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Q: I’m remodeling a bathroom that will need a thick mud floor for a flat, level tile base. It’s in an old house and I reinforced the sagging floor with sistered joists and solid bridging. But I’m worried about adding the weight of a traditional mud floor. Could I just lighten the mud mixture with vermiculite and sand?

A: Michael Byrne, a veteran tile installer and the moderator of JLC’s Ceramic Tile online forum, responds: Because your floor construction is not standard, you need to measure both concentrated and uniform deflection before attempting a tile installation. The measurement for concentrated deflection is taken from the subflooring mid-way between two neighboring joists, while uniform deflection is measured from the bottom of an exposed joist located more or less in the center of the whole floor. If there’s no access to the joists from below, uniform deflection can be measured from the subflooring, directly above a joist located near the center of the whole floor.

Concentrated deflection should not exceed L/360 of the distance (span) between two adjacent joists; uniform deflection shouldn’t exceed L/360 of the longest span of the whole floor that can be measured. If the deflection is less than L/360 of the span (whole floor or between joists), you may safely float a mesh-reinforced mortar bed, according to ANSI A108 specifications for tile.

In the past, I’ve rigged up a dial indicator for the most precise measurement. But now I use a collimated green laser whose target beam diameter is a bit over 1/32 inch (1 mm) at 50 feet. For “ballpark” guesses, I use a red beam emitted by a laser pointer or laser measuring device. For your floor, I suggest making at least two or three measurements of uniform deflection and a half-dozen or so of concentrated deflection

I never recommend using vermiculite, because vermiculite mortar has repeatedly failed in compression tests and is not recommended by the tile industry. And, while you may not mind cracked tiles, I’m sure you would be bothered by respiratory diseases caused by the asbestos found in some stocks of vermiculite.

If your renovated floor isn’t level, you can use a self-leveling underlayment such as Levellite, made by Custom Building Products. Designed for use with tile, it is 40% lighter than conventional mortar mixes and can be 2 inches thick. The company also makes a lightweight tile thinset called Megalite.