Ok, this is for all the pro's out there. Please explain to me why it is that when ceiling joists run parallel to rafters, they must be tied in at every rafter, but if ceiling joists run perpendicular to rafters, rafter ties can be installed a minimum of every 4ft?? It doesn't add up? Why then couldn't you use ceiling joists every other rafter or every 3 ft if they run parallel??? Ceiling joists running perpendicular do nothing to stop wall thrust, so why would the code state rafter ties needed only every 4 ft??

Ceiling joists at 3 feet on centers or even 32 inches on centers would produce a very wavy ceiling.

Perpendicular rafter ties can be four feet on centers because the double top plate of the wall acts as a horizontal beam, which spans between the rafter ties. When rafters and ceiling joists are parallel, the rafter ties are required every rafter because you'd need the ceiling joist anyway, so why not tie the rafters as long as it's there?

That's what I thought Richard. So, what your saying is that a dbl 2x4 top plate acts as a beam to resist wall thrust in the space of 4 ft. Ceiling joists running parrallel with rafters *could* be spaced on 3 or 4 ft ctrs and prevent rafter thrust, but would not accomodate sheetrock or whatever very well.

Yup...even more so in this day of 2x6 walls with a double 2x6 top plate.

Hey Richard, what do you think???

I think the picture is way too big, and it"s screwing up the whole thread.

Why pairs of ceiling joists every second rafter? Same amount of lumber, but insufficient support for a gypsum-board ceiling.

Because I'm leaving the ceiling exposed and boxing them in as beams.

You'll have trouble getting enough insulation in that roof and still leaving an air space above, that is, assuming you'll have soffit vents and a ridge vent.

Yes, I know. Right now I have soffit vents/baffles/ and about R-25. It's not code, but it's such a small place, it stays plenty warm. My only concern was preventing rafter thrust. Do you think it will prevent the walls from spreading?? I still could put c.joists all the way across and put up a sheetrock ceiling, add more insulation, but it would only be a bit over 7 ft high. Unless I left the c.joist exposed and put the sheetrock on top of the joists instead of underneath??