Hi
I’m setting a sub floor over an existing slab as part of an addition.
I have no option but to use 2×6’s on 16″ centers if I’m to plain through with the existing floor. This shouldn’t be a problem because I’m only spanning 8′ over all and I can shim the floor up from the slab a 1/8″ to 1/2″ varying gap to reduce the span. I planned to use steel shims, but a friend suggested packing mortar under various points of each joist, like a plumber might set a tub? I’m still thinking steel shims or perhaps cedar but thought I would ask just in case I’m missing something.
Thanks
Replies
There's a reason why the codes disallow any other than PT wood in direct contact with masonry foundations
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Piffin
Thanks for your reply. I did use pt joists just in case they tuched the slab at any piont, but that dosnt mean i want to pack mortar around them. So you would use cedar not steel shims?
Make your own pt shims on the tablesaw. It's exactly what I do in this situation.
steel or pt
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I did this in a previous house with 2x4 floor joists to reduce the ceiling height.
PT shims worked fine.
Pat
Put an asphalt shingle or a piece of starter roll on the concrete, then shim with cedar or whatever. Sometimes just a piece or two of starter roll will do the trick where the height to be shimmed is scant.
mike
Use steel with subfloor adhesive between concrete and shim also between shim and joist. as the floor moves, as it naturally will, the shims will stay in contact with the joist.Steel is better than PT because it won't shrink or degrade.
I wonder how long a steel shim will take to dissolve in direct contact with the new PT. I've heard that regular joist hangers might last less than a year. I'd stick with PT shims.
I'm the kind of person who would wear suspenders and a belt at the same time, so this may be frivolous.
If I had some of the concerns mentioned in this post, I would stick some steel shims (or PT) in under the end of the boards, simply because it is easy. Then I would mix up a batch of structural grout and put a little next to the shims, for about a foot. Drop in the joists, and you are ready to go.
Maybe I could be an advisor to FEMA.
I would be amazed if PT actually disolved steel.
The PT doesn't actually "dissolve" it, but the greatly elevated levels of copper in the wood certainly accelerate the corrosion of the iron in the steel (and aluminum flashing as well) which is why HD galvy (or better yet, stainless), fasteners/hangers are essential with PT lumber. It's been a couple of years since the potion for PT has been modified and it's amazing how many people who are banging nails still don't get it.
would the subfloor glue I suggested not provide a barrier between steel and wood and prevent corrosion?
It was my understanding that the corrosion issues were only pertinent to fasteners that were exposed to wet conditions.
blue
The way I understand it iswet or dry....it's the stuff they putin the p.t. stuff that dissolves the nailsand pretty quick from the info. I've seen on it(if it's not the proper type nail)
Could you make your shims from Trex deck material?
I would use construction adhesive.