Underlayment Help

[shubox56]

Hi guys,

Once gain I'm over thinking things. My local big box didn't have a true .25" (or VERY close) underlayment grade plywood, so I went with an exterior grade BC plywood. This above the carpenter's patched subfloor. Any issues with that grade?

I hope this post finds everything doing well and Covid free.

 

[Mark Brown]

It really depends on what you are putting on it, but mostly no. It is not best practices to use anything other than an underlayment rated panel due to the construction of said panels. Underlayment grade plywood is free of voids in all layers, has specific densities and uses filler for knot holes that is not going to bleed through anything plus they are sanded fine and blah blah blah. That being said, i would say there are few flooring fellows who have not bent those rules from time to time. With vinyl plank, I have abused all kinds of rules and gotten away with it. If you plan on any kind of floating floor, it will be entirely ok.

 

[shubox56]

Hey Mark, thanks for the info. MUCH appreciated.

Interesting stuff. Should I be less concerned about the .75 plywood patched subfloor panel installed by the carpenter? Feels ROCK SOLID with his added bracing, but I guess it's a little late now anyway. Installed with flange installed.

You have me thinking about the .25 underlayment, though. Reason being, the final choice of flooring has not been determined. Maybe I should search for a true .25 underlayment rated panel, and as much as I don't feel like recutting, recut anyway.

 

[Mark Brown]

Its probably more than ok. I mean hell, i have put sheet vinyl on top of OSB sheathing panels in my own house.... just because you shouldn't does not mean you cannot. If what you have is available and already done, well i can tell you i wouldn't be changing it.
There is a whole slew of best practices, as a tradesman i follow them to the letter lest i get burned and own a job. Then there is the "this will most likely be ok" side of things that we all know will be fine by i cannot sell it to you because someone once said "there is a better way".
Here in my own house, i have fibersheet flooring installed on osb, over an unconditioned crawlspace in an unconditioned room. Going on 5 years now and it is still rock solid, that room has been flooded numerous times whilst i was mixing salt water for my fish tanks... its amazing how often you can fall asleep while making RODI water in a barrel... flood. Its where we wheel barrow wood into the house to burn..... you get the idea, abuse. That floor still looks brand new.
I am thinking in such a small space, just roll the dice with what you have.

With a BC panel, you will more than likely have to do some patch work to get the face smooth enough for a sheet vinyl or glue down plank however, and that is where it gets ugly. The moisture from patch has a tendency to raise grain on plywood, which then leads to more sanding... but it is all possible.

 

[shubox56]

You are teaching an old dog new terms. First I've heard fiber sheet flooring. Something like the old linoleum except for a vinyl fiber composite something something? So you laid this sheeting directly on top of OSB subflooring without any underlayment?

Good stuff! Thanks!

 

[Mark Brown]

We call it "LSV-Luxury Sheet Vinyl" its got so many names, glassbac, fiber floor, foam back.... thats all that come to mind right now. It is basically any sheet vinyl that is not mineral backed. They are much more forgiving in working terms than old mineral back/paper back vinyl. It is 100% on 23/32 T&G osb, which for the record as a daily activity is a terrible idea however i am cheap and like experiments. I put a lot of patching work into the osb and happen to own a 16" oscillating sander so there is not much i cannot make sexy.
I would not suggest doing such a thing, just that such a thing can be done. It really all boils down to the amount of work that you are willing to put in.