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Old 09-28-2015, 06:49 PM   #1
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Why does the subfloor have to be flat?


I'm installing engineered wood flooring. Everything I read, from the installation instructions to every single resource on the internet says that the subfloor needs to be flat within 1/8" over 6' (or similar), even if you're doing nail-down. What I can't find is why- I mean, I get it if you have something sticking up that's going to be severely in the way- but what happens if there is a bit more of a dip (or hump)? I can't find anything that says why.

I've been struggling for several days now on trying to make the subfloor flat, and everything I've done seems to be making it worse. I get it flat in one direction, and that makes it non-flat in another. So, then I tackle that, get it flat in the first two directions, but it's not flat at a diagonal! Because I'm doing nail-down, I can't use leveling compound, and I don't want to fill my bedroom with asphalt, so instead of the shingle method, I'm shimming with pieces of thin plywood, then planing down the edges so they taper to the existing floor. But it is a pain, and I'm tired and frustrated, especially when I think I'm done, and then the diagonal isn't flat.


So if someone could tell me please, what will happen if it's not as flat as it's supposed to be? I seriously want to know the answer. Because frankly, I hate being told I need to do something that causes me this much stress without at least knowing the why (and don't say because it voids the warranty- I want the real reason).
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Old 09-28-2015, 08:07 PM   #2
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Hollow places under the wood will allow it to flex--eventually causing fatigue to the tongue and groove---

I have cheated the 'rules' a bit---but I did have a heck of a time fixing one hollow many years back--it was right in a main walk way and the customer found it irritating.
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Old 09-28-2015, 08:13 PM   #3
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What's there now for a subfloor?
How old is this house?
Just a few reasons.
Humps in the floor will show.
Going to end up with a squecking, gap filled floor.
Guess you've never tried to assemble this type flooring on a non flat floor before, not fun.
What are you using to see if it's flat?
How far off is it?
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Old 09-28-2015, 08:33 PM   #4
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Thank you for your replies- it is helpful.

I've got 3/4" plywood subfloor- the house was built in 1967. Sadly, they built it on the cheap, which has been a bit of a frustration. The joists are 4' apart, and it appears that the subfloor dips at the joist in the middle of the room.

I have been struggling with this- but I've used 2 methods for checking flatness, the string held tight against the floor, and a 6' piece of bamboo flooring on edge, which is about the most trustworthy thing I've got for flatness.

At the joist in the middle, the dip appears to be about 1/2" lower than the edges, though the floor actually dips a little at the edge of the room leading to the hall, too, but not that much. The trouble is the dips don't run the length of the room, so the subfloor isn't flat in either direction. That's why when I've got it flat in one direction, it's suddenly off in the other.

I've got it now about as good as I think I can get it, but it does look a little Frankensteined together, with all these pieces of thinner plywood attached to the original. I just hope once I put the moisture barrier down, the floor seems flat (right now I've got the optical illusion where the places with extra plywood look higher).

But I do appreciate the answers- it is far more helpful to me to understand why I'm doing this.
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Old 09-28-2015, 09:15 PM   #5
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Quote:
Originally Posted by preciouschristy View Post
The joists are 4' apart,
are you SURE of this ?

go to the store and get a 12' straight board. pay anything for it, your just going to return it when done. 6' isn't nearly enough.
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Old 09-28-2015, 10:09 PM   #6
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Quote:
Originally Posted by preciouschristy
The joists are 4' apart,
Quote:
Originally Posted by Fix'n it
are you SURE of this ?
The house is certainly on the left coast, I'll bet. Probably Northern CA, Oregon or WA. That's why Mike and others ask; New members: Adding your location to your profile helps in many ways.

It would really help give better answers.

" out of plane is very noticeable, especially within 4-5 feet. I guess your method of using thin ply is fine when it's that bad. You can tweak the last bit with tarpaper.

Jaz
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Old 09-28-2015, 10:37 PM   #7
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Quote:
Originally Posted by JazMan View Post
The house is certainly on the left coast, I'll bet. Probably Northern CA, Oregon or WA. That's why Mike and others ask; New members: Adding your location to your profile helps in many ways.

