact them to get a detail to use. Explain that the goal is to accommodate the misoriented subfloor and that you don't have any walls underneath or above. I could imagine they would tell you to do something like one of these for each block:

1) Just use a single flat 2x4 against the underside of the subfloor, 2 toe nails through the 2x4 edges into the I joist top flange at each end of the 2x4.

2) Use a full dimension block (I joist piece or kiln dried solid sawn lumber), 2 toe nails at each corner of the block into the I joist flanges (8 toe nails)

3) Add filler blocks (e.g. plywood of the appropriate thickness) to each side of the I-joist web to fill out the web flush to the edges of the flanges. Use joist hangers with full dimension blocks

I feel like I know enough about I joists to say option (3) would definitely work, but it is also the most effort, and interleaving new joists could well be easier than option (3). The manufacturer can tell you if options (1) or (2) might be acceptable, I don't know enough to say.

Cheers, Wayne
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Wayne
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