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Old 08-20-2019, 11:14 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, 11:23 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, 11:26 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, 11:30 AM   #4
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Read my edited reply above
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Old 08-20-2019, 11:37 AM   #5
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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, 11:39 AM   #6
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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.
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, 11:41 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, 11:48 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, 11:48 AM   #9
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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, 11:52 AM   #10
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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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Old 08-20-2019, 11:53 AM   #11
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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.
Yeah, thatís a lot. The best way to get there is to shim between joist and metal floor, taking care to do it on top of the supporting cross members that the sheet metal is attached. You can use wood shims or stacked washers held in place by a screw.

You need to know whatís straight. Screw a 1 foot piece of 2x4 on edge at each end. Youíll need to do this for each joist. Put a screw in the top of each of this blocks, standing proud a little and then stretch some masonís twine as tight as you can across the top of this blocks. Now you can use a tape measure to check flatness. Keep in mind that you will introduce deflection unless you fill the gap between joist and sheet metal. Probably the best thing to fill running gaps the entire length would be to apply a fat bead of PL400.
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Old 08-20-2019, 11:56 AM   #12
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Yeah, thatís a lot. The best way to get there is to shim between joist and metal floor, taking care to do it on top of the supporting cross members that the sheet metal is attached. You can use wood shims or stacked washers held in place by a screw.

You need to know whatís straight. Screw a 1 foot piece of 2x4 on edge at each end. Youíll need to do this for each joist. Put a screw in the top of each of this blocks, standing proud a little and then stretch some masonís twine as tight as you can across the top of this blocks. Now you can use a tape measure to check flatness. Keep in mind that you will introduce deflection unless you fill the gap between joist and sheet metal. Probably the best thing to fill running gaps the entire length would be to apply a fat bead of PL400.

Beauty, man. Problem (even if imagined) solved
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Old 08-20-2019, 11:58 AM   #13
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I think they want it flat to prevent the vinyl from flowing toward the downhill side, because it's not really hooked down, it's just sitting there, right?

3/16ths of an inch in 10 feet is not much. If you park your bus on a slope like that you wouldn't even notice without measurement tools of some type, and that's an adequate reason not to use vinyl planking in my opinion.

[edit] I've got a stack of 2X4s in the garage, local Douglas Fir, all no. 2, tight knot. I'd be willing to bet that none of them are perfectly straight, and most would be somewhere between an eighth and a quarter of an inch off straight in 8 feet.
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Old 08-20-2019, 12:08 PM   #14
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I think they want it flat to prevent the vinyl from flowing toward the downhill side, because it's not really hooked down, it's just sitting there, right?

3/16ths of an inch in 10 feet is not much. If you park your bus on a slope like that you would even notice with measurement tools of some type, and that's an adequate reason not to use vinyl planking in my opinion.
That's an interesting perspective. I figured the rationale was to prevent buckling or other issues during expansion/contraction, but that was just my gut feel.

I will not be floating it. It will be glued down. The warranty states that for temps above 140F that's the only acceptable installation procedure. I'm not worried about the warranty (it also states it must be in a temp-controlled environment, which it will not always be), but I figure if it keeps them from paying for failed flooring in typical installs, it's a good measure of insurance.
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Old 08-20-2019, 12:09 PM   #15
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I'm going to go out and measure it now.
I think a better way of characterizing this than 3/16 per 10' would be to think of the planks interlocking to form a flat sheet: whatever a flat vinyl sheet could conform itself to (with minimal stretching) is what the planks can handle. So if (for example) your floor was bowed up in the middle by an inch all along the length of the bus, the planks would still work fine.

People use these planks all the time in skoolies and there don't seem to be any problems with them related to the flatness of the bus floors.
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Old 08-20-2019, 12:14 PM   #16
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Somehow I got the impression that stuff was not glued down. If it's glued it won't flow and they are probably more concerned about buckling.
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Old 08-20-2019, 12:27 PM   #17
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I think a better way of characterizing this than 3/16 per 10' would be to think of the planks interlocking to form a flat sheet: whatever a flat vinyl sheet could conform itself to (with minimal stretching) is what the planks can handle. So if (for example) your floor was bowed up in the middle by an inch all along the length of the bus, the planks would still work fine.

People use these planks all the time in skoolies and there don't seem to be any problems with them related to the flatness of the bus floors.

I don't know, music. What you've just said directly contradicts the manufacturer recommendations. I see what you're saying (assuming a lengthwise arrangement), but would this not alter the alignment of the interlocking grooves? Could this not potentially cause problems?


I'd prefer to not take any chances. You say people use these planks in skoolies all the time without problems, but I've read numerous accounts of people having problems they couldn't explain. Perhaps the flatness of the floor was part of their problem?


The floor is one of those things I only plan on doing once. If I end up doing more work than necessary I'm OK with that. I just don't want to find out after everything's in I have to rip it up because I did less than I should have.
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Old 08-20-2019, 12:30 PM   #18
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Somehow I got the impression that stuff was not glued down. If it's glued it won't flow and they are probably more concerned about buckling.
It's designed to allow for a floating installation, which is a big part of its popularity. But a glue-down install remains a viable option. Apparently the only manufacturer-supported option in extreme high or low temp environments.
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Old 08-20-2019, 12:43 PM   #19
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I don't think vinyl is technically a solid. I think it's considered a supercooled liquid, like ice, and therefore would not act like a solid during temperature changes.
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Old 08-20-2019, 01:11 PM   #20
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I see what you're saying (assuming a lengthwise arrangement), but would this not alter the alignment of the interlocking grooves?
Think of it like a jigsaw puzzle. You could bend it pretty considerably along either axis, but you couldn't get it to sit down in a bowl-shaped depression. I think it's just easier for the manufacturer to explain it as "gotta be flat".
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