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Old 04-06-2013, 11:13 AM   #1
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Attaching new sub-flooring over existing sub-flooring


I just purchased a manufactured home that uses cheap 3/4" particle board for its sub-flooring, and all the "wet" rooms have some sort of damage (some significant).

Instead of ripping out the entire 1,500 sf of sub-floor, I plan on overlaying new 3/4" OSB T&G over the existing sub-floor.

What is the best fastener to use, ring-shank nails, staples, screws, or a combination?

Should I use any adhesives?

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Old 04-06-2013, 02:15 PM   #2
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Attaching new sub-flooring over existing sub-flooring


Very bad plan. That partical borad is going to keep rotting and form mold.
I's also going to keep falling apart so all the new wood will be moving once it does.
All that old partical board needs to go if you want this to work.
You can rent a toe kick saw or buy a cheap on at Northern Tool or Harbour Freight to cut right along the walls.

Also just adding more wood will through off all your trim, door openings and any other floors it meets.

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Old 04-06-2013, 10:01 PM   #3
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Attaching new sub-flooring over existing sub-flooring


I'm going to add to everything said above with a little more information on why this is a bad idea.

Manufactured/Mobile Homes are built on frames and to be economical. This means the frames are built to handle a certain amount of weight. Yes, they get blocked under the frames to help the frame support the load, but they also have outriggers that go from the frame to the edges, and by adding 3/4 OSB to the mix overtop of those cheap saw dust sub floor you have your going to destroy the home.

The outriggers & frame can NOT handle this much weight and if it doesn't buckle it right away it WILL buckle it over time or if anyone ever tries to move the home to a new location. Do NOT overlay OSB over top of the original floors, instead cut out the damaged areas and lay in 3/4 OSB or plywood in it's place. Trust me, I've worked the MH industry for years and there is nothing like trying to pick up the ass of a mobile home that's buckled it's frame or outriggers because someone over loaded it in a diy project.

To give you an example are normal route of fixing one of these is to gut the entire home of all walls perpendicular to the frame, then bring in 2 12 x 12 wooden beams twice the length of the sagged section. Using 1 1/2 " I-Bolts we then loop them under the frame and secure them to the beams inside the home and slowly suck the frame back up. Your talking a lot of man hours here and the risk of collapsing the roof & exterior walls as you do this, once the frame is sucked up to where it's supposed to be we then have to sister new frame i-beams up to the old ones and weld it all together.

Now, if you want to go through all of that then go ahead and lay the osb overtop of the trashed floors, otherwise cut out the old and put in new the proper way. Also, be sure to remove all insulation from under the home in the affected area and replace it with new and use treated materials or composites in the wet areas to avoid this issue in the future.

Sorry for such a long post, but I think it's best if you know the true risks of what your contemplating doing.
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Old 04-06-2013, 10:27 PM   #4
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Attaching new sub-flooring over existing sub-flooring


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Originally Posted by joecaption View Post
Very bad plan. That partical borad is going to keep rotting and form mold.
I's also going to keep falling apart so all the new wood will be moving once it does.
All that old partical board needs to go if you want this to work.
You can rent a toe kick saw or buy a cheap on at Northern Tool or Harbour Freight to cut right along the walls.

Also just adding more wood will through off all your trim, door openings and any other floors it meets.
I should clarify that all damaged subflooring, to include any mold issues, would be replaced. But the remaining particle board subfloor is not acceptable to me. And since this subfloor is over 16" OC joists, I don't want to deal with cutting out the entire subfloor around the walls then trying to find ways of installing bracing where OSB sheet ends don't land on a joist, especially along the walls.

So think of this as "underpayment" for the finish flooring. Just very thick underlayment.

I am going to redo all trim anyways and all floor finishing so the only "additional" work would be shaving the door bottoms. And by doing the entire house I would ensure an even floor height everywhere.

Last edited by Augster; 04-07-2013 at 12:24 AM.
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Old 04-06-2013, 10:34 PM   #5
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Attaching new sub-flooring over existing sub-flooring


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Originally Posted by TheJerk View Post
Manufactured/Mobile Homes are built on frames and to be economical. This means the frames are built to handle a certain amount of weight. Yes, they get blocked under the frames to help the frame support the load, but they also have outriggers that go from the frame to the edges, and by adding 3/4 OSB to the mix overtop of those cheap saw dust sub floor you have your going to destroy the home.

