Roof over Roof again

[Examiner]

Read the other one posted but what is your opinion of this one.

Existing Church with a semi-vaulted ceiling. The existing finished ceiling creates an attic space below the original roof. There does not appear to be any attic ventilation in the existing system. Plans call for new roof several feet above the old roof without taking the old roof off.

I question allowing the new roof over the old period. Not to mention how do you vent both attic spaces if the old one is left in place? Talk about cutting holes in the existing roof for venting. The building is Type II-B built under the old SBC. The roof is wood decking both existing and proposed. The framing will be steel.

Any good code sections I can use about new roof constructed over old. Oh, the building does not have a sprinkler system. Now possible draftstopping issues????

Architect and Structural Engineer are on the project. This roof over an existing roof is puzzling to me.

 

[texasbo]

I think that attic ventilation provisions would definitely apply, and they would apply to *both* attics. I also think at the very least you could apply 717, Concealed Spaces, firestopping/draftstopping. In this case, draftstopping would be required to separate attic spaces into no more than 3000 sf. I don't think there is a code section that would actually allow you to prohibit the "roof over a roof".

 

[ggmarch]

i've done this several times and on churches. there's no need to vent the existing roof, it becomes like a sloped floor (and we don't vent floors). if there is tile or metal, there's probably no venting as it is for that one since there's no reason for it. if there is, make sure they block off the original venting or any insulation you put above ther roof is an exercise in futility. the new roof structure just can't be higher than the max building height and you're good since now they can insulate this new attic space without opening a huge plaster/drywall ceiling to the elements. i probablly wouldn't consider the space above an attic, but would lay r30+ insulation on the old roof and allow above that roof to vent through the soffits to the ridge. there are many benefits to their approach. there's essentially no new load added that the structure can't already handle, save the interior, insulate the crap out of it, new straight level framing, and a new roof soffits downspouts, and unaffected use of the building while under construction. totally worth the money! i did a really cool one where we had a 30,000 sf factory that wanted to expand so we built a 75, 000 sq ft building over it then dis-assembled the old one from the inside. they never shut their door for even one day 3 shifts per day. they were happy.