07cbc 704A.1.2 vs 1507.3.9

due to location we need to comply with both sections. not being a roofer, does one layer of 72 lb mineral surface cap sheet (per ASTM D3909) provide the same protection as two layers of #15 Type I organic felt paper (ASTM D226)?

to get the two layers #15, they get lapped so that you really do get two layers. but i get a feeling that going with two layers of 72 pound cap sheet is going to be a problem.
Original Post
Brent,
The code sections you referr to are for tile roofing application with an exterior wildfire exposure and valley flashing. Since most tile roof applications require battens with weep notches, the 72lb roll lap seams can be positioned in the gap spaces between the battens if the batten to seam layout is figured out before installation. If used for valley flashing, the roll roofing should be one piece. If a single piece is not possible, joint lap should be per the manufacturer's listed instruction as 1507.6 does not directly address minimum lap dimension.
72lb roll is more than equivalent to 2 layers type I felt.
yes jim, you are correct about the application. thanks for the input. for the others, i'll clarify my question further.

in my case, for the mission clay roofing tile, we are using the seismic wires. so i would have to provide the 72 pound paper over the entire roof. so can i default to standard lap requirements, or do i really have to lap the paper so that two layers are provided?
Brent,
There are installation specs for roll roofing in 1507.5.3 that require underlayment, type I. Since roll roofing is not "paper" but a product suitable for a wheatherproof roof it must have it's own underlayment.
During the urban wildland interface committees I attended, the main reason for the mineral surfaced roll roof is to keep flying embers from readily ingiting the roof sheathing and underlying structure when embers get into the gap between tiles and deck. Roll roofing product standards require the roofing product to be tested and listed to at least class C for fire resistance, the fire class testing includes the "burning brand" set directly on the roofing surface.
So the overall situation is; 2 layers type I underlayment (typical 15lb felt 1/2 lapped resulting in double layer) then the 72lb mineral surfaced roll roofing over the underlayment with the roll roofing lapped per the manufacturer's instructions. The 72lb does not need 1/2 lap as it has it's own underlayment.
Likes (0)
×
×
×
×
×