Adding Additional Tile to a Floor Truss system
I am looking at a tile job where the HO wants to add new tile over an existing tile floor in their kitchen. The current floor tile is set on a bed of 1 1/2″ material (it looks like concrete but could be, thinset, slc, or gypcrete). Instead of tearing out all of the tile and setting bed down to the subfloor, the HO and I would like to go over the existing tile with Ditra and set our new tile. My concern is the additional weight on the substructure. The underlying floor system consists of 3/4″ osb subfloor set on 12″ deep open web floor trusses (2×4) spaced 24″oc. The trusses span 15′ under the tiled area where the tile will be set. I am trying to find an engineer to inspect it, but would like to be able to give the HO an idea of whether or not we may be able to proceed. Anyone have any thoughts?
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Floor Trusses
The depth of the current trusses seem a littel shallow to me, but I'm not a truss designer.
Take picture of the trusses and measurements to a truss plant or local lumber yard that commonly orders trusses for builders. Thier truss guy can give you a better evaluation better than a bunch of us guessing for you. Your 15' span is not bad for a 12" deep truss, but the fact that it may be gypcrete suggest you may be near the upper end of the design load for them.
There isn't any easy way to figure out how much loading the floor trusses were designed for. You could go to a truss plant and ask for help, but they'll most likely just refer you to an engineer.
Offhand I don't like the thought of adding all that height and weight to a floor system. Seems to me you'd be better off to tear it out and do it right.
tile and floortrusses
12" depth with 15' span is not out of the ordinary.
What I would be concerned is the spacing. The most we span any trusses would be 19.2".
I do not know what lumber was used for the trusses, but even with topgrade your floor design is pushing the limit. On top of it you have OSB sheathing which is not the greatest to prevent deflection
My recommendation is to contact an engineer to verify the capability of the proposed work