New construction, using 1 1/8″ Advantech subfloor panels. 2nd floor room that is the subject of this post is 13′ x 15′. Floor joists are 14″ I-joists, 16″ OC. The I-joists run across the 13′ dimension, i.e., the I-joists are about 13′-6″ long. The subfloor panels cannot “run long” on either end of the 13′ dimension.

Here’s where it gets weird. The decision to finish the 2nd floor room was made after one of the 15′ long walls was already up (it was needed early on for roof support). That wall is a regular 2×6 framed wall, one bottom plate, two top plates, etc., and it sits on top of  Advantech subfloor. However, on the side of this wall where the room is located, the subfloor panel only sticks out about 6″, and there’s no tongue or groove, i.e., the panel’s 8′ edge has been cut off on that side. So as it stands, I’ve got 12′ 6″ to cover. 

I’m trying to decide the best layout for the subfloor panels. Bottom line is I have a 13′ span that already has 6″ of subfloor along one wall. So given the 4′ dimension of the panels I’m working with, what’s the best layout? I can probably cut back the subfloor on the one side a few inches, but I can’t get it flush with the wall. Since the existing “strip” of subfloor in the room doesn’t have a tongue or groove, I can cut down one of the panels to allow the other side to be wider. For example, starting at the existing subfloor strip, I can rip a panel to be 2′ 6″ x 8′ and butt the ripped edge against the existing strip’s ripped edge, put in two more full panels, and rip the one against the opposite wall down to 2′ x 8′. Or, I could use three full panels and another 6″ strip on the opposite end, which doesn’t seem like the right thing to do. 

I guess the real question is… how narrow can a strip of Advantech 1 1/8″ subfloor running perpendicular to 16″ OC floor joists be before it becomes structurally deficient? I read the Advantech Installation document, and it doesn’t address this issue. FYI, whatever flooring we use won’t add to the subfloor strength/stiffness, and I will be putting blocks between the floor joists wherever there is a subfloor seam that isn’t T&G.

Thanks,
JL