Floor structure is in question

oldhousetimeFebruary 1, 2010

Well, I am in a dilemma. Bought an old House, 1905, gutted the 2nd floor bathroom and would like to put a cast Iron tub in. The tub with water will weight about 500lbs. When ripping out the bathroom, the subfloor , which was diagonally placed 2 x 6s that went from one room to the other, have been taken out. The floor joists have large holes from old plumbing plus holes for new plumbing. I am worried that the diagonal subfloor boards did more then act as just sub flooring, but distributed weight to a larger surface making less load in the bathroom...

Can I sister the joists and call it a day or should I think about something more invasive.

This problem has been keeping us from moving forward.

Thank you in advance.


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No one can tell you what to do without seeing the situation but if joists are properly sistered they should be as strong as the original structure. Of course, you can replace the subfloor with 3/4 plywood which should be stronger than the original sub floor.

    Bookmark   February 1, 2010 at 9:07PM
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3/4 in plywood is not even near the strength of diagonal 2x6 sub flooring.

Even 1x6 diagonal is stronger than 3/4 in plywood.

    Bookmark   February 2, 2010 at 4:02PM
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The plywood, installed properly, would act in compression along the top of the joist where the 1x6's would not. If replacing 2x6's then 1 1/2" of plywood would be even stronger.

Plywood would also be stronger as a subfloor because it would be the same thickness but span a shorter distance not that it would necessarily matter in this case.

    Bookmark   February 2, 2010 at 6:48PM
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Only half the plies in perpendicular layed plywood are providing any real strength.

Solid wood acts just fine in compression and tension, with its full cross section doing work.

The reason everyone switched to plywood is for the reduced labor of installation of 4x8 sheets, not strength.

    Bookmark   February 3, 2010 at 9:46AM
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I think the OP is just concerned that the floor is solid, not a debate of wood vs plywood.

Plywood is a perfectly acceptable subfloor and will provide all the strength you need as long as the underlying framing is solid. You should check the spacing on the floor joists though.

    Bookmark   February 3, 2010 at 10:21AM
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