Flatten under 1/2 underlay?

[bitbanger]

Hi folks -

I'm installing ~1300sq ft of LVP. Subfloor is 23/32 T&G ply, in sound condition (built in '98).

There are quite a few sections where a high or low joists, setling, repairs etc have resulted in needing to flatten up the floor, up to about 1/4" over 6'.

Complicating this is I need to build up the underlayment height about 1/2" so I purchased 7/16" APA OSB sheathing for the entire area. I would have preferred a ply but at the time prices were nearly $65+/sheet for BC.

Plan is to staple down the OSB with 7/16" x 1-1/8 16g divergent point staples. Atop that 1.5mm underlayment then 6.5mm lvp.

I've had good luck with Henry troweled patch/leveler, but I'm at a bit of an impass. My instinct is to level at least minimally before I lay the OSB, because the staples are not likely to draw down the OSB in the low spots between joists and I'm afraid I'll end up with voids and a spongy floor after all this effort. However I'm not sure it's a good idea to be running all those staples through what could be some large areas (16"x6' for example) of leveling compound (I e. Feather finish).

Should I do some type of felt or tar paper under the OSB, then final leveling compound after OSB is down? I anticipate needing to do some joint transition flattening because the OSB thickness is not always 100% at the edges.

Many areas are maybe 1/8" out over 6'. Am I splitting hairs by bothering to flatten under the OSB in that areas?

Can you staple through feather finish?

Thanks for reading.

 

[Floorist]

Stapling through any floor patch will cause it to crumble.

 

[bitbanger]

Suggestions for leveling under the 7/16 OSB?

 

[Floorist]

Shingles?

 

[C.J.]

Shingles or roofing felt in layers are good options. You can go up to 6 layers thick of roofing felt. 30# roofing felt is 1/32” so 6 layers will buy you up to 3/16”. Combine that with a little sanding here and there and I believe you can achieve an acceptably flat substrate without having to use any patch, or at least minimal patch.

 

[bitbanger]

Thanks CJ, I agree, I picked up a roll of 30# felt earlier today.

First room down (attached). I'm leaving little less than 1/8" gaps between panels to avoid squeeks in the future but many panel joints will need filled due to thickness variations at the joints...so this begs the question, is the gap pointless if I'm filling + feathering out the height difference? They won't be able to expand into the cement based filler? I am leaving generous room against the walls.

 

[bitbanger]

Shingles?

Thanks Floorist. Seriously considered this, but I knew I'd use up felt but couldn't see using up all the shingles (hope I didn't just jinx myself, I have about 500sq ft of existing flooring to rip up yet)

 

[C.J.]

Floating LVP? I would prolly not leave gaps, or at least not that big of gaps, between the panels myself and sand things smooth. No need to patch unless you actually need to patch the joints. Maybe even a little sanding of the subfloor so you don’t have as much sanding to do on the OSB.

Smooth and flat is what LVP likes. As you’re installing the OSB I would check your progress with a straight edge to highlight where you need to sand things smooth. Slide the straight edge around and highlight any highs and lows with a sharpie scribble on the floor. Maybe even mark it with a H or L. Makes it easy to see what’s going on and a you won’t overlook the sharpie scribble when you get up and do something else then come back to what you were doing.

Personally I use a pencil to mark my attention spots on my subfloors and slabs but I say sharpie because a pencil may be hard for some to see.

 

[bitbanger]

Sorry I should have specified yes a rigid core vinyl plank.

Thanks for the input on the panel gap - I'll forego the gap, but will definitely need to flatten between panels...This OSB, while I'm sure technically meeting its APA PS spec, is liberal in the thickness variance... Considering we're fishing from the bottom of the barrel these days with wood products I'm not entirely surprised. I am giving them plenty of time to acclimate in the house, not that has much bearing here, but I'm doing my due diligence.

And yes this was a good excuse to pick up a 6' level/straightedge.

i said subfloor was sound, which I believe it is... Not soft and no major deflection, but I think the ply T&G may have been low quality. Something is up with it. Several localized "bubbles" where top layer had delaminated and for seemingly no reason. Thankfully they're only 1x2-3", but there are some panels with lots of what I can only describe as "dry rot" looking cracks in top ply. More often than not when I cut that top bubble off I find there is a void just below in the next ply down. From beneath (dry, unfinished basement) panels show zero evidence of moisture or wear like above.

