Floor, ceiling joist and roof rafters size and spacing are a direct function of span.
If you have an approved set of plans, the size of those elements have been called out and approved for your building, by the local building authority.

However, if you want the answer to this question;"What would be the cost differential to specify 16 inches?", you should ask your builder, as he will be purchasing the extra material and providing the extra labor and will be due additional profit.

24 inch on-center is fine if the rafters are sized properly. I did a 625 SF room addition spaced 24 inch on-center with 2x10 rafters so I could get R-30 insulation into them and have a cathedral ceiling.

Thanks. That is food for thought.

We are working on drawings now and I realize That the 24 inches OC is within code. However, this does not necessarily mean it is the best way to go. There are parts of the code that many professionals would consider a bare minimum practice at best.

I am an old timer and cannot get used to thinking in terms of less than 16 inches OC. A 7/16 inch OSB sheathing would be quite springy with a 24 inch span, and there is less framing to withstand stresses, such as a huricane. And occassionally someone will have to walk up there. The longest span is about 13-14 feet.

The pitch is 6-12 and the ceiling is a traditional flat ceiling (no cathedral ceiling). The framing will probably be a combination of on-sight built trusses and stick built. My question is how much strength am I really giving up? and should I go to a thicker sheathing, or use plywood instead of OSB.

Thanks.

Site-built trusses? Are the plans for them stamped and/or approved either by a structural engineer or your local inspector?

7/16ths OSB is not rated to span 24" on a roof unless it has additional edge support. That may be satisfied by T&G, by "H" clips, or by blocking. "H" clips are normally used.

Do make sure that the OSB is properly installed. I've seen WAY too many roofs where the sheet edges have swollen from moisture, and the outlines of the 4'x 8' panels telegraph through the shingles. Looks pitiful.

Advantech is a step up. Or ply.

Mongo

Part of this, is being smarter then what you're working on.

"The longest span is about 13-14 feet."
"Span", is the distance between structural supports.
"Run", is the length of the rafter, measured at the center of the width, from the ridge or plumb cut to the apex of the seat cut or birds mouth.
"Carpenters Truss", consists of 2 opposing rafters and a horizontal framing member, fastened together at each end with 5, 16d's and additionaly attached to the top plate.

Here are a couple of links;
http://www.homefocus.com/410/framing_square_know-how.htm
http://www.tpub.com/content/construction/14044/css/14044_59.htm

personally I would not let him use the osb rule of thumb that I was told is 1/2 for walls and roof that are 16oc 5/8 with clips for roofs that are 24 oc
I also think this is code in most places
As for the 24 oc rafters sized right they will be fine. That said I would build it 16 oc but I have been known to over build things. Better over built and strong than just barley code and bouncy

Compromise at 19.2. That should please everyone. OSB will like it better too.
Casey

The 7/16 OSB should have a 24/16 marking on it. The 24 denotes the span for roof sheathing & the 16 denotes the span for subflooring. So 24" span for 7/16" roof sheathing applications is approved by most residential codes.

The 2002 IRC code does not require H-clips for 24" spans. Maybe some local jurisdictions require that. I do agree they should be used but again not typically required by codes which obviously are minimums.