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Conventional Lumber Diaphragms

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I have a project for an addition to an old house and garage.  New 
construction is to match the old, or old style to the extent possible.

1.  Sections 2315.3.1 and 2315.2 of the '97 UBC give instructions for 
construction of diaphragms of diagonal 1x lumber.  Table 23-II-G specifies 
span-width ratios for diagonal-board-sheathed horizontal and vertical 
diaphragms.  I can't find allowable values.  Am I overlooking a Table or 
Section of the Code?  

Appendix Chapter 1 on the UCBC has allowable values for horizontal diaphragms 
of wood diagonal sheathing with either roofing or hardwood flooring 
coverings, but this is a code for existing construction, not new, and does 
not include values for shear walls.

2.  Section 2320.12.9, Roof Sheathing, allows spaced roof sheathing for 
conventional construction.  Chapter 15 allows spaced board sheathing under 
tile and wood shingle roofs.  Why does re-roofing universally include plywood 
sheathing over existing spaced sheathing?  (At least in S.CA it does.)

Nels Roselund
Structural Engineer