A BIT O’ BACKGROUND INFO:

a) House built in 1960, single-story, raised foundation, southern California, 2×4″ stud walls, plank & beam subfloor.

b) Floor support structure: 4×6″ beams on 48″ centers, with posts & piers on 48″ centers along the lengths of the beams (piers are 8″ diameter concrete, poured in place but I don’t know how deep they go). House has withstood three earthquakes over 6.5 on the Richter scale with no damage, so I know it is well-built.

c) Subfloor planking is tongue & groove, actual measured thickness is 1.5″, width is 5″ (not incl tongue). Length of each plank appears to span at least three beams, maybe more.

PROJECT: Renovating a small bathroom. Removing the original floated-concrete, ceramic tile shower enclosure back to the studs, new hotmop pan, re-scratch-coat, re-float and re-tile. At the same time, we will tear out the room’s existing vinyl sheet-flooring and 0.25″ plywood sheathing down to the subfloor planks. We want to replace the floor with the same tile as is being used for the shower, and also run the same tile up the wall behind the sink, as a waterproof wainscot. Out of respect for the style of construction of this house and it’s performance for over 40 years, I want the tile installer to float the floor for the tile base at the same time he floats the shower enclosure. I would also like the new tile floor to be flush with the wooden floor in the bedroom into which the bathroom opens.

TWO QUESTIONS:

1) As long as I make sure the proposed dropped section is well-attached and well-supported, is there anything wrong with cutting out that section of subfloor planking and dropping it 1.5″ or so? I plan to cut the planks underneath the bathroom floor clean to the walls, screw 2×4″ cleats to the sides of the 4×6″ beams, drop the planks 1.5″, screw them down to the new cleats, then screw 0.75″ plywood to the lowered planks. In addition to screws, I’ll use the appropriate adhesive between layers for added strength/quietness. On top of the 0.75″ plywood goes the floated concrete, thinset and tile. Goal is to allow for vertical build-up of the concrete float, thinset and tile, so that the finished tile floor is level with the surrounding wood floor in the bedroom. I believe this technique would also stiffen the floor due to the added 0.75″ plywood atop the existing (but lowered) planking. I know I can support the soon-to-be-dropped flooring very well in that local area, but I’m a bit fearful that cutting the planks at all will somehow compromise the overall structural strength of the subfloor and therefore the house itself (remember, earthquake country).

2) I’d also like to drop the bottom of the shower pan so that it is a “step-down” shower, rather than having a raised curb to step over. I have the vertical room to drop the subfloor under the shower and make the pan 3.5″ deep, is that deep enough to meet code? I can add beams and posts/piers in the area underneath the shower pan if necessary for strength. It never made sense to me to put a water-containing pan sitting on top of the structural flooring anyway, which is a secondary reason I’d like to drop the pan below floor level. It should be the lowest point in the system so any leaks don’t run along the floor into adjoining rooms.

I would appreciate any insights and/or professional advice as to whether my plan is a good thing or a bad thing to do to this house.

Thank you,

Ray