Attic Span and Plywood Thickness

Old 11-05-01, 03:19 PM
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I have an unfinished attic which spans 17 feet to centerline. The joists are 2 x 8 nominal, 16 inch on center. I wish to put down plywood flooring for typical dead load loading (holiday decorations, off season clothing, etc). I have a few questions:

1) Do I need to "shorten" the span by "hanging" the joists to the rafters near the outside wall?

2) If so, how far do I shorten? a foot or two, etc?

3) If so, are 1 x 4's screwed in with deck screws to hang the joists adequate?

4) Once this is done, what is appropriate for flooring? OSB, plywood? and what thickness?

Old 11-11-01, 01:40 AM
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In most house structures, loads are distributed through torsion stress. The underlament, sheathing, plywood or whatever name you wish to choose is considered to be a sub-straight. As the name implies it is something that goes under something, like your roof shingles, siding or flooring that keeps things straight. The correct type of sub-straight (plywood) for your application is sub-flooring. Any type of flooring you put over the sub-flooring is merely costmetic. To illustrate load distribution, let's say you put a heavy object in the middle of your floor. Compression and expansion stress is being applied but not being distributed. The sub-flooring distributes it by pulling and pushing on the floor joist adjacent to it and keeps on pushing and pulling to subsequent floor joist until the stress is dissipated. In order to apply this properly, the nailing should be no more than 4 inches apart and the screws should be no more than 6 inches apart. Contractors use a construction adhesive to avoid squeaking floor noise.
Old 11-12-01, 03:42 PM
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Thanks for the info. I understand the load distribution mechanism. I am looking more for what the allowable span length is for 2x8's,16 on center, for attic dead loading, as well as the recommended "flooring" thickness. Any suggestions?
Old 11-17-01, 08:35 PM
some help
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If you are not going to use this for living Use 3/4" plywood it will help with the span and you can go 14' to 16' with 2"x8" with no problem...insolate before you cover and make sure you do not cover wires running over the joist or nail into them.....Good Luck..
Old 11-19-01, 02:30 AM
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Thanks for the info.

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