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Old 06-05-2007, 08:15 AM   #1
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Smile Structural Issue - Removing Plank Subfloor & Replacing with Plywood?

I'm planning a gut and replace on our main bathroom. Our house is a 40 year old bungalow. The subfloor is 3/4" T&G planks running diagonally across the joists. After reading a number of threads here I was planning on cutting out the planks and replacing with 3/4" T&G exterior grade plywood. I don't have the luxury of building up the existing subfloor as there would be too much of a lip when meeting the adjoining floor.

My question is - if I'm goign to be cutting out the existing subfloor and replacing it, will that cause any strucutral issues, specifically around support for the walls that surround the bathroom? ie. I'm not sure if the floor helps tie in and hold all of the walls together. I was planning on leaving 1/4" around the perimeter of the room for expansion purposes.

Any advice? Is my quesiton even clear?
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Old 06-05-2007, 08:43 AM   #2
Brian in San Diego
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Yes, your question is clear and the first part of your answer is no. The subfloor doesn't support the walls per se. The problem that you may experience and have to address is the lack of support for the edges of your new subfloor. You may have to do some blocking and make provisions for supporting those outer edges. Once that is done, you should be able to install your 3/4" exterior plywood.

It seems you've done some reading and I'm assuming you've run your joist dimensions through the deflecto. If so, (and you have enough structural support)then with the new subfloor you should be good for a ceramic installation. Stone requires two layers of plywood.

If that doesn't work, I'll always think it should have.
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Old 06-05-2007, 09:06 AM   #3
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Thanks - that's great info. I was planning on blocking the perimiter to ensure that all points of the subfloor were supported. I was concerned that removing the existing subfloor would somehow weaken the structure or lead to "flexing" of the walls.

I've used the Deflecto and am clear for installing ceramics. I'm actually putting down a porcelain tile (3/8" thick, not glazed) with a DITRA underlayment so I am confident I won't have any cracking there. I just didn't want to go ahead, replace the floor, tile everything, then have sagging walls or something!

Thanks for the advice.
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Old 06-05-2007, 04:37 PM   #4
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If your walls don't sit on a joist, you'll want to run blocking support underneath the wall, especially if the wall runs parallel with the joists.
Jim DeBruycker
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Old 06-11-2007, 07:43 PM   #5
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jadnashua - I think I get what you mean. If I'm cutting out the subfloor, and the wall is overhanging the joist, then I should block underneath the wall to support it, right?
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Old 06-11-2007, 07:59 PM   #6
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[quote]If I'm cutting out the subfloor, and the wall is overhanging the joist, then I should block underneath the wall to support it, right?[/QUOTE=AaronB]

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Old 06-11-2007, 08:11 PM   #7
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More often than not you can take out that old subfloor that runs under those NON-BEARING walls and nothing will happen. The walls are tied into the ceiling joists or the bottom chord of the roof trusses or raffters, they're tied into the walls they intersect, they are even held in place by the drywall or plaster that is on the wall on the other side from the bathroom.

I'm not saying that you shouldn't try to tie everything back together, but sometimes is next to impossible to get blocking into some or all of the space that you'll find under the wall you're describing.

One trick we use is to get the new subfloor in and fastened to the joists. Then we install Simpson framing angles along the bottom plate of the wall and tie it into the subfloor. This arrangement will support whatever slight sag the wall might have over time. If you are going to put backer board on the floor then use a router or a chisel to plow out a space for the bottom part of the angle. If going with a mud floor you don't have to worry about it.

If you can get some framing under this wall , by all means do it. But if it seems extremely difficult, post some pictures and we might be ab le to think of some options.
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Old 06-12-2007, 08:41 PM   #8
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I've been doing this in my house for the past year and let me impart some advice and similar to what Rob was saying.

After much trial and error, what I found worked best is to locate the nearest parallel joist to the interior/exterior you’re working with and only cut the existing sub-floor to that joist. You can either split the difference along the center line of the joist or sister a new along side it and attach the new sub-floor the either method. The reasoning for this is two fold
  1. Providing proper support - If you cut out the sub-floor where an interior wall lays parallel between two parallel joists, the now unsupported sub-floor will now sag and you'll have to support it which may involve jacking up the interior wall a bit, just to slip in the new joist materials. If the interior wall is load bearing or supporting some decent load, it will sag. Way too much effort and complex.
  2. Defined square - It's by far easier to work in a known defined square area, then trying to match up uneven dimensions. Trust me, you'll need these pieces to go in seamlessly.

In the image below, you can see how I cut out the existing T&G sub-floor. The old existing floor remained on the left side of the room and I installed a sistered joist to make the new sub-floor install easier. I'll also tack down the old T&G with 8d ring sank nails to secure them.

Also, you'll need to note that 3/4" from 40 years ago isn't the same as it today, you'll be 1/32" higher on the old floor than the new floor. Nothing but a belt sander can't take of in a few seconds


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Old 06-15-2007, 02:41 PM   #9
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Mike - you're the man! That's an awesome idea, and I will certainly be using that approach. Makes a lot more sense, looks easier, and looks like it will do a better job of supporting the walls (no need for the blocking now).

Thanks a million for the idea!
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Old 02-21-2008, 05:37 PM   #10
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mike does the sister member on the left run the whole span? or are you bolted where access was available?


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