The outriggers & frame can NOT handle this much weight and if it doesn't buckle it right away it WILL buckle it over time or if anyone ever tries to move the home to a new location. Do NOT overlay OSB over top of the original floors, instead cut out the damaged areas and lay in 3/4 OSB or plywood in it's place. Trust me, I've worked the MH industry for years and there is nothing like trying to pick up the ass of a mobile home that's buckled it's frame or outriggers because someone over loaded it in a diy project.
Now this does concerns me. Though I should again clarify that this home is on a "permanent" foundation with permanently attached garage (the entire park is this way as it is specifically a manufactured home park), so it will never be moved.

The issue remains: most of the subfloor (where it is carpeted; tiled areas are just fine, but I don't want tile except the kitchen, bathroom, and laundry room) flexes too much for my taste. Hence my desire to overlay OSB to alleviate this issue.

Last edited by Augster; 04-06-2013 at 10:44 PM.
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Old 04-06-2013, 10:39 PM   #6
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Attaching new sub-flooring over existing sub-flooring


Edited as you posted the above while I was posting, sorry about that.

While your in a park on a foundation, your still a manufactured home that was built as good as it could be while still being as economical as possible. Do NOT put the OSB over the sawdust particle board for the reasons stated by myself and others above. You still run the risk of bowing the frame and outer supports, though the permanent foundation does reduce it some.

In addition, the particle board you don't remove will continue to self destruct over time even if you remove all the rot/mold sections. This stuff is just crap, no other way to put it and it quickly turns to sawdust at the first sign of moisture.

Use the sistering of a 2 x 6 or 2 x 8 up to any joists where the osb/particle board are not matching up well and you'll be fine. But do not under any circumstances treat this like a stick built house and try to put that much more weight into it without removing the old first.

One other thing, your flex isn't coming out of the sawdust/particle board, but rather the frame. Putting 2 layers of sub flooring in this thick will not fix that issue, it's actually going to make it worse. I realize it doesn't make sense, but it's the way these homes are built. Your walls are 2 x 2's, not 2 x 4's as just an example, all of this is to maximize room, reduce cost and especially weight.

Last edited by TheJerk; 04-06-2013 at 10:51 PM.
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Old 04-06-2013, 10:53 PM   #7
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Attaching new sub-flooring over existing sub-flooring


Patching the 3/4" particle board with plywood or OSB won't work very well. The 3/4" particle board used in those trailers is actually 3/4" thick, but 3/4" OSB and/or plywood are not actually 3/4" thick.
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Old 04-06-2013, 11:09 PM   #8
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Attaching new sub-flooring over existing sub-flooring


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Patching the 3/4" particle board with plywood or OSB won't work very well. The 3/4" particle board used in those trailers is actually 3/4" thick, but 3/4" OSB and/or plywood are not actually 3/4" thick.
Yes, I understood this, which is why I was going to patch with particleboard then overlay everything with OSB.

But now it seems the "frames" of these homes is not designed to handle the additional weight of the extra 3/4" OSB. So it sounds like I'm stuck patching with cheap particle board unless I cut out the entire subfloor wall-to-wall and replace with OSB.

Although I'm still not convinced the "spongy" feeling that I get when stepping around the problematic floors is the joist flexing as well as the deflection is way too much and it "feels" like just the immediate area I'm stepping on is sinking, like it's between the joists. That's my impression at least, but I could be entirely wrong in my assessment.
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Old 04-06-2013, 11:15 PM   #9
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Attaching new sub-flooring over existing sub-flooring


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Your walls are 2 x 2's, not 2 x 4's as just an example, all of this is to maximize room, reduce cost and especially weight.
Is this just the exterior walls or both interior and exterior?

My house was built in 1991 and all walls are built at least 4.5" thick, as can be determined from window ledges, closet spaces, and door ways. It uses standard drywall as well, not the wood paneling of the 60's and 70's. So could it be that the information you are conveying is not applicable to my home?

Perhaps representative photos would better indicate how the house is constructed.
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Last edited by Augster; 04-07-2013 at 12:33 AM.
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