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Old 08-20-2019, 10:14 AM   #1
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How to build a perfectly flat floor?

1) Our metal floor is not perfectly flat.
2) Our chosen SPC vinyl plank flooring recommends a perfectly flat floor.

We'll be using joists w/ a plywood subfloor. Can anyone clue me in as to what techniques/materials are used to make it flat. I think I've got a handle on how to check for flatness. I just don't what the best way is to adjust it in this context.
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Old 08-20-2019, 10:23 AM   #2
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How unflat is it?

I think what they are referring to is not the gentle roll, but lumps and screw heads and such. When you first put the floor down over stuff like that it will look fine and then it settles and goes the shape of the lump or screw. Unevenness in your plywood seams will also be noticeable.
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Old 08-20-2019, 10:26 AM   #3
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Lol. I'm not sure how unflat it is. I just know from the imperfections it won't be flat. I also don't know how flat flat needs to be, but I want to be armed with the knowledge of how to flatten it before I begin
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Old 08-20-2019, 10:30 AM   #4
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Read my edited reply above
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Old 08-20-2019, 10:37 AM   #5
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Quote:
Originally Posted by TheHubbardBus View Post
1) Our metal floor is not perfectly flat.
2) Our chosen SPC vinyl plank flooring recommends a perfectly flat floor.

We'll be using joists w/ a plywood subfloor. Can anyone clue me in as to what techniques/materials are used to make it flat. I think I've got a handle on how to check for flatness. I just don't what the best way is to adjust it in this context.
Your plywood will be flat enough for the vinyl planks. Your main concern will be making sure the seams between plywood sheets "break" over an underlying joist (i.e. you don't want a seam to be free-floating between joists) and that attaching screws (if you use them) are flush with the top of the plywood.

They don't mean like laser-leveled flat here; "smooth" would probably be a better word, since bumps or slight edges underneath will eventually show through. I think if you left a quarter underneath it would eventually show.
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Old 08-20-2019, 10:39 AM   #6
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Originally Posted by Danjo View Post
How unflat is it?

I think what they are referring to is not the gentle roll, but lumps and screw heads and such. When you first put the floor down over stuff like that it will look fine and then it settles and goes the shape of the lump or screw. Unevenness in your plywood seams will also be noticeable.
No, they're referring to the overall slope. According to their install instructions - at most - 3/16" over 10'. I'd like to aim for better if I can.
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Old 08-20-2019, 10:41 AM   #7
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No, they're referring to the overall slope. According to spec - at most - 3/16" over 10'. I'd like to aim for better if I can.
Why would the slope of the floor matter? Especially when your bus will frequently not be level itself? Do you have a link to the spec? Maybe they explain the need for this.

Edit: I see, they don't really mean the slope, they're referring to potential undulations on that scale, which could be enough to mess up how the rows of planks interlock and lay flat. FWIW I don't think your metal floor is going to vary by that much, and as long as you keep your joist heights consistent you'll be fine.
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Old 08-20-2019, 10:48 AM   #8
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Use a proper T&G flooring plywood, it will be stiff enough to not have imperfections in levelness.
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Old 08-20-2019, 10:48 AM   #9
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Originally Posted by musigenesis View Post
Your plywood will be flat enough for the vinyl planks. Your main concern will be making sure the seams between plywood sheets "break" over an underlying joist (i.e. you don't want a seam to be free-floating between joists) and that attaching screws (if you use them) are flush with the top of the plywood.

They don't mean like laser-leveled flat here; "smooth" would probably be a better word, since bumps or slight edges underneath will eventually show through. I think if you left a quarter underneath it would eventually show.
Good info. I appreciate the advice. I plan on doing as you describe, so that should minimize problems there. But as posted above, the install instructions are referring to the overall flatness of the floor. If the joists aren't flat with respect to each other, the plywood won't be either once it gets screwed down.

I may be making a mountain out of a molehill. I'm having a hard time envisioning what 3/16 over 10' looks like. Think I'll go out there and see what I see before I get carried away here.
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Old 08-20-2019, 10:52 AM   #10
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Originally Posted by musigenesis View Post
Why would the slope of the floor matter? Especially when your bus will frequently not be level itself? Do you have a link to the spec? Maybe they explain the need for this.

Edit: I see, they don't really mean the slope, they're referring to potential undulations on that scale, which could be enough to mess up how the rows of planks interlock and lay flat. FWIW I don't think your metal floor is going to vary by that much, and as long as you keep your joist heights consistent you'll be fine.
Yeah, 'slope' was my word, and I knew it wasn't the correct term, but I couldn't think of a better one to describe what they meant

I'm going to go out and measure it now. I think you're right.

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Use a proper T&G flooring plywood, it will be stiff enough to not have imperfections in levelness.
That's the plan, Marc. Muchas Gracias. They make that in marine-grade, si?
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