I wouldn't sweat the diaphragm ratio limit for something this small no matter what the code says. Despite appearances, structural engineering is still very much a game dominated by judgment and reasonableness in my opinion. If a cranky code reviewer calls you out on this, I suppose that you could just install a discrete, steel strap bracing system under the roof deck to make it a truss rather than a deck diaphragm. If your walls are concrete or block coming up from below, you might also be able to just make those cantilever.

Thanks for quick reply. The plan checker actually calls me out for the diaphragm ratio limit. My building is wood-framed building. I will add a bracing system under the roof deck as you have suggested. Thanks.

You're welcome. I guess 4' is pretty darn narrow.

If it's wood, I suppose that you could prefab or site build a flat truss to lay horizontal under or over the framing. It won't be working too hard.

Another thought. You're roof deck could easily be constructed with very few plywood sheathing joints (as few as two). If you placed framing members beneath those joints and supplied chords/flanges etc, I feel that you could claim that your roof beam is really a plywood webbed, custom beam rather than a diaphragm as conceived of in the usual sense. I couldn't say if this would fly with your code reviewer or not.

I've done KootK's alternate method and provided a horizontal plywood box beam. The AHJ in our area was leery, but ultimately accepted it.

Do you actually have an AHJ that checks stuff in Manitoba? If so, I'm jealous. In Alberta, we have no such thing.

Depends on the day.

All right, thanks guys! I think I need to call the plan checker up and present him the box beam idea. If that doesn't work, I will just tell the architect to widening the penthouse.