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Community and Q&A

Rake flashing on a low slope roof.

Tommy87 | Posted in Energy Efficiency and Durability on


I am reading up on guidelines for rake/gable flashing for snaplock and most say to install the trim directly over the snaplock, with butyl tape underneath where the screws go. See here:

Is this really the best way to install rake trim? Having exposed screws on a snaplock roof seems to defeat the purpose. With that being said, has anyone ran into any issues with this application?

I have also seen rake trim installed over z flashing, where there is no exposed screws.

Interested to hear your guys thoughts.

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  1. Expert Member

    There are a couple of ways to avoid the exposed gasket-headed screws from penetrating the roof panels.

    - You can (as you say) use a Z flashing much as you can at the peak.

    - You can either run your fascia 1" high, or install a block at the perimeter of the same height, and secure the flashing into that.

    Rather than screw through tape into the roof panels, I prefer to use a second washer between the flashing hem and the panel below.

    The few penetrations all occur at overhangs where the consequences of a failure are relatively minor. That said, I've never had even a minor leak in any of the 40 odd Westform metal roofs I've installed over the last couple of decades.

  2. Tommy87 | | #2

    Hi Malcolm,

    I think I will just go with the exposed screws option, if you have never seen any issues. I assume it is VERY important to not overdrive the rubber washer screws?

    Also, can you further explain the benefit of using a washer rather than butyl tape? From what I understand, you still screw into the metal roof (and butyl tape seems like it would protect from water better?)

    And just to be clear, these penetrations will not be over an overhang.

  3. Expert Member

    It is very important not to over-drive the screws. Use a cordless screwdriver with a clutch, not an impact driver, which also can take the finish of the screw-heads.

    i don't rely on tape or closure strips for any of the joints in the roofs I install, preferring to bend up the panels or flashing to create dams. Without the butyl tape you can get some water moving under the gable flashing, but it is stopped by the rib on the first course and bent up panel edge on the last one.

    My thinking on the tape vs gasket is that if the gasket is the preferred method of sealing the screws in all other situations, why not rely on it in this one? That said, I've never heard of any leaks through the butyl either.

    If you are doing the work yourself, and are securing your ridge caps into the top of the panels ribs (rather than using Z flashing and stitch-screws), you might want to use Westerman panels instead of the Westform. They have a flat top on the ribs that is much easier to hit through the peak flashing.

  4. Tommy87 | | #4

    Copy that on the screws.

    I'm still having a hard time picturing your washer method :/ You still screw through the metal roof (as well as the trim) right? So it seems like the butyl tape would seal that penetration better than a washer.

    Can you perhaps send me a picture of this washer you are talking about? Is it a rubber gasket washer?

  5. Expert Member

    If you screw through the hem of the trim, and through the panel, into the sheathing below, there is no gasket between the trim and the panel. I back off the screw until the tip is visible between the two, take a washer from another gasketted screw and place it so the screw sandwiches it between the trim and panel.

    So from top to bottom: Screw head with gasket, trim hem, gasket on screw shaft, roof panel, sheathing or furring.

  6. Tommy87 | | #6

    I see. So you still screw into sheathing/furring, but use a rubber washer instead of butyle tape?

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