Best practice for installing floor joists on slab for a raised floor? Want to avoid pressure treated.
I need to build a raised floor surface that is one step up from a concrete slab (slab is polished and finished floor surface), and I would to overthink this with you to avoid moisture issues and to keep it flat.
As the diagram shows, for sizing it’s convenient to use 2×6 sawn lumber as sleeper joists to get to the 7-1/2″ step height with subfloor, radiant heat panels, and finished floor.
I’d prefer to not use pressure treated 2×6’s for the joists—though the general consensus is always to use treated lumber when in contact with concrete. Even if the treated joists were fastened to the slab, they’ll inevitably shrink and bend… and I don’t have time to adequately dry them before installing.
So, if I were to use non-treated kiln-dried lumber:
1) What’s the best barrier to use between the joists and slab?
2) Should the barrier be continuous under the flooring area between slab & joists, or just strips under each joist? (I would worry that a continuous vapor barrier might trap latent moisture under the barrier on top of the slab, even though the slab is very well vapor-protected from underneath and sides and has had some time to dry.)
3) To prevent joist movement, should I fasten the joists to the slab? (For example, fasten treated blocking to the slab mid-span and then joists to that blocking. Also, the rim joists will be treated.)
I have a bunch of 60mil EPDM from a flat roof project I could use to cut a strip under each joist.
Would love to hear any wisdom on how to build this to perform long-term.
Replies
“[Deleted]”
I have the same issue, but in my case the slab has in floor radiant heat and is 27 years old. I too would prefer to not use treated joists. What solution did you end up installing?
I would simply put down a 2 inch thick layer of XPS on the concrete and use 2x4 sleepers on top of this. The XPS will be a barrier against moisture from the slab and would also stop thermal bridging to the slab. XPS has enough compression strength to support the floor, which would be "floating". Since your diagram doesn't indicate any insulation under or around the slab your radiant floor will be more efficient if the whole floor is isolated from the slab. I also assume that this slab is a basement floor and you have good drainage and proper foundation coating in place. If not then I'd guess the walls are framed with a PT plate, or are you about to frame from the slab up? If so, you can't use the XPS under the wall framing, but you could at least use the rubber there, and use a water resistive barrier on the wall sheathing that overlaps the plastic on the side of the slab.
I recently replaced all the 2X4 sleepers under a hallway floor for a friend. They were completely rotted, non-PT, and in direct contact with the concrete pad underneath the floor. Food for thought.
Gary
Thank you all for the replies. I’m not the OP, and my job is a little different, but it still involves framing over a concrete slab. I need to build a roughly 4’ x 6’ elevated platform to hold a freestanding tub in a master bath. The reason for the platform is to allow room for plumbing. As I said before, the house is 27 years old and has in floor radiant heat. The sunken jacuzzi tub we removed was framed with non-PT lumbar and there was no sign of rot or deterioration, so I’m not worried about having wood on the interior in contact with the concrete. Perhaps that’s one of the advantages of living in the high desert of NM. I do like the XPS idea. Thanks.
If you put non-PT directly on the slab, it will very likely absorb moisture over time, then of course it will rot.
I've done this before - I put asphalt shingle between the wood and the concrete. Moisture isn't going to cross that barrier.