I'm rebuilding kitchen 15X13 platform on slab. Joists are 2X6, 12 inches on center laid on flat 2X4 strips. While all the joist read level one or two of them seem a bit high, perhaps 3/16ths from the next. There will be 3/4 inch t/g plywood screwed and glued over them, and 1/2 inch durarock over that, then tile will be laid. What is the tolorable vertical variance between joists?

Three sixteenths is a lot. If I read your description correctly, the joists aren't really joists, since they are not spanning any distance. Therefore, there's no reason why you should not plane the high ones to reduce the differences. Otherwise, a bump in the joists will be a bump in the floor.

Look at the grade stamps on the lumber. They should all say either S-DRY or S-GRN. Those marks mean "surfaced dry" or "surfaced green", and they MAY reflect a difference in moisture content in the lumber. If some say one or the other, then the shrinkage rates could be different, and your high spots, if you plane the joists, might turn out to be low spots. If that's the case, I'd change a few joists so they are all the same. Doesn't much matter in terms of one or the other; it matters more that all are the same.

This message has been edited. Last edited by: Richard Hetzel,

The joists mentioned above are 15 feet long and there are 13 of them spaced 12 inches on center. I'm in the process of shimming each so as to match the next. Based on a laser measurement accross the joists there appears to be less than an eight of an inch cummulative accross the 13 foot cross section. Your comments now would be appreciated.

A variation of +/- 1/8" between joists made of dimentional lumber is considered to be "right on", since, as Richard Hetzel pointed out, these come from trees, and may vary in moisture content. Also, the "crown" of the joist, which is how much the board deviates from dead straight, when tipped up on it's edge and sighted from one end to the other, should always be up, so that the bow, if there is any, will tend to flatten out as drying, shrinkage, and settlement occur.

Many contractors have gone to using man-made I-joists, to get away from the often gross variation between joists out of the same pile, since the wood is coming from second, third, or (?) growth trees.