subfloor for tiles?

Hi,
most places (literature) I looked, durock is recommended as a subfloor for floor tiles. I have currently 5/8" subfloor, and was gonna screw on 5/8" on top of it to get the recommended 1-1/4" for plywood subfloroor, but now i am not sure if I should skip the 2nd layer and just install duroc. The main reason to go with plywood is that I have about 5 sheets of it. The main reason for duroc, is that it is thinner, and wouldn't increase the floor height so much. Plywood is supposed to be flexing more, but if you secure it at recommended intervals with proper screws, is that still a problem? So my questions are: 1 - are both options acceptable? (1-1/4" plywood) or 5/8" plywood with duroc on top? 2 - is the durock instead of plywood going to make huge difference?
The area to cover is cc. 100sqft, - 28 sqft for tub island. Does the additional flooring need to go under the tub island, or can the island rest directly on the main subfloor?
Thanks.
m.
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Martin,
Keep in mind there are other backerboard products on the market today that offer height as well as density advantages over the old duroc stuff. I've used the Hardibacker products on both floor and a countertop on several occasions and am well pleased.
Your proposed 2 layers of 5/8 plywood (over a well constructed subfloor) is a sufficient and acceptable base, but with such a small area the cost for backer wouldn't be extremely expensive either.
As for the island cabinet, your underlayment and tile don't need to go underneath it as long as your toe space is adequate.
Martin wrote:

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Hi Martin:
I just finished ripping out the old tile floor in my bathroom. The builder used 2 layers of 5/8" plywood, just as you suggested. However, at every plywood seam, the tiles cracked because of seasonal expansion and contraction of the plywood. Because of this, I will never recommend installation of tile directly onto plywood.
I have replaced the upper layer of plywood in the bathroom with a layer of 1/2" Durock and a layer of 1/4" Hardibacker (it had to be at least 5/8" thick for cosmetic reasons). It feels like a concrete slab, now.
Three years ago, I replaced the hardwood floor in my laundry room with tile. I used 1/2" Durock on the 5/8" subfloor, and have had no issues whatsoever with the floor. This room sees heavy through traffic every day, and a heavy, vibrating washing machine.
Regards, John.
Martin wrote:

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