Adding Tile To Existing Floor Trusses
We are planning to add 1/2 in. cement board and 3/8 in. quarry tile to a kitchen floor. We need to know if the floor trusses will handle the additional weight. The floor trusses are 19.2 in. O.C. and the loading numbers are 40-10-0-5. What do these numbers mean?
The numbers 40-10-0-5 refer to the design loads for the truss in pounds per square foot (psf). There are two sets of numbers, one set for the top chord of the truss and one set for bottom chord of the truss.
• The top chord live load is 40 psf; this is standard loading for residential floors. Live loads account for loads from people, furniture, moveable objects, etc.
• The top chord dead load is 10 psf, which accounts for things like the weight of flooring materials and part of the weight of the truss.
• The bottom chord live load is usually zero, as it is in this case.
• The bottom chord dead load is 5 psf, which accounts for dead loads applied to the bottom chord like gypsum board, insulation, ductwork, lighting and part of the weight of the truss.
By adding cement board and tile to the floor, you will be increasing the top chord dead load. Tile loads are usually higher than 10 psf, so chances are your trusses were not designed to account for this. A couple other considerations are the on-center (O.C.) spacing of the trusses. Most sources say that sub floor for tile should not span more than 16 in. O.C. to reduce deflection of the sub floor and minimize cracking of the tile. So, there are many things to consider here: What is the span of the truss? How far is this extra load from bearing? What deflection criteria was the truss designed for – l/360? l/480 or higher? You might install the tile and find that it is problem free; on the other hand, it might not be. It's difficult to make recommendations without seeing the assembly.
You should determine the load from the tile and the cement board (and all the other materials on the floor). The manufacturers of each product should have that information. Then you should call the truss manufacturer and ask them to do an analysis of the truss with the additional loading. They may be able to suggest some options for this installation if you choose to proceed.