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Community and Q&A

Converting a flat roof to a pitched roof?

tina81 | Posted in General Questions on

I’m in the process of remodeling my 1950s ranch house. I have a modified bitumen flat roof, which is about 15 years old. The roof is in okay shape right now, however it’s sloped incorrectly in about half the roof and has some ponding of a few inches most of the time. There has been some damage to the roof sheathing near the chimney in the past. The rest looks to be in good shape. I’d like to improve the roof and be able to add more insulation to the ceiling as the first part of my renovation. I’d like to get your opinions on the best way to improve the roof. Right now I am considering converting the flat roof to a hip roof with at least a 4/12 pitch.

I met with a contractor and he suggested putting trusses over the top of the existing roof assembly to achieve the pitch. He said a tear off of the membrane and ship lab sheathing wouldn’t be necessary. He would cut through the membrane and roof sheathing at the end of each rafter to blow cellulose between the sheathing and the ceiling below it (about 6 inches) and blow more cellulose on top of the sheathing and membrane. Right now the flat roof is vented through the sofits around the whole perimeter of the house. Vents would be added to the top of the roof. Would this set up cause moisture problems?

Thanks for any suggestions, let me know if I can provide more info.

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  1. tina81 | | #1

    I'm in zone 6a

  2. GBA Editor
    Martin Holladay | | #2

    If this were my house, I would insist on the removal of the existing roofing. It's fine to leave the shiplap sheathing in place, however.

    It's also important the ensure that the existing ventilation channels are sealed or somehow redirected, so that when the job is complete, the soffit vents direct air to the attic, above the top layer of cellulose (and not to an intervening air space somewhere in the middle of the insulation).

  3. tina81 | | #3

    Thanks Martin!

    What's the reasoning for removing the old roofing? Moisture concerns?

  4. GBA Editor
    Martin Holladay | | #4

    Under certain weather conditions, it's conceivable that the roofing membrane might be cold enough to be a condensing surface for moisture. The likelihood is low, but the roof will perform better without the roofing membrane.

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