tiling help needed -- thanks!

Greetings,
I need to know what my best underlayment option is for ceramic tile based on MY PERSONAL CRITERIA. Please help. I have tried to outline all the information below. If some going back and forth is required I will be glad to clarify or answer any questions you might have.
A special note:
The Tile Council of America doesn't have my personal criteria in mind. The Tile Council of America has emphasis on quality beyond economic sanity. I have to run my business as a business so I am seeking your help instead of just doing what the Tile Council of America tells me.
A made up example: <= this example is fictional
Tile Council of America would never suggest tiling once knowing it will crack in 10 years. Even if tiling over the cracked layer again will produce a lasting product. (The first layer of tile in addition to the underlayment will prevent the second layer from ever cracking.) If the money saved from the corner cutting in the first tiling invested for 10 years would more than pay for the second tiling then the Tile Council of America isn't making economic sense.
Economics:
If I am not looking for "the best" tile job what am I looking for? I am NOT LOOKING FOR THE CHEAPEST JOB. I am looking for the most economical tile job IN THE LONG RUN. For instance, I wouldn't want to install tile only to have to replace it in three years. I would also not want to pay an extra 10% to extend the life of the tile from 50 to 300 years because I'll be dead. If I can cut the cost of the job by $1000.00 but I'll have to spend $30.00 fixing popped tiles every year it is worth it. If I cut the cost by $1000.00 but I have to spend $250.00 replacing cracked and popped tile every year it is not.
The Goal:
I would like to tile the first floor of several rental properties to reduce future costs.
Facts:
The tile:
Type: Ceramic
Size: 6x6 and 12x12
Amount: roughly 5000 sq. ft.
Note: I already purchased the tile, in bulk. It cannot be returned.
The houses:
Brick Row homes
Built in late 1800's
Floor spans: 12-15 ft
Joist spacing: 16 inches on center
Joist size: 2x10 (some 2x8)
Basement present: Yes, on all
Current floor: painted and unpainted floor boards 1x6 and 1x4 (3/4" thick)
Labor costs:
Low
First question:
How do I determine the amount of weight the floor will safely support?
Second question:
What underlayment should I use? Below are several choices under consideration.
PLAN A
================ Use 1/2" wonderboard and thinset
Cost: $3300.00 + latex modifed thinset
PLAN B
================ Put down 15 lb felt and pour as thick a mud bed as the floor will support. If metal lath is absolutely required that would probably price the mud bed option out. If no metal lath is required this might be the cheapest option. A 1 part Portland, 6 part sand mud bed costs $60 per cubic yard. If I added $10 worth of fiber reinforcement per yard a 2 inch thick mud bed would still cost less than 45 cents per square foot or $2250.00 for the whole project. A 1 1/4 inch mud bed would cost about $1400. The problem is that I think it might be too heavy and I don't know if I could increase the strength of the floors for the money I would be saving over Wonderboard. I also don't know if fiber cement is going to allow me to not use steel reinforcement and lath.
PLAN C
================= Pour a thin layer of "high performance" mortar. If I could match the properties of Wonderboard somehow that would be ideal. Does anyone know what I need to do to make a similar product? How many parts sand / lime / cement? Is that fiberglass matting and how could I obtain it? Does Wonderboard use some special magical ingredient that I do not know about? At $430.00 per cubic yard Wonderboard is phenomenally expensive concrete. Sand is only $30 per cubic yard. Pure Portland cement is about $175 per cubic yard. That leaves a lot of extra money to purchase fiberglass matting and still come out ahead. I also like this option over Wonderboard because some rooms on t he first floor are 1/8th inch higher or lower than ajoining rooms and could easily be "made level" during the process.
PLAN D
================= Put down plywood / OSB over the floorboards and then tile directly onto the plywood. The cost, of course, depends on how thick a sheet of plywood used. I imagine the cost is probably slightly less than Wonderboard. Since I have reason to believe the floor may be exposed to moisture from time to time I would opt for Wonderboard
PLAN E
================= Put down 1 / 4 inch Luan and tile over. Cost, around $1720. This is the cheapest option other than a mud bed which might weigh too much for the floor.
A final note:
Please don't say: "do this, do that". Explain WHY and where possible use numbers.
Thank you all very much for your help,
William
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William,
Here is my opinion. And just for the record, I USED to lay tile and did so for just a couple years. Most of my tiling experience was new construction but did do a few remodels as well.
OK Here goes....
Situation: It doesn't sound to me like the floor/area to be tiled is too expansive and sounds like it should be fairly sturdy "Floor spans: 12-15 ft; Joist spacing: 16 inches on center; Joist size: 2x10 (some 2x8); Basement present: Yes, on all; current floor: painted and unpainted floor boards 1x6 and 1x4 (3/4" thick)". Though this is hard to say with any certainty just by reading a few specs.
