STRUCTURALLY SOUND FLOORS:
First of all, floors must meet a building inspectors minimum code, which is typically 2X10 jounces, 16" OC (on centers) and this also depends on the floor joists wood type, and span, with at least 3/4" (depending on the age of your home) plywood sub-floor. And this could be particle board, OSB or any grade of regular plywood. And that is only minimum code. Click here to see what Tim Carter from Ask The Builder had to say about minimum code. You also have to take into consideration, the length of the span involved. This might work OK if you are installing carpet, vinyl or even hardwood floors. But, if it's tile and it's a large room (say 15' span) and up stairs with no other support than the floor jounces themselves, it could very likely be a problem. There are ways to strengthen the floors after the home is completed. Such as sistering 2X10's to the existing floor jounces. If you have a crawl space, you could add 2X10" support beams perpendicular to the existing joists with support poles, at about $100/Ln. ft. This is the extreme way to go and all are expensive. Some more than others, and most home owners don't want to spend that kind of extra money to do this. The simplest and least expensive fix is to add plywood to the floor. The thickness would be up to the owner, but 1 1/4" is the recommended thickness to handle the tile load. It could be an additional 1/2" if this does not pose a problem with height. And if you use an exterior grade (EGP) plywood, and a good thin set with polymers, or use an additive, you can put tile right over the plywood. This is not recommend for anything other than exterior grade plywood. If you have cracked or cracking tiles and or grout, you need to address this problem first. I have to agree with Mike Holmes from the HGTV show "Holmes on Homes", when he says builders should use common since when building homes. And I say they should considering the extra weight of tiles, granite counter tops, jetted tubs, and tiled walk-in showers.
Here are some minimum codes for 2"X10" floor jounces:
The strongest wood - Douglas fir-larch SS - 16" OC = 19' 1" max span (go to 12" OC and gain 2 feet)
The cheapest wood - Spruce-pine-fir #3 - 16" OC = 13 feet max span (go to 12" OC and gain 2 feet)
Span Tables for Joists and Rafters