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Roofing material over sheathing which can’t dry to the inside?

mikkelsen | Posted in General Questions on

Underside of roof sheathing has polyurethane foam on it, so if it were to get wet, there would be no drying to the inside. Would it be problematic installing asphalt shingles on this roof? Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Eric,
    People do it all the time. In another decade or two, roofers will have more experience with this type of roof, and they'll be able to report on whether these roofs have more problems than usual or fewer.

    If you prefer to include ventilation channels, you can create them above the existing roof sheathing. The usual way this is done is to install 2x4s on the flat, with one 2x4 above each rafter, to create 1.5-inch-high channels that run from the eave to the ridge. Then install a second layer of plywood or OSB, and any type of roofing you want.

  2. Expert Member
    Dana Dorsett | | #2

    The basic premise that "...there would be no drying to the inside" is false.

    Polyurethane foam has a wide variation in water vapor permeance, depending on it's thickness and density. The most common closed cell polyurethane foams run about 2lbs per cubic foot density, and are usually not quite a class-II vapor retarder at 1" thickness, running about 1.2 perms. The same foam at 3" thickness runs about 0.4 perms, which is comparable to the kraft facer on a batt (but usually much more air-tight.)

    Half pound density polyurethane foam is still 10 perms or more at 5", which is a bit too permeable for keeping the sheathing from accumulating wintertime moisture in cold climates.

  3. mikkelsen | | #3

    It'll be 3.5" of polyurethane then cellulose...for the cold climate that I'm in. This is actually the roofing material that I'm thinking of installing: http://bellafortebydavinci.com/products-shake.php My bigest concern is if the plywood sheathing will have sufficient drying potential. This roofing material sheds snow. and the pitch of the roof will be 10/12. The roof manufacture said that there is formed air space behind the tile, but as far as air flow....I'm not too sure.

  4. Expert Member
    Dana Dorsett | | #4

    Cold is relative. Where is that cold climate, exactly?

    What density foam? At 3.5" most 2lb foam is going to run about 0.30 - 0.40 perms, which isn't a lot of drying capacity toward the interior, but some.

    With a shingled roof the permeability of the underlayment might give you some relief for drying to the exterior, but if this is a location where the roof will be covered in snow for 12-20 weeks, even for a modestly permeable felt it won't be drying until the snow is fully gone (nor will it dry when the shingles are wet with dew or rain.)

  5. mikkelsen | | #5

    Sorry, 2lb foam. location; Central Idaho @ 5,000' elevation. First frost is usually second week in August and last frost is usually second week in June. usually over a hundred inches of snow a year but again, if I go with this material it sheds snow, and there are no valleys in the roof. I Just don't want rotting Plywood sheathing. Summers are quite arid. I'm planing on using 30# felt paper and this particular roofing material calls for a cap material to go over that for the fire rating.

  6. Expert Member
    Dana Dorsett | | #6

    First & last frost dates aren't reliable indicators of the length/depth of the winter temps ( especially at 5K', where the radiational cooling on clear nights can bring on frost even when the daily average temps are above 50F) or an indication of the duration of rooftop snowpack. Got a zip code?

    While #15 felt is semi-permeable when wet, #30 felt is fairly vapor retardent at any moisture content.

  7. mikkelsen | | #7

    Is there any underlayment NOT to use over this kind of a roofing situation? Is there a prefered underlayment?
    Thanks!

  8. GBA Editor
    Martin Holladay | | #8

    Eric,
    Manufacturers of most synthetic roofing underlayments do not allow their products to be installed over unvented roof asseblies. The exceptions are the (few) synthetic roofing underlayments that are vapor-permeable (and expensive).

    Above an unvented roof assembly, the best roofing underlayment is plain old-fashioned asphalt felt.

  9. mikkelsen | | #9

    Thanks, that is what I was thinking...
    However I find myself in a situation of either using a tarp over the roof for the winter or something stronger like grace triflex... thoughts? Would this product fit the bill? Or tarp for the winter and put on felt in the spring. No one will be living in it and it is unfinished.

  10. mikkelsen | | #10

    I think I will tarp for the winter then felt in the spring...out of money for roofing materials right now. When I continue in the spring, is a single layer of #30 felt good or should I consider a double layer? Seams like I've heard of people doubling their felt. Are some plastic cap nails better then others, the ones I've seen are not very impressive (tear easily) and often the caps are on upside down.

  11. GBA Editor
    Martin Holladay | | #11

    Eric,
    One layer of asphalt felt is adequate under asphalt shingles. As long as the shingles are installed on the same day as the asphalt felt, you don't need cap nails. Ordinary roofing nails are fine.

    If you expect a tarp to keep your building weather-tight for the whole winter, however, you'll probably want two layers of tarp, and you'll probably want to install 1x3 or 1x4 battens to hold the tarp in place. Even so, you'll still get a few leaks.

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