Building Structures Illustrated - Patterns, Systems, and Design_下.pdf

ROOF STRUCTURES
As with floor structures, the nature of the
roofing material and the manner in which
it is laid to shed rainwater determines the
pattern of the secondary supports, which in
turn dictates the direction and spacing of
the primary spanning members of the roof
structure. Understanding these relationships
aids in developing the framing pattern for a roof
structure.
• Roofing shingles, tiles, or panels may require
either solid or spaced sheathing.
• Sheathing spans across the roof slope.
• Sheathing supports span down the roof
slope.
• Depth and spanning capability of sheathing
determines the spacing of its supports.
• The plicated method for framing
a sloped roof plane uses relatively small,
closely spaced rafters that span down the
slope and support solid or spaced sheathing.
Wood Rafter Span Ranges:
• 2x6 can span up to 10' ( m);
• 2x8 can span up to 14' ( m);
• 2x10 can span up to 16' ( m);
• 2x12 can span up to 22' ( m).
• Primary roof beams can span either across or
down the roof slope.
• Roof beams spanning down the roof slope
can support structural decking or panels.
• Depth and spanning capability of the
structural decking or panels determines the
spacing of the roof beams.
• Note that the spanning direction of the
roof beams is perpendicular to that of the
structural panels or decking.
• Depth and spanning capability of the purlins
determines the spacing of the roof beams.
VERTICAL DIMENSIONS / 191
ROOF STRUCTURES
There are alternative ways in which to frame steel
and timber roof structures, depending on the direction
and spacing of the roof beams, the elements used to
span the beam spacing, and the overall depth of the • Metal or cementitious roof decking
construction assembly. • Roof beams
Roof Beams Parallel with Slope
The steel or timber roof beams may be spaced 4' to
8' (1220 to 2440) . and spanned with steel or wood
decking. The beams may