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Discussion Starter #1
I'm putting in a floor over a concrete slab. Going to raise it up about 4" to be even with adjoining room. Got to looking up subfloor and got confused. RTD, CDX, OSB, Advantech, etc. What should I use? I had read that for 16" oc joists I should use 1/2" but couldn't find as many options. Should it be 3/4"?
Basic question but couldn't find a straight answer. Thanks!
 

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Done a moisture test on this slab yet?
I'd lay down 6 mil. plastic first over lapping and taping any seams, and run up the walls a few inch and tape.
1/2" would be useless, use Advantech, 8P ring shanked nails and constrution adhesive on top of the joist for a rock solid floor.
 
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Discussion Starter #4
Thanks for the help! Yes, going to use 2x4s on edge, or maybe 2x3s if I don't have the room with subfloor and flooring to make it flush. Slab is pitched away from house, so that side will be raised up a bit. Will either use shims or some kind of bracket to support the joists (maybe something like Floor System or Klevaklip or just 2x4 "Ls").

Will definitely use 6mil plastic. Is it OK if the screws that hold the shims/brackets go through the plastic?
I'll get the 3/4 Advantech if that works best. Just need to adjust my dimensions 1/4" now!

Also was planning on using this project as an excuse to get a framing nailer as I'm redoing some of the walls and sistering some ceiling joists. Would that be suitable for nailing down the subfloor?

Thanks again!
 

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That Floor System looks like an expensive product, something an architect would spec. Be best if you could cut tapered joists, but an alternate solution may be easier. Measure the high spot in the room, subtract 3/4” for the advantech and more for the finish floor. Rip all the joists to that dimension. Snap chalk lines on the floor either 16 or 24” on center.
Lay out the joists, use shims every 12” or so to level the joists. Use liquid nails to glue the shims to the floor and the underside of the joists. Now you should have a level grid of floor joists ready for the advantech.

Everybody needs new tools ... but a framing nailer would be the wrong tool for this project. Problem is the nails are likely to be too long, all you need is about 2”. It would work well for framing the walls and ceiling. Use deck screws to secure the flooring.
 

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I use my PortaCable FC350 framer to shoot a 2 3/8 ring shank nail on sub floors. +1 on scribe and cut full length taper joists. I have 2 thoughts on attaching, 1.) use treated joists, glued to the concrete along with concrete anchors every 12-16", then use plastic above the joist. or 2) plastic first, regular untreated lumber for joists , concrete anchors every 12-16", glue on top to Advantech sheathing.
 

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Yoda is correct, you can shoot the correct size nails into you subfloor. But they are typically sold in boxes of 5,000 and you aren’t going to need nearly that many. And they’re too short for your wall and ceiling work, so you would have to buy 5,000 nails of 2 sizes.
 

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Discussion Starter #10
Thanks for all the help! I've been playing around with different designs for a while. I'll take some more measurements and see what might work.
For the anchors, would I be drilling through the joist vertically? Would I countersink them so the sub floor is flush? Only ever used tapcon screws never the other type of anchor

Thanks!
 

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Discussion Starter #11
Just took some more measurements. On the side with the step down there is about 4.5", minus 3/4" subfloor leaves me with 3.75". I guess that would be best for a 2x6 scribed. On the opposite side (12' across) it's about 7.75", so the 2x6 wouldn't be on the ground. Would it work to hang a ledger board on that side then either rest the joists on top or use joist hangers? I would need to figure out how much of a span I could have without having support underneath, but would 6' be OK?

Also, why do I need to achor to the concrete? If both ends are attached somehow, and maybe if I add blocking in between, there's no way they could move, right? So what is the purpose of attaching to the slab? Almost like a low deck at this point...

Thanks again for all the advice!
 

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7 3/4 minus the sub floor is 7" just go to 2x8.
I would be using treated lumber and as you know it can warp and twist so a row of solid blocking would be in order and they can warp up so a few blocks pinning them down might be a good Idea,
Pinning is a 3/16" hole a couple pieces of wire or nylon string from a weed eater and a 3" nail.
 

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Also, why do I need to achor to the concrete? If both ends are attached somehow, and maybe if I add blocking in between, there's no way they could move, right? So what is the purpose of attaching to the slab? Almost like a low deck at this point...

Thanks again for all the advice!
Sounds like the joists will be bigger and heavier than originally expected. I would still anchor to the concrete in the center of span to prevent possible upward movement. Not sure I understand Neal's method, I'd probably use blocking jbetween joist turned flat against the floor and then anchor bolt thru it. Then add another vertical block above the flat to prevent joist twist. Essentially a upside down "T" block with a bolt passing thru a short leg anchored to the floor. Lumber pricing is ridiculously high right now, especially treated, hope it's better where ever you are.
 

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Discussion Starter #14 (Edited)
Sounds like the joists will be bigger and heavier than originally expected. I would still anchor to the concrete in the center of span to prevent possible upward movement. Not sure I understand Neal's method, I'd probably use blocking jbetween joist turned flat against the floor and then anchor bolt thru it. Then add another vertical block above the flat to prevent joist twist. Essentially a upside down "T" block with a bolt passing thru a short leg anchored to the floor. Lumber pricing is ridiculously high right now, especially treated, hope it's better where ever you are.
That idea of the T or L bracket is what I was thinking. Read somewhere that having a vapor barrier over slab was bad, but most everything else said to use it. Always confused about barriers, trapping moisture, etc. If I run the barrier up the wall a bit will it be a problem for the bottom of the wall? Or can they transfer the moisture outside to dry off?
Thanks again

Edit: and yes, prices look about double. Not great timing for a my big project. Might run the joists 24 oc to save a bit of money but not sure if it's worth it
 

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If I run the barrier up the wall a bit will it be a problem for the bottom of the wall? Or can they transfer the moisture outside to dry off?
Thanks again
Much has been written on using VB's. It's not as straight foreword as you might think. First, your new floor directly on the concrete. Couple things to consider like how dry is the slab? How old is it? Do you know if a vapor barrier was placed under the concrete? If so, and your concrete is always dry you may be ok to skip the floor poly. You can check for concrete dampness by duct taping some 24" squares of ploy down to the floor then check back in a day or two. Regarding the concrete walls I believe most intelligent designs involve building your wall with a air gap between concrete and new wall. No poly VB, and allow drying to the inside. But a vapor retarder is ok like the paper on your faced insulation. Most like to use Roxul insulation here due to its moisture resistance. You can also use open cell foam board (white large bead, no foil). But to throw a wrench in, some areas require VB down here. So best to check with your local code office. Some reading on the topic of VB's BSD-106: Understanding Vapor Barriers
 
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