An engineered I-joist has a large hole cut through its top flange. Why is this more serious than a hole in a solid joist?

An engineered I-joist is an efficient shape precisely because it puts material only where the forces are. Bending produces compression along the top and tension along the bottom, and the flanges — narrow strips of solid or laminated lumber — carry nearly all of it. The thin OSB web between them carries shear and holds the flanges apart at a fixed distance. Cutting the top flange therefore severs the compression member of the assembly at that point, which is categorically different from boring a hole in a solid joist, where the material is continuous and a properly placed hole in the middle third removes lightly stressed fibre. Manufacturers permit holes in the web within published size and location limits and prohibit any cut, notch or hole in a flange, which is why this finding is reported and referred rather than measured against solid-lumber rules.

The flanges carry almost all the bending force, with the web resisting shear and holding them apart. Cutting a flange removes the member doing the structural work, whereas a solid joist has continuous material and tolerates correctly placed holes.

Engineered I-joists concentrate bending resistance in the flanges and shear resistance in the web. Manufacturers publish hole charts permitting web penetrations within limits and prohibit any notching, cutting or drilling of flanges.

Flanges Bend, Web Shears. Holes belong in the web and never in the flange — the flange is the part doing the lifting.

Do not apply solid-lumber notching and boring rules to engineered lumber. The exam tests whether you know they are governed by the manufacturer's tables instead.

• •Applying solid-joist hole rules to engineered lumber
• •Treating a flange cut and a web hole as equivalent
• •Assuming a cut I-joist can only be replaced