Q.: We are a structural engineering firm specializing in the design of residential light-frame housing in Southern California. We specify both conventional and post-tensioned slabs on grade, but the slabs often develop some cracks. In a typical 30x50-foot slab, what is considered to be a tolerable width for a shrinkage crack?

A.: Tolerable crack width is related more to slab performance and aesthetics than to slab size. To some building owners, any crack is too wide. At the other extreme, the Home Owners Warranty Program at one time allowed a maximum permissible crack width of 0.188 inch for basement floors. That's a pretty wide crack by anyone's standards.

Crack widths exceeding 0.035 to 0.04 inch are generally believed to make aggregate interlock ineffective in transferring load across the crack. To minimize potential corrosion of reinforcing steel, crack widths may be limited to 0.015 inch.