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Planning for a standing-seam metal roof: Getting the right plan

severaltypesofnerd | Posted in GBA Pro Help on

I’m planning for a standing seam metal roof, for the building pictured, and have some envelope and vent questions.  I have read https://www.greenbuildingadvisor.com/article/does-this-roof-need-to-breathe

For the new roof, standing seam was chosen for durability, fire resistance (it’s a fire area), and relative light weight (home is 100 feet from an active earthquake fault). San Francisco Bay Area fog zone 3C+, so mild summers, mild winters.  Solar is intended to attach to the standing seam ribs, on the south side.

The existing roof is end of life asbestos shingles on 2×6 rough beams over 1×4 skip sheathing with 1 inch gaps between boards.

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The attic is insulated, and will be air sealed.  Lathe and plaster below.  Will have a Farco insulated attic ladder ( https://www.fakrousa.com/products/attic-ladders/ ).

The home is 2 story, 12 foot ceilings, forced air natural gas heat.
The attic space is about 6 feet tall, and is semi-finished and used for storage.
It’s the storage aspect of the attic, and potential use for an air handler, that bring up the most questions.

* The attic gets cold in winter, super hot even on mild days now.  How can temperature be moderated, so stored goods are not cooked?

* From a cost and weight point of view, standing seam metal directly on the existing 1×4’s is attractive.  But should there be more layers?  Will the roofers insist on adding a plywood/osb/tar paper/radiant layer, and will they be right?

* Are there alternatives to rubber plumbing and solar/electric penetration boots, that don’t get rotted out due to UV light?

* Is there such a thing as standing seam compatible temperature sensitive vents, that will open more in summer and close down on foggy/cold days?

* What quality metrics do I look for in a roof contractor?  On my list is “using backer rod on center of each panel to reduce oil canning”.  Anything else?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bryce,
    Q. "The attic gets cold in winter, super hot even on mild days now. How can temperature be moderated, so stored goods are not cooked?"

    A. If you want a conditioned attic, you can have one. Right now, it's unclear what you are planning -- all you told us is "the attic is insulated, and will be air sealed," without telling us whether the insulation follows the sloped roof or is installed on the attic floor. I assume that you meant to tell us that the insulation is on the attic floor, meaning that this is a vented unconditioned attic. If you prefer an unvented conditioned attic, you can have one. It just costs money. Here is a link to an article that explains the necessary work: "Creating a Conditioned Attic."

    Q. "Will the roofers insist on adding a plywood/OSB/tar paper/radiant layer, and will they be right?"

    A. Whether or not the metal roofing can be installed over skip sheathing or requires a continuous sheathing layer (like OSB) depends on the requirements of the roofing manufacturer. So contact the roofing manufacturer. What you call "tar paper" (more accurately called asphalt felt) is a type of roofing underlayment. Roofing underlayment is a code requirement -- it's not optional. Installing a radiant barrier between the roof sheathing and standing-seam metal roofing wouldn't make any sense, unless you are planning to incorporate an air space adjacent to the radiant barrier.

    Q. "Are there alternatives to rubber plumbing and solar/electric penetration boots, that don’t get rotted out due to UV light?"

    A. Wiring penetrations can occur at gable walls, rather than through the roofing, if that's what your prefer. You can also run conduit down the outside of your walls, if you like the look. A good sheet-metal shop can also make you copper flashing boots to cover the rubber boots if you want a top-of-the-line installation.

    Q. "Is there such a thing as standing seam compatible temperature sensitive vents, that will open more in summer and close down on foggy/cold days?"

    A. Not that I know of. Nor would that type of vent be particularly useful.

    Q. "What quality metrics do I look for in a roof contractor?"

    A. Choose a contractor that has been in the business for many years, and check references.

  2. severaltypesofnerd | | #2

    My first thought on the attic was:

    1) Continue with the attic floor as the primary conditioned space air barrier, and deepen the rafters to get to R50 or so.
    2) Put maybe 1 inch of foam on top of the skip sheathing and under the metal, in order to moderate the attic temperature.
    3) Add vents and a window, operable, to allow further moderation of the summer temperature.

    So the space would end up "semi conditioned". It's not big enough, open enough, or tall enough to make sense to spend a lot of money "finishing" it. At the same time it's useful space, and currently used for both storage and "hanging out".

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Bryce,

      It may not make a material difference if your walls aren't strong in shear, but sheathing the roof with plywood, and fastening it with an aggressive nailing pattern will significantly improve it's performance in a seismic event.

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