Q: In our area, masons sometimes start laying block before concrete in the footing has set up. I've looked for a code provision that prohibits this but haven't found any. Do you know of a recommended practice, building code, or specification that says block should not be laid on green concrete?
A: We don't know of any document that specifically prohibits this practice. But common sense indicates that you wouldn't lay block until the footing could support the weight of the block without deforming. Otherwise it would be difficult to keep a straight mortar joint.
I would like to comment on the February 1992 question and answer (p. 136) regarding the timing for laying masonry units on a concrete footing. The practice of laying masonry units on a fresh, plastic concrete footing is commonly called "wet setting" or "first coursing. "In California and in most Western states, the Uniform Building Code (UBC) is the governing standard and there is a provision in Chapter 24, "Masonry," that states: "The initial bed joint (the mortar joint between the masonry units and their supporting members, such as footings) thickness shall not be less than 1/4 inch nor more than 1 inch. "Since the mortar is placed between units to provide bonding and shear strength rather than compressive strength, this code requirement serves to ensure proper bonding between first courses of masonry and footings, an area of high tensile and shear stress. If masonry units are "wet set" after the concrete footing has begun to harden, there is a good possibility that the masonry will not absorb adequate cement from the concrete to produce this needed bond. Therefore, in regions where the UBC governs, "first coursing" is not allowed. The practice of "wet setting" is, however, commonly used on federal reservations in this part of the country because these facilities are not bound by any specific building code.
James R. Thomson
Spring Valley, Calif.