kkes Tile Floors September 14th, 2018 - 05:49:33
There are formulas used in the industry to determine if the subfloor has excessive `deflection` [bounciness. lack of rigidity]. The most cited one is the Tile Council of North America standard for deflection. which is stated as L/360 as a minimum. before tile underlayment is installed. L/360 means that the floor should not bend under weight more than the length (expressed in inches) of the unsupported span divided by 360. For example. if the span between supports runs for 20 feet then the deflection should not be more than 2/3" between the center and the end. L=20 x 12" = 240". L/360 = 240"/360 or 2/3". So 2/3" is the maximum amount of movement the center of the span should be allowed to move.
If a subfloor displays excessive deflection. it can usually be remedied by installing more plywood on top of it before tile is laid. and by reinforcing the joists from below. While it may make the floor higher than before. think of it as a sort of `insurance policy` against flooring failure.
Can you cut into the layers on top to get a cross section of the existing floor? If there is a heating grate that you can remove. that may show the layers the floor is composed of. What will be reassuring to see is a thick layer. ideally over 1 ½ inches thick of plywood. Alternatively. with the property owner`s permission. we sometimes cut in to it to check what it`s composed of.
Is there an unfinished ceiling below to look up and measure the distance between joists and the condition of the wood below and how long the unsupported span is? A few minutes in the basement with a flashlight and tape measure can let you know if you have a winner (thick and deep joists. spaced closely together. in good condition. with a narrow span). or a loser (thin and shallow joists. irregularly spaced or spaced far apart. in bad condition. with a long span).