Here is what I might consider:
Plan A: I have used wonderboard before to tile over subfloor and have used it on stairways that were somewhat rickety. I layed the wonderboard onto a spread of latex added thinset and then screwed it down. Used thinset with latex to set 12x12 tiles and in 4 years of frequent traffic, no cracks at all. I think this would be a good option because, though more expensive materials, your labor costs would be lower because I think it would take less time. (though this may not be the case depending on your experience and if you are working alone. A person can spread mortar pretty fast and with a helper, could make quick work of this type of installation.) http://www.ontariotile.com/cbuwonder.html
Plan B: Though 'mudding' does produce a pretty strong/hard surface, I'm not so sure you could get away with not using metal in it for stregnth. Also, I think this would take alot longer, therefore increasing costs of labor by having to prep, mix, haul, spread, screed, etc. This also requires a good bit of skill to do preoperly. I'm not sure if using fiber cement is enough to not use metal, like you said.
Plan C: I don;t know the 'chemical/or mix' properties of wonderboard but I do know that it does have a fiberglass mesh on both sides for stregnth. You pay more for convenience. Buy it, deliver it, install it. no mess, no mix, etc. As for the 1/8 difference in floor height. There are 2 thicknesses of wonderboard, 1/2 and 1/4 and you might be able lay wonderboard on a thicker spread of mortar where the floor is higher using a deeper trowel to spread.
Plan D: Be careful using OSB. OSB isn't made for wet applications and may separate with the wet thinset and therefore may not provide a good secure bond. If you suspect any moisture problems, I would not consider this at all.
Plan E: Just read the link......... http://www.c-cure.com/servref/techsupp/115.htm
Again, It is hard to know exactly what your up against but these are my opinions based on what I got from your descriptions.
I hope this helps.
Doug
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Greetings,
Thanks you VERY MUCH for your input. I was worried that my request might have been a little too "heavy" to elicit any worthwhile help. I am very thankful for your time.
No one seems to object to the idea of me making my own "high performance mud bed" (except it might be take extra time and prove difficult to produce a flat surface). Everyone seemed to champion the official Wonderboard product.
I did a little research and ended up purchasing 5.40 ounce/M2 6X6 mesh/inch white coated alkali resistant fiberglass mesh fabric. Based on my calculations I will be able to lay a 3/4 inch mudbed of 1:1:4 (portland/lime/sand) fiber reinforced mortar with fiberglass mesh embedded near the top and bottom for a cost of $1835 plus labor. This is $1332 less than the cost of Wonderboard. I am gambling the extra work will be less than $1332 in labor costs.
I now have two options:
1.. Attempt to manufacture 334 sheets of Wonderboard in my basement and then install as regular Wonderboard 2.. Embed the fiberglass mesh within a monolithic mud bed I would like to hire someone for $10/hour to make the Wonderboard in my basement. If they could average a sheet every five minutes the total labor cost would be about $280. The total savings would be $1052. This would allow me to wet cure the 11.57 yards of underlayment in my basement for a full 30 days to ensure full strength was achieved. Any shrinkage could occur before I installed the boards. This method would allow me to ensure their flatness. Before making the 334 sheets I will make a test board or two and allow it to cure for a week. This will allow me to compare it to the official Wonderboard to ensure it isn't obviously inferior. If the board is inferior (or seems labor intensive) I am only out the cost of the one or two boards (<$10).
Thanks again,
William
Note: I still don't know what ratio of Portland / Lime / Sand to use. The ratio above was only used for cost calculation reasons.
Legal Note: Wonderboard is obviously a trademark and I obviously cannot produce it any more than Cannon could make a Xerox machine.

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You are more ambitious than I would be. :-)
I'd be interested in hearing how it turns out, if the homemade sheets work ok. Pictures would even be cool also(if you're feeling that much more ambitious). I'm just curious to see how they are made and how it works out.
Good luck!
Doug
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William,
You can also use an addative to the grout and that can help prevent cracks as well. Your local tile distributor/shop should have different additives you can purchase.
Doug
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the right way to do it

the hard way to do it. much more labor intensive to get flat and get right.

another hard way to do it

wood is not to be used for laying tile unless you're going to use exterior, and even in that case, you won't get a good bond using thinset. the wood sucks the water out of the thinset too quickly and you get a crappy hold.

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