The pipe to be soldered is about 1" down from the underneath of
plywood flooring and 1 inch from the top side of the joist.
I thought of bending a 12 x 12 sheet metal 90 degrees in the center.
Drilling 2 holes on the end of the bend that goes under the flooring.
Another hole near the top crease. Screw the top side to the floor
letting it hang down about 1/2" AIR GAP. Leave A 1/2" air
gap to the side of the joist
Before sweating the pipe I will apply water to all the exposed
wood to saturation.
Then, I will use paste flux and normal propane torch aiming the torch down
from the top and applying the solder around both sides of the repair.
For additional safety I will have a bucket of water available nearby.

Do you think that is a workable plan?

Thanks
George

yes

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

It's possible, but I always try to solder these tricky joints ahead of time out in the open first then install the half made contraption with joints in easier spots, sometimes it's not possible.

I would also advise be careful of the heated air in a confined space, it rises up and travels sideways once it hits the subfloor, and if your head is at the same height, that super heated air can take out your eyebrows.



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From the angle I will be lying on my back
with my head slightly below the sweat joint about
one joist over. The hardest part may be the effort
to bend the solder so that I can apply it 360 degrees
around the two joints.

it is not allowed on potable water but soldering one little joint with 50/50 lead bearing solder will be alot easier than 95/5 solder.
less heat needed and it flows easier.

I have a roll of aluminum flashing that I cut a piece off of and use as a heat shield as necessary. Watch your flame and the metal will keep the wood from burning. If the joint has been properly cleaned and fluxed, and once the joint is properly heated, the solder will "run" around the joint, so you don't need to physically apply solder at every point on the circumference of the pipe.
Also, pay attention to what's on the floor and under the joint you are soldering (besides you). Hot flux can liquefy and drip and hot solder may drip as well. In my crawlspace I have heavy poly sheeting on the dirt that was put down as part of radon mitigation. I use a heavy canvas drop cloth on the floor to keep a drop of hot solder from igniting the plastic.

I hate soldering in the crawl with low head room. Last time running my PEX had the PEX in one hand crawling forward with the other, head lamp on my head with dripping sweat running down my eyes and a tool box tied to my ankle as I drag it along. While holding the torch if you feel a spider on the back of your neck do not react.