Question: Some of your recent articles ("Can Your Floors Pass This Test?," June 1981, page 512; "The Floor Tolerance Conundrum," August 1981, page 673; and "A Time Bomb in Floor Tolerances," November 1982, page 865) are making me a little more cautious on my estimates.

Realistically, tolerances of 1/8 inch in 10 feet appear to be hard to meet. Has anyone ever made a study of the relative costs of various tolerances on floor flatness?

Answer: We have recently come across some data compiled by a large floor contracting firm based on their own operations only.

The chart shown here, based on their data, shows how the deviation allowable under a 10-foot straightedge affects the cost of materials plus labor for a 6-inch floor.

The plotted points represent deviations the company considers acceptable for various kinds of floor use, and these points show the relative costs. (Since there are a number of possible ways being used to make tolerance measurements, it should be said that these data are based on the departures from a 10-foot straightedge supported onshims of equal thickness.) The cost of an abrasion-resistant floor for foot traffic only, with a tolerance of 0.5 inch in 10 feet, was taken as the standard cost. Relative costs of abrasion-resistant floors with lower tolerances are shown along the upper curve, and they increase progressively up to 150 percent for a floor for turret truck aisle traffic. The company found costs for all floors with ordinary, nonabrasion-resistant finishes to be lower (see lower curve). Percentage differences for all floors would have been greater, the company reports, if the costs shown had been for labor only. If such a plot is representative of most contractor's costs one could readily conclude that the all-too-commonly specified tolerance of 1/8 inch in 10 feet is well beyond many owners' price range, but that such a small tolerance is not often needed anyway.