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Community and Q&A

Roof valleys – open metal or closed cut?

Debra_Ann | Posted in General Questions on

I want my roof to be as leak-proof and long-lasting as possible. Can’t afford standing seam metal, though, so it will be mid-grade architectural asphalt shingles installed with meticulous detail. I’m in a mixed humid climate zone 4A. Roof is 5:12 slope, with a couple of valleys. I’m trying to decide on the best way to do the valleys – open metal valleys, or closed cut ones.

The closed cut valleys are cheaper and commonly done in my area, but they tend to wear out sooner due to the high volume of water in the valley washing away the protective grit on the shingles there before on the rest of the roof.

I’m leaning towards an open metal valley, but I’ve had contractors disagree – saying that the metal expands and contracts too much. But I’ve also heard that, properly done, they are very effective and can last a long time.

I’ve been somewhat confused about the installation instructions for metal valleys. Most state to use clips on the long sides of the valleys to allow the metal to expand/contract as needed, and to not nail the metal in place. BUT the shingles seem to be nailed through the metal about 6″ outside the bottom of the valley, which contradicts the “no nailing through the metal”.

Any thoughts, experiences, or recommendations for roof valleys?

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  1. dankolbert | | #1

    Will you be laying the roofing yourself?

    If you have a decent architectural shingle, I don't think it's worth the added money and labor to do a metal valley. If not done right, a metal valley can be a disaster.

    If you're not doing it yourself, ask the roofer what they think.

  2. Debra_Ann | | #2

    We're doing it ourselves.

  3. Expert Member


    I would never use closed valleys for a number of reasons:

    - Much less chance initially of leaks due to installation errors, and any problems over the long-term are immediately apparent and visible.

    - Less debris accumulates in the valley, and that debris has less of a chance of causing leaks by capillary action.

    People working on your roof, especially less skilled trades or homeowners, invariably use the valleys to walk on. Closed valleys are very vulnerable to mechanical damage.

    - Repairs to closed valleys are a lot more involved than working on an open valley - as is re-roofing or making changes to a portion of your roof in the future.

    As to installation:

    I don't see nailing through the valley flashing as that big a deal. If the metal expands and contracts you could get some minor oil canning, but nothing that effects the integrity of the valley. Metal roofing and flashing details are a bit funny that way. Installation instructions often call for avoiding fasteners except clips or through slots, but roof penetrations, and many flashings, like drip, gable and end-wall flashings are fastened directly and seemingly do just fine.

    If you do want to avoid fasters, here is a link to a FHB article on how:

  4. dankolbert | | #4

    Then definitely don't do a metal valley. It's pretty hard to screw up a cut valley, pretty easy to screw up a metal one.

  5. user-2310254 | | #5


    Would you also recommend installing a peel and stick membrane in the valley as a backstop for the metal?

  6. Debra_Ann | | #6

    All installation instructions I've read have recommended peel and stick membrane up valleys first, no matter which method you use. I suspect code requires it, too.

  7. Expert Member


    Yes. The membrane acts as additional protection if there are problems with the valley, keeps condensation away from the sheathing, and acts as a buffer if nails in the sheathing work their way out a bit.

  8. GBA Editor
    Martin Holladay | | #8

    I'm not quite sure what you meant when your wrote that peel-and-stick membrane in a roofing valley "keeps condensation away from the sheathing."

    Condensation happens when warm humid air contacts a cold surface. Of course roof sheathing can be cold -- it usually is cold in winter -- but at that time of year, the dangerous warm humid air tends to be indoors. So when warm humid air rises through cracks in the air barrier and encounters cold roof sheathing, the cold roof sheathing absorbs moisture. And the peel-and-stick is on the wrong side of the roof sheathing to prevent that phenomenon -- which is similar to condensation but more properly called adsorption.

    All of that said, I'm not advocating that we put peel-and-stick membrane on the interior side of the roof sheathing. I'm just saying.

  9. Expert Member


    I meant keeps the condensation that can form on the underside of the valley flashing away from the sheathing - although underlayment or building paper would do that just as well - and thinking about it I wonder how much moisture actually forms on the underside of flashing installed directly on top of the the roof deck? I've opened metal valleys built without membranes and never found any damage to the sheathing.

  10. GBA Editor
    Martin Holladay | | #10

    OK -- I see what you were driving at. But I agree it's not really a problem.

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