Q: Can you please solve a conflict I’ve been having with a few of my carpenter buddies? We all disagree about the purpose of the little diamonds that fall at 19-3/16-in. intervals on a tape measure. What are they for? - A.J. Cain, Vassalboro, ME
A: The diamonds fall at 19.2-in. intervals, and they are reference marks for laying out I-joists or trusses that are engineered to be spaced 19.2 in. OC (to support floors). If you follow the diamonds, you'll see that the fifth one is at the 8-ft. mark, meaning an 8-ft. sheet of floor decking would be supported by five members spaced 19.2 in. OC. This contrasts with six members spaced 16 in. OC or four members spaced 24 in. OC.
The 19.2-in. spacing may seem to be a compromise between the strength of 16 in. OC and the economy of 24 in. OC, but it should be used only with floor support systems. Unlike floors, walls and roofs contain many components (insulation, fixture-hanging hardware, baffles, vents, etc.) that are designed for installation only in standard stud cavities. - HANDY