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MitchellScott 04-29-2009 12:37 PM

New Roof and Insulation over Existing Roof
Hi. I recently purchased a home built in 1950 in Raleigh, NC. I have removed the interior walls and ceiling to expose the beautiful tongue-and-groove roof boards that cover the gable structure. The 2x6 rafters below the decking are held in place by collar ties, and the entire load is distributed to the outside walls, which are constructed of 2x4s.

I would like advice for the best way to add insulation above the T&G deck boards (since I will keep the ceiling cathedral and expose the T&G boards from the bottom). Here are my options so far:

1) purchase scissor trusses with a bottom slope of 8-12 and top slope of 10-12 to completely contain my existing 7-12 roof - then use spray foam and/or batting. I may build a knee wall on the exterior walls to provide some separation, in which case I may be able to get by with 7-12 bottom, 9-12 top trusses.

2) frame a 9-12 roof above the existing 7-12 roof and install collar ties above the existing ridge board to stabilize the new, higher-pitched roof - then use spray foam and/or batting. I may build a knee wall on the exterior walls to provide some separation between the two pitches and additional length for the collar ties.

3) simply place new 2x6s on top of the existing deck boards to line up with the existing 2x6s below and attach the new 2x6s with nails - then use spray foam on top of the deck boards before attaching OSB and roofing.

Given my climate zone, 5.5 inches of spray foam will give me approx. R-38.5...fine for this area. The existing structure has practically zero overhang, so whichever option I choose will simply include extending the new boards to create my 1'8" to 2' overhang.

I am leaning toward option #3, but I haven't heard of anyone else doing it that way. As long as I include vents to connect the eves to the ridge vent I should be fine, right?

Any advice you can provide would be greatly appreciated!!!

Ron6519 05-03-2009 09:24 AM

What you are attemping is basically a, "cold roof". Try this link:
This roof would need to be engineered with your current structure. Modification might need to be made to the current roof structure to support the substantial added load.

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