I've filled these delam spots, and re-screwed with deck screws along every joist, nearly every nail. of course this left a lot of puckers despite best efforts, that I've had to then hit quickly with the sander.

Backstory: this house was built by the local trade school and while I'm sure it met code, it goes without saying meeting code doesn't mean meeting industry best practices/standards of construction. Budget materials I'm sure. I was told the second story was built off-site at the school and craned in, which makes me wonder if the 1st story (shown) got left in the elements for too long. As mentioned it's 23/32 Exposure 1 PS1-83

I digress... Picture below is after screwing, before hitting any high spots or filling.

 

[bitbanger]

Case in point:

 

[C.J.]

Loose lay the sheets of OSB and see how they fit before you fasten them. You can build up underneath the lower one with roofing felt to reduce or eliminate the need for sanding

 

[C.J.]

Looking a little closer at the pic and it looks like the thickness decreases within just a couple inches of the edge. I would prolly patch that if it was just a couple inches at the corner as opposed to an uneven thickness across a wider span of the sheet. Check it with your straight edge to confirm that you need to fill it vs flatten it. Carry on.

 

[bitbanger]

Yep learned my lesson on the above - already fastened, but I'll know to do it for subsequent rooms. Seems the OSB is actually thinner along the edge, that's not a result of a complete panel being that thin. Point is I'll know to use the felt to try to address thickness issues also.

I'm starting to realize that going with a cheaper material (OSB vs ply) is going to cost me more time. Still, at 1/3 the cost... Guess I'm (my labor) "cheap"

Thanks again for the input.

 

[Mark Brown]

You would probably not be surprised to learn that plywood also suffers these inconsistencies as well. It truly never ceases to amaze me what we have to accept as building material these days.

 

[C.J.]

Time vs money is nothing new. This is why we will often charge more money to work with less expensive materials due to the additional time that they may require to properly install. If you have the time to give then you’re dollars ahead in the end. Luckily your rigid core floating floor will be a little more forgiving than other products. Personally I like it when people get to experience situations like this for themselves. It gives them a better understanding of what we have to go through when someone is trying to save a buck or two and why we charge what we charge.

 

[Floorist]

You can run into that if you mix too. 3/4" particle board is always a different thickness than 3/4" OSB or plywood.

 

[C.J.]

You can run into that if you mix too. 3/4" particle board is always a different thickness than 3/4" OSB or plywood.

Yep. A lot of products are 23/32” vs a true 3/4”. The thinner underlayments are often in MM vs standard.

 

[bitbanger]

Time vs money is nothing new. This is why we will often charge more money to work with less expensive materials due to the additional time that they may require to properly install. If you have the time to give then you’re dollars ahead in the end. Luckily your rigid core floating floor will be a little more forgiving than other products. Personally I like it when people get to experience situations like this for themselves. It gives them a better understanding of what we have to go through when someone is trying to save a buck or two and why we charge what we charge.

For sure. I put down about the same amount of laminate plank in my last house back when a 30yr Pergo w/underlayment was as premium as you could get, and I got very lucky - newer house, installed and looked like a dream. So, I didn't come away with an appreciation for how much prep would be necessary for a non-ideal subfloor. This has also opened my eyes to how much can/could be skipped by a less-than-up front installer. I raise an eyebrow every time I hear a friend or family had floating plank floor installed for seemingly next to nothing. No disrespect to your trade, it's the case for all of them, but it makes me glad I am doing it myself because 1)there's no way I could have afforded to pay someone to do the 'right' job, and 2)it's like rolling the dice with contractors in my experience regarding quality of work.

 

[Mark Brown]

Laminate is a whole lot more forgiving that vinyl plank ever will be. The unfortunate part is they look the same so the vast majority of homeowners/installers/salespeople do not recognize this distinction.

 

[bitbanger]

Laminate is a whole lot more forgiving that vinyl plank ever will be. The unfortunate part is they look the same so the vast majority of homeowners/installers/salespeople do not recognize this distinction.

I was dead set on laminated based on my positive experience installing it in the past. But, I was surprised everyone I talked to seemed to turn their nose up at it, like in the ten years since I out mine down the industry had discovered some hatred for the product. With kids and pets on the way, I didn't want to be constantly stressed about the flooring, so admittedly I was 'sold' on the idea of LVP... I'm fully aware the surface water resilience says nothing about the underlayment/subfloor, but... Let's not go down that rabbit hole - I'm trying to make peace with the product we picked.