design for 2 layers of asphalt shingles

huangyhg

design for 2 layers of asphalt shingles?
is there a code requirement that requires an asphalt shingle roof to be designed for 2 layers (original plus one overlay).  i work in california which still uses the 1997 ubc.
thanks
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i don't think there is a code requirement....it would simply be a decision that the owner and engineer would want to discuss up front.
there may not be a code requirement, but i use about 5 or 6 psf for shingles, anyway.  it seems to be a common belief that a roof can accept re-shingling once, but not twice.
daveatkins
i would check with the local building official.  i believe they limit the maximum number of layers of asphalt shingles by building type. you need to design for the maximum number of layers.
typically - only one re-shingle is allowed either by code or shingle manufacturer.  
you buy shingles by their weight - say 220 or 240 which means 220 or 240 lbs per square and a square is 100 sq ft.  theoretically - the heavier the longer they live......
therefore - a typical roof has about 2.5 lbs psf for one layer of shingles and add some for felt - like .5 psf
in roof truss design - 10 psf for dead loads if very typical and allows for two layers of shingles.
i have always felt that the 10 psf dead load for roofs is low.  some of the higher end composition roofs can run 400 pounds per square (4 psf).  when you add plywood (1.5 psf), insulation (0.5 psf), 5/8" gyp board (2.8 psf), mech & elect (0.5 psf), misc. (1.5 psf), trusses/girder trusses or 2x12 rafters/beams (3.0 psf) and the slope factor, i usually end up with 15 psf as a minimum design dead load for composition shingle roofs.  that is for one layer of shingles.
i remember reading in one of the codes (can't re  
kramer -
i tend to agree in that i was talkling about what most truss guys use - 10 psf dead top chord (shingles, plywood, etc.) and 10 psf dead on bottom chord (drywall, insulation, lights, etc.)unless otherwise specified.
if i was doing simple rafters - i would use something in the 20 psf range.
yes, irc only permits two layers of asphalt shingles.
don phillips
with a gable wood truss roof, i use 10 psf top chord dl, and 7 psf bottom chord dl.  these numbers are used by all of the truss designers around here.
daveatkins