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Window detail question: flashing over bottom nail fin?

James Howison | Posted in Green Building Techniques on

I just finished installing some new windows and am having second thoughts about one flashing detail.

Particulars: Austin, TX, Zone 2, South East facing wall. Concrete block construction, rough openings brought to size with treated lumber bucks. Windows are Milgard fiberglass, with integral wood inside. Wall has 2 foot eave (but up quite high since it’s a gable end). Eventually we’re going to finish out the wall with either foam or mineral wool, rain screen and siding.

We ran beam of caulk between bucks and concrete before fastening them. We then flashed the bottom buck before install using Protecto Wrap BT-25XL Butyl Peel/ Stick , paying attention to the corners as shown in GBA window flashing vids. We installed the windows using a continuous bead of caulk as in the manufacturer’s instructions, we didn’t need many shims thanks and screwed the nail fins into the wood (three sides, concrete one vertical side due to RO constrains).

Once in I applied more of the Protecto Wrap right around the windows, starting with the bottom then the sides and finally the top, so the lapping is right.

Throwing the Protecto wrap away I noticed that they show the bottom fin not flashed on the outside (only under the fin). At first I thought this might be for drying or drainage if any water were to get inside and was about to cut out and remove the bottom outer flashing.

But then I realized there’s caulk all around under the nail fin anyway and so it really can’t matter. Thus I left the flashing (but will cover it with painted wood trim in the next week or so, claims 120 days in sun ok).

So, the question is: right thing to leave it (realizing it’s unnecessary but not harmful) or should I remove it? Does it make sense to make any provision for drying from between window and buck (window has weep holes at bottom, those are totally uncovered), especially when caulk is applied under nailing fin?

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  1. Dick Russell | | #1

    Your concern about how water that makes its way onto the rough sill flashing can drain to the exterior is valid. Correct installation of a flanged window unit avoids sealing the bottom flange to the sill flashing or housewrap. Caulk should not have been applied under the bottom flange or peel/stick sealant over that flange.

  2. David Meiland | | #2

    You should be installing rough sills that are sloped to the exterior, with either membrane or sheet metal protecting them, and the windows should not have caulk or self-adhesive flashing across the bottoms. Windows often leak through the frames, usually at the bottom and usually at the corners, and that water needs to drain out, otherwise it will build up and come inside.

  3. James Howison | | #3

    Thanks, that does make sense to me. And I do have peel and stick wrapped sills, with a very slight slope outward.

    Annoying that the manufacturer's instructions very clearly specify a continuous bead of sealant, including the bottom fin. I can remove the flexible tape no problem but it's going to be difficult, if not impossible, break the caulk. Do people think I should bother sticking a knife or something up there to break the caulk connection on the bottom side?


  4. James Howison | | #4

    Moreover, the relevant AAMA standard (2400-10) says that a continuous bead of sealant should go around the whole flange before install, saying nothing about leaving a drainage channel (it does, however, not specify peel and stick over the lower flange, although since this basically does the same thing as the sealant it doesn't seem important).

    Of course a standard might be wrong! I understand the logic of not sealing the bottom, but can anyone confirm that I'm reading the standard correctly?

    AAMA 2400-10:


  5. davidmeiland | | #5

    the relevant AAMA standard (2400-10) says that a continuous bead of sealant should go around the whole flange before install ...... I understand the logic of not sealing the bottom, but can anyone confirm that I'm reading the standard correctly?

    This got me curious enough to ask, so I sent an email to AAMA and got this response:

    AAMA 2400-10 is simply a recommended standard practice for products exposed to low wind/water exposures. FMA/AAMA 100 is a standard practice specifically designed for installations subject to extreme wind/water climate exposure, particularly in the coastal southeast United States, and addresses buildings that will be at high risk for water intrusion. Thus, preventative measures are called for that are above normal installation practices. Within FMA/AAMA 100 it calls for a more defined sill pan flashing design as well as providing voids in the sill sealant for possible drainage.

    Obviously, if a window “system” incorporates a dedicated sill pan underneath to
    catch water from poorly sealed corner joinery, then there would have to be
    provisions made to evacuate this water leakage while preventing a pressure drop (air
    leakage) created by the weep system.

    However, in accordance with the language contained within the International Building
    Code, the default answer in all cases is that windows shall be flashed and installed
    in accordance with the manufacturer’s written installation instructions.

    I looked briefly, but did not find a copy of FMA/AAMA 100 that I could download, so I don't know the particulars, but it would seem like good reading for anyone concerned that they might have more than low wind/water exposure on their installation. There does appear to be a demo video of the standard on the FMA site.

  6. James Howison | | #6

    Thanks David, very interesting.

    So Milgard says that it's instructions are AAMA 2400-10, see this page:

    They don't make any provision for exposure type or location in the country, just refer one to AAMA 2400-0. No other instructions came with the windows. Which means that in some situations, at least, the "manufacturers instructions" directly contradict the opinion of the people who wrote the instructions :) I'd hate to get into a warranty discussion with Milgard on this topic (it excludes any liability from "improper installation").

    Thanks again for the additional info. Seems like a warranty catch-22 to me.

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