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How to waterproof the top course of shingles on a shed roof

grmp945 | Posted in Energy Efficiency and Durability on

Hi all,

I’m about to put an asphalt roof on a single-pitched building I’ve just built. I’ve attached a photo for clarity (not my build unfortunately).

What is the best way to shingle or flash this style of roof to prevent water intrusion at the top course of shingles? I have taken great care so far to properly waterproof the rest of this build, and I am surprised to find only amateurish results on google on how to deal with the top edge of a shed-style roof.

One solution would be to nail a piece of metal flashing with a 90-degree bend, but the flashing itself will then have exposed nails which seems less than ideal. I don’t trust sealants to offer long-term waterproofing, so simply sealing the nail heads doesn’t pass muster in my mind.

I am not a pro builder and any advice on how to finish this detail is appreciated! Thank you!

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  1. Expert Member


    Use a peak cap similar to the one shown in this link. If you don't trust sealants, you can use gasketted screws to keep it in place.

    If wind-blown rain can still get at the exposed rafter ends, it's a good idea to cut a kerf in the underside of them as a capillary break, so the water can't run right down to the exterior wall.

  2. walta100 | | #2

    I like this detail with the drip edge on top of the shingles the question is how and where to attach the drip edge.

    It would be good for your facia to hang below the bottom of the rafters.


  3. ERIC WHETZEL | | #3

    We used the bent metal flashing with screws that had a gasket as Malcolm suggests. The hemmed bottom edge of the flashing stops about halfway down the face of the fascia board as Walter mentions. So far, it's been holding up well.

  4. grmp945 | | #4

    Thanks for all the replies!

    I am going to go with metal flashing with gasketed screws as suggested.

    Any suggestions on spacing between the screws? I usually like overkill for fasteners, but since every screw is a potential leak, it seems like less is more here.

    One other question: does using some roofing cement under the flashing seem like a good idea, or just additional mess?

  5. gawdzira | | #5

    I appreciate the aesthetic of the exposed rafter tails at the top side. It will leak into the wall. Covering the tails with a trim board will greatly increase your chances of not leaking as much as if they are exposed. I would seal the rafter at the wrb level with liquid applied flashing in at least double the recommended thickness. Then caulk the siding layer like your bank account depended on it.

    The kerf cut suggested above is also a great idea to aid in the water flow.

  6. grmp945 | | #6

    Just to clarify again - the attached photo in the first post is not my build. I only included the photo as an example of the roof style I am referring to (single-slope or shed roof). My build does not have the exposed rafters as the photo shows. I agree with the advice given for the owner of the building in the photo though!

    My own build's question/issue is limited to the flashing detail on the top course of shingles as described in post #1 and post #4.

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