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Community and Q&A

Gap Between Window Trim and Frame

jamesboris | Posted in Green Building Techniques on

Hello, with a flanged window, what size gap should I leave between the trim and the window frame? If there is a gap, won’t the flashing tape show? I see a gap on some details (like p. 164 here), but no size. Also… Should I caulk it? Is there any concern with puncturing the nail fin with trim nails?

Specs of my project, in case it helps:
-Fiberglass windows, mounted to the sheathing, and projecting ~1 1/8″ beyond it.
-Flashing tape will lap from WRB, across the nail fin, onto the window frame.
-Rainscreen will be 1/2″ plywood strips.
-Siding will be 1″ rough boards, live edge, not beveled
-Trim/casing will be 2 3/4″ thick

Thank you as always!

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  1. Expert Member


    There are two schools of thought on this. Martin likes to prioritize drainage paths and drying, so he advocated no sealant between the trim and window. Others think stopping water intrusion should come first, and caulk around the window frame. If you are caulking, leave a suitable gap (3/16"). If you aren't, go tight.

    You don't have to worry about trim nails penetrating your flanges, but even though it's quite thin, the flange will cause your trim to lay at an angle. If the trim is being applied directly to the sheathing, you should rabbit out the back so it stays flat. If the trim goes over the cladding or furring, I like to keep the furring off the flange so the wall is co-planar.

    1. jamesboris | | #2

      Thanks Malcolm. Especially appreciate the suggestion to keep the furring off the flange. I'd actually prefer to make it tight in this case, but making it tight raises 2 worries for me:
      1) What if the wood or the fiberglass expands a bit (Zone 2A) and causes some binding or other problems?
      2) I feel that rain tends to get into the smallest of gaps, but only tends to get *out* of 3/16"+ gaps... but you deal with much more rain than I do... what do you think?

      Any other downsides to shooting for tight? "Tight" is never as straightforward a word as I'd like when working with wood... largely why I avoid miters on anything exterior ha.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I've never had problems with expansion - and to be honest I go tight, but do caulk the joint. I just suggested a gap because that's best practice. Even in our wet climate, it's a rare occurrence to have wood expand over the dimensions that you buy it from the lumberyard. Maybe in hot humid regions that differs?

        When I take over as world dictator all miters will be banned, along with half the socket sizes and differing pressure-washer hose connections.

        1. jamesboris | | #4

          So you run it tight -- kinda like pic 3 et seq here -- and then run a fillet joint of caulk on the frame itself?

          1. Expert Member
            MALCOLM TAYLOR | | #5


            Yes exactly. I've attached a photo showing three windows on my own house, but I'm not sure you can see much.

  2. Expert Member
    Michael Maines | | #6

    I leave about a 1/16" gap, mainly because I pre-assemble the exterior trim but also to allow for expansion since I typically use real wood. My intention is always to go back and caulk it but I rarely do and don't think it's a problem.

  3. seabornman | | #7

    I have seen wood window trim expand and cause double hung windows to bind.

    1. jamesboris | | #8

      I've heard of this too, but there are a lot of variables. For example, I'd think steel or fiberglass frames are less likely to bind than vinyl or wood ones, since the latter expand too. Can you give some specifics on when you've seen this happen, and where? What kind of gap would you recommend? 1/8" maybe? Thanks!

      1. seabornman | | #9

        Yes. The siding was cedar clapboard. Trim was 5/4 pine. All wood was finished on all sides before installation. Windows were Andersen Narrolines, which have a vinyl wrapped frame and painted wood sash. I ended up routing out the trim where they met.

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