Bill, thanks for the reply.. Attached is a photo of the exterior wall. The tub lip will be near the top edge of the lower piece of blue insulation. With the window, the additional furring around it, and the cross furring there is not that much rigid insulation. The plywood on the exterior wall was to take the place of furring strips to move the durarock out to fully overhang the tub. This would also give me more to screw the durarock to. As a byproduct, I hoped to gain a bit more insulation. Is the concern more the plywood (as opposed to furring strips which would allow more air movement), or is it the rigid insulation trapping moisture, or the combination? Would plywood be ok if there was no rigid insulation? From other posts, it was recommended that if using fiberglass with kraft facing, to cut the kraft paper face every foot. This would allow moisture to escape. Since the rigid insulation is not wider than a foot I thought it would allow sufficient moisture migration. As an alternative, I could just peel 1 inch of unfaced fiberglass insulation and place it on the exterior wall instead of the rigid insulation. I did this in the paneled basement that has a similar exterior wall (and it helped). Or if the concern is more the plywood, I could put furring strips in. Yes the existing furring strips were not very level. I replaced 2 (both sides of window), and added another just above the tub lip (for screws). Then with the plywood, shims and screws planned to make the surface more level for the durarock. From your post, I interpret that you may also be suggesting durarock directly on the existing furring strips (with vapor barrier). Thus no plywood or additional furring strips to move the durarock out farther and fully over the tub lip. Floating the wall might be easier, but I have my experience with thinset and tile so hoped to stick with that. For the 1.5 2 inches on the ends, I was going to again use plywood to furr the wall out. I planned to split the difference and make up some of the gap on each end. However, based on your suggestion I now plan to sister the 2x4 on the back wall and put most of the 1.5-2 inches there. Attached is a photo of the floor for completeness. I added another joist (2nd out from wall) between a steel I-beam and a wall that sits on a concrete floor below. The new joist was cross braced to the existing joist along the exterior wall. It was placed to sit under the left set of feet on the tub. The position of the tub feet are marked with white paper in the photo. I then used joist hangers to support the joist to be cut (3rd out from wall). I placed the cross support to sit almost under the right front tub foot. I then cross braced to support the tub left back foot. So all the tub feet will sit directly over a structural member. The tub feet are referenced from someone sitting in the tub facing the drain.

The building code for 1 & 2 family dwellings is mostly prescriptive which means it actually sets minimum and maximum values and sizes structural members for a given location in the house. This is not intended to be used as a design guide but to place limits on what you might design yourself. Using the code as a design guide can result in deficiencies but that is not the fault of the code writers. The joist size tables do not consider long span bounce. The wall systems described are the ones most commonly used but the code does not intend to forbid the use of others. The waterproofing requirements are not extensive or adequate although the addition of the requirement to install materials according to the manufacturer's instructions was intended to fill that gap. Concrete foundations and slabs should be designed to ACI 332 since the IRC does not consider many common detailing problems. Placing pipes in an outside wall is a design problem that may be addressed by some codes but there is no reason to prohibit that condition. If there were to be a restriction it would have to be based on a map or list of locations similar to the requirements for wall vapor retarders. You can base your decision about the location of pipes on anecdotal information and other people's opinions or you can design the wall to avoid freezing of the pipe. If you choose the latter, you can use the formula normally used to locate the dew point in a wall in order to determine where the temperature would be 32 degrees or lower. Ignoring air infiltration for a moment, a water supply pipe 1 1/2" deep in a 2x4 stud wall and insulated with 3 1/2" of fiberglass insulation would probably freeze at about -10 degrees F. Ironically, removing 1 1/2" of the insulation on the warm side would lower the outside temperature needed to freeze the pipe to about -50 degrees but for fiberglass insulation air-infiltrations makes that prediction unreliable. So the design solution is to air seal the stud cavity and place R-10 or greater insulation on the cold side of the pipe. This is best done with spray foam (Icynene, etc.) and if the studs are 2x6's, the R value would be as much as R-14 which would put the temperature of the pipe at about 55 degrees when the outside air temperature is -30 degrees F. At that outside temperature the house temperature would need to drop below 45 degrees for the pipe to freeze. This design is roughly equivalent to a double 2x4 stud wall with no insulation in the interior wall and 3 1/2" foam or dense-pack cellulose insulation in the outer wall. The way it was done in my house back in 1903, was to put a "double wall" of wood lath and rough plaster at the middle of the full 2x4 stud cavity throughout the house and at the shower pipe the space behind the pipe was filled with vermiculite plaster. This pipe has never frozen but because of the risk of air infiltration I don't count on protection below -20 degrees. Remember that these calculations assume that the interior temperature is 68 degrees or higher, so if you set the thermostat at 55 degrees or the bathroom is under heated or you close the shower door you should adjust the calculation.

Drain pipes can be relocated. A plumber can help discuss your specific issues on site. Could be easy to break a little concrete and move it over, depending...

Ummm I'm a contractor too. The "perfect center" that people think they get out the Franklin is no better than the "center" that you get when looking for the screws. And contrary to popular belief, you don't need to screw your cabinets or whatever into the dead center of the stud. You've got 1.5" of width to work with. Unless your screw is threaded 1/2 in and 1/2 out into the stud, you've got plenty of strength. As long as the screw screws in tight then you're in the stud and the screw is just as strong as if it was in the exact center. I have been doing this a long time and have never had anything fall off the wall because I was off of the "perfect" center that you think you get with these supposed "edge" detectors.