It would really help give better answers.

" out of plane is very noticeable, especially within 4-5 feet. I guess your method of using thin ply is fine when it's that bad. You can tweak the last bit with tarpaper.

Jaz
Wow- dead on. San Jose, actually. Didn't realize there were that many differences between regions.

But yeah, we measured from the crawlspace, I was appalled that they were that far apart. I guess my biggest worry right now is the floor is so Frankensteined with extra pieces of plywood, even though it appears to be "flat" against a straightedge, or string, I just hope the subtle differences between the plywood shims and the original subfloor don't cause problems.

I kind of wish now that I'd just used leveling compound and done a floating floor. Though I've heard that nail-down is a stronger floor. I just don't want the floor to buckle.

And I do know that 6' straight boards aren't as good as 12', but that's why I used the string, too- At this point, I think I just have some small gaps to fill in, then put down tarpaper.

Thanks for all the advice
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Old 09-28-2015, 10:45 PM   #8
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Quote:
Originally Posted by preciouschristy View Post
Didn't realize there were that many differences between regions.


I kind of wish now that I'd just used leveling compound and done a floating floor. Though I've heard that nail-down is a stronger floor. I just don't want the floor to buckle.
you have no idea.

you do know that this flooring has weight to it ? do you thing those few joists are going to hold this weight, AND remain straight (as you made it ).
or is the flooring to be part of the structure ?
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Old 09-28-2015, 10:46 PM   #9
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Your installation instructions probably tell you to install tarpaper under the flooring, as a vapor retarder My mention of tarpaper to tweak has to do with making the floor flat where needed. Obviously adding tarpaper on the entire floor isn't gonna change the flatness of the floor.

Care to share the exact model of your wood selection?

Jaz
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Old 09-29-2015, 12:26 AM   #10
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Joist 4' apart..?

As soon as you walk on it it's gonna sag.
Step off of it and it goes back.
Can't level something like that.

Got a few pictures.
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Old 09-29-2015, 01:18 AM   #11
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So I guess I'm going to have to reinforce the joists before going further. I was hoping to avoid that, since I hate going into the crawlspace.
The installation instructions say if the joists are that far apart, I need to add another layer of plywood to the subfloor. But that would add more weight, and would raise the level of the floor so it would be higher than the floor in the adjoining room. So it seems like adding joists perpendicular to the originals is the answer.

The brand is California Classics, 1/2" engineered Carlsbad Beach.
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Old 09-29-2015, 01:50 PM   #12
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Adding another layer of plywood is not going to help you.

What is the unsupported span of your joist.?

What size joist.?

We already know the spacing.
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Old 09-29-2015, 04:57 PM   #13
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Actually that style of construction is very common on the left coast and will support ceramic tiles just fine. But, the framing materials are not the same as we use in the rest of the country.

The joists are usually 4x4 or 4x6", not 2x8 or 2x10", and they span a much shorter distance. And where we use " subfloor, this system uses 2x6" or 2x8" laid flat with " ply as the top layer, sometimes more.

There's no written spec for this system that I know of and so our usual questions concerning spacing and span do not apply.

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Old 09-29-2015, 05:22 PM   #14
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Hmm. I'm not sure if there's anything but the 3/4" plywood over the joists, I'll have to look when I get home tonight. I believe the joists are 2x8's, and they are pretty long. Can't remember specifically, it's been a few weeks since I measured them, but I believe they span 12'. I thought the plywood was attached directly to the joists, but if you think there might be another layer of construction, I should find out. This project has become something of a nightmare, and I really just want it solved, so I can get the flooring put down and move on with my life.
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Old 09-29-2015, 08:19 PM   #15
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Your latest info does not jive with the earlier info we had. There's no way a single " sheet can span 4' or anything close. We'll wait and see what you report.

It's a good idea to get under the floor and inspect, but you don't have to do that to measure the thickness of the subfloor system.

Jaz
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