I am building a steel framed house. The first floor joists (over a basement) and second floor joists are STEEL Z's on 16" centers. Our plans are to lay ceramic tile on the first floor, but Hardwoods on the second floor. My plans are to use 3/4" T&G AdvanTech for the subfloors, glued to the Z's with Liquid Nails (or something appropriate for Wood to Steel), and of course the subfloor will be Screwed to the Z's. For the first floor, I'll need an underlayment since we'll be laying down ceramic tile. I'm not sure I'll need an underlayment on the second floor since we're laying down hardwoods. I've seen discussion both ways on that. Questions: What underlayment would you recommend to use over Advantech and under ceramic tile or hardwoods? What other considerations do I need to make in my decision process? I've not done this before and I'd like to do it right the first time. So any and all detailed info will be greatly appreciated. Thank you in advance. Regards, Larry

I have not run across your situation before. You would not need an underlayment for the hardwood if the flooring is ran perpendicular, but, if using 2" fasteners they will poke thru the subfloor a little and the steel worries me.

Under tile you need to know what your subfloor deflection is. Try calling the engineering dept of the steel Z joist. If it meets an L/360 deflection you can use tile. If you go with a large 18 x 18 tile or any natural stone the deflection would need to meet an L/720. I mainly use 5/16" Durock, 1/4" Hardibacker or Ditra. That all depends on whether I have to match up to something and also how many sq. foot is involved.

I agree with Jerry. Lay the wood flooring at right angles to the steel joists. If you need to add height to the wood to meet up to the tile so they are flush, use plywood at the correct thickness to acheive that. Typically, I'd simply staple it down but with steel joists, you may need to glue and screw. If the plywood underlayment was 1/2", you could use 1" staples without protruding through your 3/4" Advantech subfloor. As far as nailing down the wood flooring, IF you do not use any plywood underlayment and nail it directly to the Advantech subfloor, then use 1&1/2" fasteners (staples, cleats) to avoid hitting the steel joists. Underlayment for tile are as Jerry mentioned; Durock, Hardbacker, Ditra, etc. I have nailed and thinsetted 1/4" Hardbacker then thinsetted the tile to that. It was very stiff with zero defelection. I used a modified thinset for the backer board that allowed one to thinset to the subfloor. That is what made the backer board so stiff.

Hi Everyone,

Trying to understand something. Do I need an underlayment overtop of my T&G OSB subfloor or not? I am sure it is obvious and I am not seeing it, so speak to me like I am 5 please.

Thanks.

Please read the sticky at the begining of this forum:
http://www.hardwoodinstaller.com/hardwo ... .php?t=385