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Im replacing the subfloor in a 2nd floor bathroom of a 1920s house. It was level and in decent shape, but covered with adhesive and backer board for tile so replacing it seemed the easier option.

After ripping up the old plywood subfloor, I've discovered many of the floor joists are not flat on top; they are damaged along the long the edges such that only the middle half inch of 1 1/2" (nominally 2") on the top of the joists remain intact and flush to where the subfloor sat. I assume these joists were damaged en the previous contractor ripped up the original subfloor.

The joists are 16" on center and have some horizontal 1x6s between them. Picture below.

The plan is to put fresh plywood subfloor, underlayment and then tile on top.

Should I do as the previous contractor did and simply lay the new subfloor on top or would it be better to sister some boards to the joists to give more surface for the new subfloor to sit on? If so, is it safe to remove the horizontal 1x6s? I see some cross blocking in other areas.

Thanks!

enter image description here

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  • Depending on the tile, it might require more than an inch of sub floor on top of the joists. The joists also need to be sized for the open span. About an inch per foot plus maybe a couple of feet.
    – crip659
    Jan 13 at 21:01
  • 1920 joists/most wood should be close to actual size, a 2x8 should measure almost 2x8 where today's 2x8 will be a 1/2 inch short.
    – crip659
    Jan 13 at 22:12
  • Thanks! Does the width at the top of the joist matter? It may be tough to see from the image, but there is some damage to the top of the joists that create a taper such that there's only about a 1/2" that would be in contact with the subfloor. Jan 13 at 22:40
  • It is better the longer the support, but good solid joists should not matter. Tiles like no bonce on the floor and flat floors(being level is nice but not important). Check the sub floor thickness your tiles require. Nail/screw into the support section, angle screws if close to the edge.
    – crip659
    Jan 13 at 22:48

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Additional surface area for bearing is unnecessary. The worst softwood for bearing capacity is Hem-Fir at 405 psi--worst among the 4 softwoods you'll find in the US being sold at corporate hardware stores, anyway. The APA advises 360 psi bearing capacity for plywood. Both of those sound comforting even with your reduced bearing area (1/2" bearing width implies a demand of (10psf+40psf)(1ft)[(16/12)ft] / [(0.5in)(12in)] = 11 psi). The extra width from sistering might be nice where plywood panels end, though.

Assuming a specific gravity of 0.45 for the 1x6s (0.45 is about right for the common softwoods, where it goes as high as 0.55 for Southern Pine), I get (0.45)(62.4#/ft^3)(0.75")(1ft/12") = 1.8 psf for their weight. If you're worried about too much weight and an extra 2 psf bugs you, then remove the stuff. Or maybe they'll be a source of squeaks under the floor? Only reasons I got, as I don't expect that they provide much stiffness in comparison to your hypothetical sheathing plus underlayment.

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