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I would like to know the maximum weight per square foot for an existing roof. The pitch is 12/12, the rafter length is 18'. There is an upstairs room with a four foot stub wall that provides additional weight bearing capability. My roof currently has two layers of 240 lb asphalt shingles. I would like to cover this with 26 ga metal without removing the old roof. What is your opinion? When I calculate the breaking force and the sliding force, I don't see a problem.
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That is one approach however the building code has restrictions on go overs. The loading depends on the area you live as you could have snow loads or wind loads to consider. These are termed as "Live" loads. Typically they design for 5 lbs/sf of roofing dead load and you are there and the shingles should come off.
Other considerations are "why did the other two roofs fail"? I suspect that by your description you have a combination of lower attic space behind the knee wall and above the ceiling with vaulted ceiling in between. This creates a severe ventilation problem and you should look to address this with the new roof as it will help keep your home warmer/cooler and reduce energy costs. For vaulted roofs, it requires a minimum of 1.5" of vented vertical free air space.
12/14/2005
Or copy this URL: https://www.metalroofing.com/spirit/comment/7224/find/Dura-Loc Roofing Systems, Inc.
12/15/2005
Or copy this URL: https://www.metalroofing.com/spirit/comment/7225/find/