Do I need to use PT plywood for subfloor over dirt?

[CanadianInIowa]

Starting listing the materials for an addition to my cabin, It will be up on cement columns and the underside is open. ( ranging from 0 to 3 feet above the dirt.) eventually enclosed with lattice.

The beams will be PT, but was wondering if the joists and sub floor has to be PT, or can I use standard 2*6 and OSB for the sub floor?

PT wood is 2* the cost of the same standard wood. Was thinking of using PT only where is is very close to the ground, and regular wood for the rest. Also, they say to use 3/4" for the floor, can I then go with 24" spacing (Less joists needed)?

Thanks.

[marksr]

The sub floor shouldn't need to be PT. Are you sure 2x6 floor joist will be heavy enough [typically not for spans over 8'] Never have floor joist on more than 16" centers. You really ought to excavate some so none of the framing is in contact with the ground.

[XSleeper]

>>ranging from 0 to 3 feet above the dirt

calling all termites!


>>they say to use 3/4" for the floor, can I then go with 24" spacing

Whatever you choose to use on the floor, it will have an APA stamp on the back which shows what spans it is rated for. If it is marked Sturd-I-Floor 24 OC, then it is OK to have the floor joists on 24" centers. If it says 20OC, then the maximum floor joist spacing is 19.2. If it says 16OC, you can't space the floor joists more than 16" apart. It all depends on what you buy and how it is rated. 3/4 T&G OSB or T&G ply is probably what you want for the floor.

[CanadianInIowa]

>>ranging from 0 to 3 feet above the dirt

calling all termites!


Actually, it would be carpenter ants in my area! :blah:

I will dig it out so the dirt is at the bottom of the beam ( so the closest the joists would get is the beam's width - 10".

And there will be two sets of beams at about 3 feet( from the house) and 9 feet, so the joists only have to span 6 feet, canterlevered out 1 foot.

So, there will be 6 pillers, the beams will span 10 feet ( four 10*2 by 10 feet) they will be PT.

So, it sounds like I don't have to go through the expence of using PT wood, except for the beams, and where the wood would touch the dirt.


Thanks, and I read in another posting to go with the OSB.

[Ed Imeduc]

You dont say are you going to heat this room???? is it open all around ??? Id sure put a 6 mil poly down on the ground there first. like said 16" on center 3/4"T&G and for that like new 19" on center calls for 1" T&G

ED ;)