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A 60+ year old house, on pier and beam, appears to need a floor replacement in the bath room. The tub has not been moved since being installed in 1951. There are some soft places in the floor. How is the best way to do this?
Without seeing exactly what is going on, it will be impossible for us to tell you how to go about fixing it.
If the damaged boards can be terminated and replaced over existing structural support, then that is the best option. If there is no support where cuts will be made, then you will have to install supporting structure to carry the repaired area.
What I normally do in pier and beam construction is to use joist hangers and 2x6 to create a new support structure where needed.
There have been very few instances where I've had to fully remove a tub to repair damaged floor/subfloor, even if the damage extends under the tub. You do repairs like this by resupporting the tub OR by replacing the damaged materials a little bit at a time so that the tub is never without bearing.
The easiest way to replace the substructure or add more supporting members will be from the top, through the bathroom floor, rather than blindly doing it from underneath. Start by creating an access hole through the bathroom floor to the crawlspace. Then you will be able to pass materials, tools, and lighting through the floor rather than making a long and arduous crawl under the house.
It would not an arduous crawl as the entry to crawl space is where the tub is. The last time I looked, someone had just stuck plywood under the tub with small nails attaching it to the sub floor. I think the subfloor is 2x6 or 8.
the current tile is vinyl.
Pull up the floor and create the access as I suggested. You'll thank me when you get knee deep into this project.
If plywood was tacked to the bottom of the subfloor, all that is doing is covering up damage, it's not doing anything to help repair or support the damage in any way.
To be quite honest, the repair process sounds far more difficult that it actually is. You start by pulling the toilet, then the finished floor all the way to the sub floor. Now that you can see the extent of the damage, you start removing it. Stay within the confines of the bathroom until you are able to get underneath and assess the full extent of the damage, then strategically work removing and resupporting things as you go.
What I would recommend is that you place new support along the edge of the tub and replace the damage under the tub first. From there you're working completely from the top. Continue filling in the hole, then lay your underlayment, float the seams with leveling compound, and you're ready to either lay vinyl tile or sheet vinyl. If you want to install tile, then you will need to be certain that the structure can support the new tile and weight.
Let me just add this to what Spruce said:
The fact that you have soft spots tells us that you need to beef up the supports, before you start with the new underlayment. Also, don't assume that you're OK with the beefed up supporters, if you want tile. Consult a professional floor man for that.
The John Bridge Tile Forum has a handy dandy deflecto-meter you can use to determine if the floor joists are good for tiling. The resident engineers and experienced home builders can tell you what you need to do to make it right.