How much to (and how to) reduce moisture under slab before repairing drywall

Question

Two weekends ago I opened up the drywall and replaced the valve on a bathtub. After opening the wall I was able to see that the plumber who installed the tub also apparently had trouble installing drains correctly.

Here is the top view looking down into the opening in the slab where the drain shoe is not connected to the trap:

And here is a side view:

Apparently the tub has been draining into the ground under the bathtub since it was installed around 7 years ago with the top of the trap probably acting as a siphon only when the hole in the slab was sufficiently full.

I've since replaced the drain and everything seems watertight now. A week later, the musty smell of stagnant water is gone but the soil is still quite damp.

So my question: how dry should I aim to get the dirt before closing up the hole in the drywall that I've been working through? And how to do in the tight confines under the bathtub?

I am concerned that once I close the wall up, any moisture will have no way to escape, so I'm also wondering if I should treat the area with something to disinfect and discourage any mold growth before sealing it up.

Answer 1

Before closing that opening, grab a blower, direct the blower at the opening, plug it in and turn it on.

Run the blower at high speed. Watch everything dry and all the smell gone.

Leave the blower on for as long as you like. The soil and everything will be bone dry.

You take care.

Answer 2

It's hard to tell wether the dirt will ever dry up. If it were a solid, intact and sealed slab you could probably close it right away.

Since you are observing continued moisture around the drain and under the tub I too would proceed cautiously.

In a situation like this I would install a vent cover in the wall or under the tub. A simple grill would suffice: they are easily installed and easily removed for future inspection.

Ideally it would be installed such that you would still have access and view into the critical moist area. You could have it vent into this bathroom, but also into a different room, such as a neighbouring closet or hallway. A bedroom would not be suitable, since the vent would pass drain noise and other bathroom noise.

Then check the moisture from time to time, and once it is reliably dry for a prolonged period you could proceed to close it up with drywall or a different solid but still removable panel such as a plastic access door. These can also be reinforced or lined to reduce noise.