Drywall – tearing out tub/shower surround

Since they're up, I agree adding blocking is a PITA.

However, if its an inside wall that you have access to the other side of, use a "BIG GAP" polyurethane foam. Drill a series of 1/4" holes every 12" down the middle of each joist bay. Spray a 3 sec burst in each. Wait 12 hr and try the flex, it should be quite firm now.

You may not want to do this near the plumbing valve, for replacement purposes. It won't hurt it, though.

That is pretty much it except waterproofing. The easiest way to do this is do your plastic**/backerboard first, then drywall, mud/tape/sand/, hit it with waterproof membrane***, then tile last.

** - I don't put plastic behind hardiboard unless I am afraid of water coming in from the other direction. With plastic if backer gets wet you are looking at longer dry times and the water most likely making its way down.

*** - You can use something like redgard. It paints on in about 20 mins and you do two coats. Its $50. Does a tub surround HAVE to have this. No. Would I suggest it. Hmmm. I don't know. This is determined by what is behind tub walls and materials. Hardiboard isn't going to let a small amount of water going through grout hit the other side. Some other brands of backer will (for instance durarock - which I would never use for showers but prefer for floors). If it is an outside wall, especially one that might be cold for a few months, I would be more apt to do the waterproofing. I just did a tub in my house a couple years ago and didn't. It is all inside walls and would have been overkill. (Old tile was on drywall that was in pristine condition after 50 years)

Another tip is to use the .5" hardiboard for tub. You will need to buy 4 sheets for a typical tub. They come in 3X5 sheets. You will simply stand each one up for each end. So backer will extend outside of the tub and 5 feet above it. You will then lie the back piece horizontally, and then cut your fourth at 2 feet to be even with the others. This advice is given you have standard American 5 foot tub. If you don't just make sure backer extends a few inches outside tub and goes about 5 feet higher than tub. For tiling I always recommend trying to tile past backer board so you have seamless transitions.