Roofing over Cathedral Ceiling

[Rich Riddle]

I have a unique ceiling. It's a cathedral made from 2 x 6 planks. This is supported by five large beams that span the entire great room. On the outside, on top of the 2 x 6 planks appears to be some sort of "sound board" about an inch thick. They placed tar paper and shingles directly over that. The best I can explain the "sound board" is something like MDF. To say the least, the roof needs to be replaced because of inferior contractor quality. Any ideas on how to properly place a new room on the great room? In the included pictures you can see the ceiling goes to the outside as well.

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And since we are discussing roofs, this roof projects further forward at the highest point and then gets closer to the house as the roof slopes down. This feature allows everyone to get drenched when walking on the stoop in the rain. Any ideas?

[Art Mann]

That type of roof construction isn't unique. I have seen it many times but it is usually a church or commercial building. My guess is the material you are calling a sound board is some kind of insulating material. It might actually need to be there. You really need to contact a roofing contractor who is familiar with this type of roof to get good advice. I'm not sure how many people here would be familiar with it.

[Joel Goodman]

I have a roof like that might be similar. It is a post and beam with the roof decking of homosote about 1 1/8" thick. The homosote is both the decking and the interior finish. It was supposed to be a miracle insulating material when it was installed 40 years ago -- the insulating properties were rather exagerated! It has broad overhangs and the homosote became damp and buckled on the overhangs after a while. I replaced the homosote on the overhangs with plywood and added 3 inch rigid insulation and plywood on top of the undamaged homosote after stripping the roofing. If you have 2X6's as decking I would strip off the old homosote and replace with modern rigid insulation and then plywood and new roofing. The modern stuff is way better and you can go thicker.

[Mark Paavola]

Rich, my house is built in a similar manner. It was built about 40 years ago. I have 2x6 T&G that is the inside ceiling. On top of that there is a layer of roofing felt. Next is 3" of foam insulation. It is the rigid foil faced insulation. Next is a layer of 1/2" plywood that has long screws that go into the 2x6 boards. On the perimeter of the roof are 2 - 2x4's that make a 3" edge. Then on top of the plywood is your normal roofing paper and asphalt shingles. There are wide fascia boards to cover the edges. Think of it as a multi layer sandwich.

[Ole Anderson]

3" of rigid iso style deck insulation gives you an R value of about 18, way better than a layer of homosote, but way less than an attic full of traditional insulation.