Non fire treated plywood sub-floor over non-combustible concrete decking in type IIB construction

[Tim Mailloux]

I have a school project under construction with one of the learning stairs that are all the rage in schools now. My detail consists of cold form metal framing with a ¾” T&G structural concrete panel floor. On top of the concrete panel I call for a continuous layer of ¾” fire treated plywood sub-flooring to act at a nailing surface for a ¾” hardwood flooring finish. I have used this detail on numerous school projects without issue until now. The FT plywood has been installed and in a controlled HVAC environment for over a month but is still reading to wet to install the hardwood flooring over. One option being proposed by the CM is to remove the FT plywood and replace it with standard plywood. Looking at IBC 603.1 and 805 my gut reaction is that the plywood is required to be FT, but reading more into things I am not so sure. 805.1.2 allows for wood flooring to be installed on wood sub-floor on sleepers per 805.1.1. And per 805.1.1 allows combustible materials to be used provided that the construction is fire blocked per 718. As the non fire treated plywood would be a continuous layer fire blocking is not an issue, and it will also be sandwiched between the hardwood finish floor and a non-combustible concrete structural floor deck. What are you thoughts?

 

[steveray]

Furring/ sleepers/ blocking...it's fine.....As long as you are not making a cavity where FB becomes an issue or at some point sprinklers (around 6").....Plus you don't have to worry about the FRTW eating the fasteners...

Maybe expose the FRTW to my sense of humor...Thats dry....

 

[TheCommish]

Moisture coming out of the concrete panels keeping the. FRT moist?

Will moisture from the concrete panel be a problem with untreated plywood?

What is under the metal frame a concrete pad another possible moisture source

 

[ICE]

FRT plywood is particularly nasty. I used it on parapets that broke up apartment buildings. There was so much chemical that the sheets stuck together. Slivers are akin to a rattlesnake bite. As to moisture levels, I don’t know if the chemical is water soluble or the “wet” is all chemical. I recall that the sheets were noticeably heavier than the generic type. That indicates a lot of chemical was soaked up.