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Unread 06-09-2008, 08:01 AM   #1
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Join Date: Jun 2008
Location: Toronto
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Appropriate base for porcelain floor tile

Hi Everyone,
A newbie here. I want to install 500 square feet of 13x13 porcelain floor tile in the kitchen, foyer, bath and laundry rooms. The sub is 5/8 ply, joists are 2x8 with 12 inch spacing and the span is 13 feet. I think the deflecto rating is L/326. There is vinyl currently and this will be removed.
Four contractors have bid on the job. Only one actually inspected the subfloor, joists and span and said the floor is not strong enough and must be reinforced by adding 1/2 inch Hardibacker to the subfloor. None of the others even asked about the subfloor or joists. Two recommended mesh and mortar directly to the subfloor, and the other said that Ditra could be added directly to the subfloor.
My original preference was for Ditra but I'm leaning towards the first solution because he actually made a recommendation based on his inspection (however, the price is much higher). Am I on the right track here? All advice would be greatly appreciated.
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Unread 06-09-2008, 08:19 AM   #2
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Location: SE Tennessee
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Welcome to the JB forums, Chris.

Your joists are currently not stiff enough to support a tile job, and no amount of subfloor layering can correct that. You'll have to shorten the unsupported span, or beef up the joists somehow (sistering, for instance).

The main purpose of cement board is to provide suitable substrate for tile. A secondary purpose is to add height. Cement board adds no significant strength to your floor.

The mortar and mesh sounds like what is referred to as Jersey mud job. Do a search on that term here. It's nothing you want.

Assuming you can get your joists up to snuff, another layer of ply with either Ditra or cement board will be fine.
Dan - a DIYer in SE Tennessee
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Unread 06-09-2008, 08:41 AM   #3
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Thank you for your reply. You have confirmed my worst suspicions. I will have to see what can be done to strengthen the joists.
Much appreciated.

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