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Community and Q&A

Roof insulation

user-1055444 | Posted in Building Code Questions on

The building: 1968 5-plex in Portland, Oregon.
The old roof: tar + gravel, flat.
The new(er) roof: asphalt rolls laid over a slightly built-up frame over the old flat roof. Probably as much as 6-8″ between the newer roof sheathing and the old abandoned roof.
The joists: 2×8, 16″ O.C., insulated with paper-faced fiberglass (a long time ago, it would appear). Though the walls are to the studs, we have not planned to demo the ceiling, so we’re hoping to leave the batts in place.

The question: what’s the economical (read: budget is quite limited) and correct thing to do here? The roof is in need of replacement (the fascia is rotten, and the asphalt is deteriorating. I have bids for a new TPO roof, but am unsure whether we should be specifying a complete tear-off down to the original flat roof (in which case tapered insulation might be necessary to ensure good drainage?), or if the built-up sheeting might be worth re-using where it’s not damaged? In which case is there a benefit to having rigid insulation over the sheeting with that much space underneath?

Looking forward to reading all your advice on the matter.

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  1. GBA Editor
    Martin Holladay | | #1

    If I understand correctly, this is your roof assembly, from the inside to the outside:

    - Finish ceiling (probably drywall or plaster and lath);
    - 2x8 ceiling joists filled with paper-faced fiberglass batts;
    - Old roof sheathing (probably boards);
    - A tar-and-gravel (hot asphalt) roof;
    - Another layer of joists, probably 2x6s or 2x8s;
    - Another layer of roof sheathing (perhaps plywood?);
    - Roll roofing (perhaps mineral-surfaced double-coverage roll roofing).

    Did I get that right?

    I have no idea what you mean by "the walls are to the studs."

    If I have guessed all the layers correctly, I think that the right thing to do is to demolish the upper layer of roofing and the framing above the old tar-and-gravel roof. Then you should remove the old tar-and-gravel roof to inspect the condition of the original roof sheathing.

    The next step would be either roofing underlayment (if the sheathing is sound) or new plywood roof sheathing (if necessary), followed by a thick layer (or layers) of rigid foam sheathing, followed by a cover board and new roofing.

    For more information, see Insulating Low-Slope Residential Roofs.

  2. user-1055444 | | #2

    Sorry Martin, I totally dropped the ball on this thread!
    You're correct, mainly:
    - Finish ceiling (drywall);
    - 2x8 ceiling joists filled with paper-faced fiberglass batts;
    - Old roof sheathing (probably boards);
    - A tar-and-gravel (hot asphalt) roof;
    - Another layer of joists, probably 2x6s or 2x8s; (not sure about this section--the roof above is not entirely flat, it tapers down toward the edges of the roof, so it's not just a layer of joists going across the old roof line)
    - Another layer of roof sheathing (plywood, maybe OSB);
    - Roll roofing (perhaps mineral-surfaced double-coverage roll roofing).

    By "the walls are to the studs" I had intended to convey that the exterior walls have had sheetrock and old fiberglass batts removed, so they've been demo'd down to the (2x4) studs. Fastened to those studs is a layer of tar paper, then OSB sheathing (perhaps old T-111 siding; building was built in '68), then certainteed lap siding, FWIW.

    Your advice seems to jibe with the plan of the roofer. Unfortunately this adds $10k to an already expensive proposition. I had hoped there was some way to avoid the expense of removing the old tar-and-gravel roof, but that seems like you think it should be removed.

    Thank you for the prompt reply/advice(!), and let me know if any of the above updates changes anything in your mind.

  3. GBA Editor
    Martin Holladay | | #3

    Sorry that my advice has precipitated the extra expense. I think that when all is said and done, you will be glad to have stripped off the old materials so that you can examine the original roof sheathing and do a good job of insulating.

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