concrete filler around subfloor?

A toilet in an unused bathroom sprung a leak that went unnoticed until carpet outside the door got wet. I've taken up the old tile, linoleum and plywood. Under the plywood are diagonal boards (sub-subfloor?) over the crawl space. After drying the room for a month I filled the termite-eaten boards around the toilet with Bondo, and now I'm ready to lay down new 3/4" ACX plywood. I'm new at this and I'm hoping someone will be generous enough to answer a few questions.
The previous plywood had large gaps between it and the walls. Any reason for this? I cut the new plywood for a tight fit.
There are gaps between the ends of diagonal boards that come together on a floor joist. The gaps were filled with what looks like portland cement (?). Wondering why filling 1/2" over a beam was important when some of the boards are spaced wider than that. Much of the cement crumbled out when removing the plywood. Should it be replaced?
The space between the old plywood and the walls was filled with the same cement. I asked someone at the local lumber yard about this but he didn't know what I was talking about - said I should just use some kind of waterproof caulking around the edges. Good advice?
Any reason for NOT using construction screws instead of nails to hold down the plywood?
Thanks, Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The large gaps are not necessary. It would be prudent to leave some gap though--1/8" or so should be fine. Too tight and the plywood might buckle if it gets wet.

Since plywood can float over a 14 1/2" space between joists--it can certainly span 1/2" spaces just fine.

Not really sure why you should use waterproof caulking around the subfloor. It would be good to caulk down your base trim to your finished floor however.

Screws are fine. Nails are used because they are faster.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks, marson. Were the property to be purchased this house would be a tear-down, but I still want to do the job right.
Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Previous workers didn't think it mattered. No reason to leave more than a half inch regardless of how complicated the cut is. Google "tick sticking" - might be one word - for an easy foolproof method for laying out complicated sheet good cuts.

That sounds to me like the remnants of the last incarnation of the bathroom. It's possibly the remains of the mud job floor, or floor patching compound (in this case, mortar) to level the floor for whatever was the original floor covering (linoleum? depends on the age of the house). No need to replace it - the new plywood subfloor will bridge any little gaps.

Not really. If you're going to waterproof a bathroom, that's not the way to do it. Google Schluter (might have two T's) products if you will be tiling the bathroom.

Generally I counsel gluing the subfloor with either nails or screws, but in the case of an older house and a bathroom adhesives are a pain in the ass to a remodeler. Use screws.
R
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RicodJour:

There are a couple of places where the new plywood touches the 1x1 boards that are at the bottom of the lath & plaster, but mostly 1/16-1/8" gap. Covered entire bath with one 4x8 - one pc 48x52", another 36x43". Haven't screwed em down yet, but thinking more trimming is unnecessary.

The floor I pulled looked like 5/8" plywood, linoleum, and vinyl tile. It looks like a few of the diagonal floor boards were cut out and replaced in front of the shower, so I suspect none of that was the original floor. I was intending to lay vinyl tile on the plywood, but now I'm thinking linoleum as it could all be one piece. There must be some kind of brush-on prep for the plywood surface.
Thanks much for all the information.
Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.