concrete filler around subfloor?

Posted by Dave on September 14, 2007, 7:50 pm

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

Posted by marson on September 14, 2007, 10:25 pm

> 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.

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.

> 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?

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

> 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?

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.

> Any reason for NOT using construction screws instead of nails to hold down
> the plywood?

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

> Thanks,
> Dave

Posted by Dave on September 15, 2007, 2:16 pm

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

Posted by RicodJour on September 16, 2007, 2:13 pm

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.

> Any reason for NOT using construction screws instead of nails to hold down
> the plywood?

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

Posted by Dave on September 17, 2007, 11:11 pm

RicodJour:

> No reason to leave more
> than a half inch regardless of how complicated the cut is.

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.

> patching compound (in this case, mortar) to level the floor for
> whatever was the original floor covering (linoleum?

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

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