Keppie - what size tiles are you planning on using?

I'd start with the calculator below which will tell you whether the framing you have can properly support tile.

While you're there, search the tile forum there for assistance with your questions about how flat your floor has to be, the cement board and All that Jazz.

Here is a link that might be useful: John bridge deflection calculator

Keppie-- is it that the whole floor slopes in one direction, or is the floor up and down? I ask because so long as the floor is flat,, and slopes in one direction, a 1/2" drop is nothing to worry about. What I WOULD worry about is if your subfloor is 5/8" plywood, you'll need to use 1/2" cement board. Only if your subfloor is 3/4" will you be able to use the 1/4".

Thanks for the quick replies:

To swmf1977: tiles will be 20 X 20 porcelain

To bmorepanic: Thanks for the link to John Bridge deflection calculator. I'll work it tonight

To bill Vincent: Plywood subfloor is three quarters inch which is nailed to 2 x 8 Doug Fir joists , 16 in. OC, span is 12 feet. If you would be willing to look at my floor plan (http://www.flickr.com/photos/w7jpg/5011009456/ ) which shows the floor elevations using points that are typically corners of the base cabinet footprint.

As far as slope goes, the points C, D, E, and F are on the east wall. If we head west starting with point F (-0.1875) to point G (-0.500), the floor drops 0.3125 inches in six feet or 0.05 in. per foot which is about 3/64" per foot. Continuing west from point G, which is the end of the peninsula, to point N, an outside corner of the base cabinet against the west wall, the floor drops .0625" in four feet (almost level ) and drops another 0.125" in the 24" run to the wall. So the major drop is in the first six feet heading west from the east wall. Going North to South starting at point A (-0.250") on the North wall across the open middle of the "U" to the closest corner at the end of the peninsula, point H (-0.500"), the drop is a quarter of an inch over eight feet.

To net it out and to circle back to my main concern : In every reference I've read the rule that seems universal when installing base cabinets is to find the highest point on the (cabinet footprint) floor plan and level the cabinets to that point. In my floor plan, the highest point is point C (0.000) in the extreme north east corner of the room, behind the sink cabinet. The lowest is at the end of the peninsula (points H and G (-0.500). Is a half inch too much of a drop to shim up? The other factor is that the base cabinets will be covered with Silestone, beginning at points A/L on the north wall and ending at the end of the peninsula which includes the cabinet run on the east wall. My understanding is that the Silestone installers want dead n*ts level base cabinetry. What do the pro cabinet installers do with this one??
Looking for more comments, please.

Here is a link that might be useful: kitchen floor plan

After plugging in my kitchen floor data into the John Bridge deflection calculator, the result was that my floor would have too much deflection for tile.

To deal with the floor drop in the middle of the room and provide additional support to the floor, I installed a beam under four major joists in the center of the floor. Piers were plumbed and leveled. A 10 ton hydraulic jack was used to raise a beam placed under four joists 5/8 inch. Two support posts were cut 1/8 shy of the total distance beween the top surface of the respective pier and the bottom of the 4 X 6 beam. Once the posts were centered under the beam above and on the pier surface, the jack pressure was slowly released. The final result is a floor with much less deflection and is 1/2 inch higher in the middle of the floor which now matches the highest elevation point of the floor.