plywood layout question

[boberic00002]

I decided to replace the plank subfloor in the bathroom that I'm remodeling, but I unfortunately didn't decide to do tat until after I installed the tub and tiled the walls.

I'm looking to tile the floor and I didn't want to raise the floor up any more than it was originally. The original tile was tiny thin porcelain tiles set in a mud bed that was all told about 1" thick including the tiles. I selected a 12"x24" porcelain tile for the new floor which is substantially thicker so I don't think a mud bed is an option at this point. Reading a lot of other advice, the suggestion was that I'd need to put substantially more wood over the planking to prevent the movement from cracking new tile, so that's how I wound up ripping out the planking that was in place.

To meet up with the old plank flooring around the perimeter, I figured I have to do a bunch of blocking anyway so I just bought square edge 3/4" plywood sheets. My intention is to leave a 1/8" gap and screw the old planking to joists and blocking to minimize movement.

The original planking was 3/4" thick, about 7.5" wide, and run diagonally across the room, so I installed blocking between joists and cut it along joists. I've attached a pic of what it looks like now.

The area to get the new plywood is about 60" x 60". No matter how I install the new plywood, there will be a seam. As far as the orientation of the new plywood... would I
a) run the 4'x5' sheet with the long edge between the door and the tub, then run the 1'x5' piece under the vanity and toilet
b) run the 4'x5' sheet with the long edge between the two side walls, with a 1'x5' edge either near the doorway or near the tub running parallel to the tub
c) make two 2.5'x5' pieces and run the seam down the middle of the space

If there's a better idea than the above, I'm open to it. I'm aware I'll have to install more blocking and sistering for all edges of the new plywood to be well supported.

The tape measure may be difficult to read and the writing is upside down but the joist to the left of the closet bend is just about 4' from the joist at the right side wall.

[cx]

Welcome, Bob.

Regardless where you elect to put your seam, you must orient your plywood with the strength axis perpendicular to the joists. If your seam is properly supported, it doesn't really matter where it's located, but putting it where you anticipate the least traffic makes sense.

Some 5 or 6-inch rips of your plywood make very good blocking for the seam. I prefer to glue and screw them under the panel edges.

My opinion; worth price charged.

[jadnashua]

The long edges of the ply need to be supported, thus, the blocking underneath the sheets. T&G ply would have been easier, at least between the two partial sheets, but you still want to block the seam where the planks and ply meet.

[boberic00002]

CX:

When you wrote this: "Some 5 or 6-inch rips of your plywood make very good blocking for the seam. I prefer to glue and screw them under the panel edges."
I'm not sure I understand what you mean--laminate multiple strips together or just use each strip as blocking individually? Would I have to drill a pilot hole between the plywood subfloor and the plywood blocking or will a screw go into the 3/4" edge of plywood easily and hold well?
I have a bunch of 2x4 pieces leftover from another project so I was just going to use that for blocking as follows:

For between the joists, I intended to just cut pieces of 2x4 to fill the approx 14" long gap with the 1.5"x14" side up, diagonally screwed into the joist on each side (toe nailed but with screws).

For along the joists, my plan is to screw the 2x4 next to/parallel to the joist along the whole length of where the plywood edge would otherwise be unsupported. The 2x4 sistering the joist would be screwed to the joist every foot?

I intend to secure the 3/4" plywood to the joists and blocking with subfloor glue and 3" subfloor screws.

[cx]

Bob, the rips of plywood are to replace the T&G, which you do not have and usually do not want when installing in a cramped space such as you've got.

Just cut some of your plywood into 5" by 14" pieces. You don't want the ends to touch the joists.

Pre-drill the edge of each of each of your subfloor panels and inch or so from the edge. Install the first panel. Put some adhesive on half your plywood rip and put it under the edge of the installed panel and screw down into it through your pre-drilled holes. Put adhesive on your plywood blocking and install your second panel. Done. And better than having T&G.

You don't need anything close to 3" screws for your subfloor. You can get some 1 5/8ths" DeckMate screws at your local home center that sufficient to the task.

My opinion; worth price charged.