So, I was in Oakland (well, Richmond) California for a week, working on my nieces place. It's a ranch style home, built as one of four models in a subdivision.

One of the tasks that she had outlined for me was to install porcelain tile in the two bathrooms and in the entryway. When we were lining this project out, I was stationary and happened to watch the floor when Andy (the husband) stepped on it. It flexed a good quarter of an inch.

That's pretty much incompatible behavior with tile, so I went down into the crawl space under the house for a look and found that there were no floor joists. Instead, the builders ran 4x6 beams the length of the building on 4' (four foot) centers. Adjustable supports were installed every eight feet or so, which rested on concrete pads. 1 1/8" plywood was installed over the beams. Probably tongue-and-groove, probably nailed down.

Clearly, this technique works well enough to have lasted since the 1970's.

I ended up adding joists under the bathrooms and entry, and improved the interim supports as well. Those three areas exhibit no flex and are ready for tile.

Anyone else ever encounter this kind of approach? I've seen it done with 2x6 T&G lumber over 6x6 beams, but only for roofs or lofts, never the main living space.