Diagonal Floor Sheathing
There really isn't any damage in this picture, I just
want it to show what diagonal floor sheeting looks like.
Diagonal floor sheeting is when 1 x 6 lumber is used in random lengths and ran at a 45° angle. One of the biggest problems with these floors, is loose knots and cracks. I have seen knots the size of your fist missing in areas where a small child could easily step through.
Over time as the 1 x 6 dries out, it can crack or split in areas where the lumber looked fine during installation but with a little time, it can create safety problems in your floor and roofs.
Home Repair Tip: If your home has 1 x 6 floor sheeting, there's a good chance that it had a wood floor on top of it. If the wood floor is removed for some reason and you choose to install carpeting or would like to reinforce the floor sheeting for other reasons, you can install new plywood on top of the diagonal sheathing.
I would recommend a minimum of half-inch plywood to reinforce your flooring. Smaller plywood if wet over time can de-laminate or buckle, easier than larger plywood's.
I used quarter inch plywood, one time and eventually it needed to be replaced. It didn't delaminate or come apart but it did buckle and eventually became too difficult to walk on.