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Replacing 1920s sub floor


MTowers's Avatar
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Join Date: Jun 2013
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CANADA

06-05-13, 11:29 AM   #1  
Replacing 1920s sub floor

Hi there,

I'm looking to replace the old plank sub floor in our 2nd floor bathroom. I'm pretty sure the existing floor is t&g pine planks, they run perpendicular to the 2x8 16" oc joists and the last time the bathroom was redone (1980s?) they glued a thin layer of plywood overtop and finished it with peel and stick tiles. I'm feeling like they need to come out and be replaced because they are pretty chopped up from previous plumbing cuts and we plan on changing the layout pretty significantly.

I was planning on tackling it like this;
Posted By: another forum 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 youre 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
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.
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

Unless anyone has a better idea. The only problem I see is that it might be tough to sister the joist for the whole span without cutting some bricks (double wythe solid masonry construction). Anything else you can think to look out for?

 
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joecaption1's Avatar
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06-05-13, 05:30 PM   #2  
You need a Toe Kick saw so you can cut out the old flooring tight to the walls.
Use a Sawsall or ossilating saw to cut into the corners.
I only use Advantec subflooring, never plywood. Far stronger and more moisture resistant.
You need to use constrution adhesive on top of the joist.

 
MTowers's Avatar
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06-05-13, 07:32 PM   #3  
If I cut the existing subfloor right to the wall on the interior side what do I attach that edge of the subfloor to? There is a joist just over a foot out from the wall, but if I cut it all the way back it will hang over nothing.

The exterior wall is less of a problem because there is a joist right beside it so there won't be any overhang in the subfloor.

 
Nashkat1's Avatar
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06-05-13, 08:14 PM   #4  
If I cut the existing subfloor right to the wall on the interior side what do I attach that edge of the subfloor to? There is a joist just over a foot out from the wall, but if I cut it all the way back it will hang over nothing.
You could add cross-blocking if the wall wouldn't sag while you did it. Sure.

I would cut the old flooring right down the middle of that closest joist.

 
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