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RE: Conventional Lumber Diaphragms

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Good question on part 1. I would be interested in the reason for that too.
In regard to part 2 I believe it has to do with the required fire rating of
the roof. Most jurisdictions in California require minimum class C rating on
roof assemblies. If I am not mistaken, to obtain class C rating you need
solid sheathing below the roof tile or shake.

	-----Original Message-----
	From:	NRoselund(--nospam--at) [SMTP:NRoselund(--nospam--at)]
	Sent:	Thursday, September 16, 1999 5:47 PM
	To:	seaint(--nospam--at)
	Subject:	Conventional Lumber Diaphragms

	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

	1.  Sections 2315.3.1 and 2315.2 of the '97 UBC give instructions
	construction of diaphragms of diagonal 1x lumber.  Table 23-II-G
	span-width ratios for diagonal-board-sheathed horizontal and
	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
	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
	conventional construction.  Chapter 15 allows spaced board sheathing
	tile and wood shingle roofs.  Why does re-roofing universally
include plywood 
	sheathing over existing spaced sheathing?  (At least in S.CA it

	Nels Roselund
	Structural Engineer