We talk about OSB at construction school a lot. OSB is an engineered lumber, kind of like particle board, made from wood shavings bonded with wax and resin. It comes in 4×8 sheets and is commonly used for sheathing, subfloor and roofing.

For as much OSB as we use, you’d think we’d have a better handle on the name. Early on, some of the guys in class called it OBS. I think they were kidding, but I don’t know. I’m not sure we’ve ever used the whole name—oriented strand board—which is why it can be difficult to remember the order of the letters. OSB, OBS, BOS, SOB…whatever.

OSB comes in various thicknesses, but we tend to use 1/2″, which is what we were using today to sheath the exterior walls of a small structure we were building to practice cutting roof rafters. Our practice structure was supposed to be 6’ x 5’, with two 6’ walls connected by a third wall, so the whole thing looked like an “H”. That meant we built two walls at 6’, then built the connecting wall at 53”, so the thicknesses of the two longer walls (3.5” each) would make up the difference, so the combined width of the ‘H” would be 5’.

Anyway, I was feeling very confident as I drew up the cut list, and we banged that “H” together in no time, slapped OSB on the exterior, then launched into measuring and cutting the rafters.

Cutting rafters is where Pythagorus meets power tools. It’s not easy. So when we calculated and cut our rafters to span a 5’ structure, we were feeling pretty smug. Then we started tacking the rafters between the walls and the central ridge of the roof, and…what the what? They were too short to span our 60″ structure.

Then someone pulled a tape measure and found the walls were actually 61” outside to outside—an inch wider than we intended. No wonder the rafters were falling short. How could that be?

Our instructor just laughed, as if we were not the first people to make this mistake. We forgot to account for the thickness of the sheathing. When we slapped the 1/2” OSB on the outside walls, it made the whole structure an inch too wide.

That’s when I realized what OSB really stands for: “Oh, S#*t. Board!”