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Community and Q&A

Confusion about window sill terms and flashing

Michael Bluejay | Posted in Green Building Techniques on

(1) What do you call the 2×4 piece of lumber that forms the bottom of the rough opening for a window? Is that the “window sill” or the “window sill plate”, or is it the same thing (or is it something else)?

(2) Are windows supposed to come with separate window sills? This article refers to, “a sloped secondary sill tucked under the sill that comes with the window…” What is the sill that comes with the window? Is it part of the window or a separate piece? I’m pretty sure my brand-name nail-fin window didn’t come with extra, uninstalled pieces.

(3) What is the piece under the window in this diagram? It’s not labeled. I presume it is what I think is called “sill pan flashing”. Incidentally, the diagram is labeled “Bird’s Eye View” but it seems to be a side view.

(4) Are rigid pan flashings still recommended? This article recommends sill pan flashing (which I presume means a rigid flashing), but a Builder Online article claims that “Flexible peel and stick flashing membranes have made rigid pre-formed pan flashings a thing of the past.”

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Replies

  1. David Meiland | | #1

    1) 2x4 or more likely 2x6 in new construction... that's the "rough sill"

    2) Depends on what the manufacturer offers, and what you order.

    3) Sorry, can't see the pic.

    4) Flexible membranes and even liquid applied membranes work well. I sometimes use metal flashing pans too.

  2. Michael Bluejay | | #2

    Thank you. Okay, if the framing member is the "rough sill", then what is a "window sill", and what is a "window sill plate"?

  3. User avatar
    Stu Turner | | #3

    Hi Michael, are you planning on installing your windows in a wall with a continuous layer of exterior foam? Note that the article you are referencing is specific to that wall assembly, which has more challenging details than a "normal wall". In any case, your point #2 above does not apply -- a nail fin window can not be installed in a "innie" configuration, which that part of the article is addressing. Your windows will not come with a separate sill piece. In terms of a sill pan flashing, there are many options available that I would deem equally effective. I think the point of the article in #4 is probably that peel and stick flashings are cheap and easy to install, compared to rigid pans.

    Big picture - at some point, assume water is going to get through the window-wall interface. It will run downhill. Eventually it will hit a flat surface: the window sill, right under the bottom of the window frame. Ideally, you want this surface to be waterproof and sloped toward the exterior (this is the sill pan). Otherwise water could pool, run into your house, or soak into the wall beneath your window, where it could eventually lead to mold.

  4. User avatar
    Stu Turner | | #4

    This picture might help. You will need to copy the link into your browser, sorry

    https://s-media-cache-ak0.pinimg.com/736x/06/e1/44/06e1449d001fb2746000d352ef522b8d.jpg

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Michael,
    I have never heard of a "window sill plate." The window sill is the horizontal part of the window that sheds water on the exterior, below the lowest sash.

  6. Michael Bluejay | | #6

    Yes, I am planning to install with a continuous layer of foamboard on the sheathing.

    Are you sure that nail-fin windows can't be innies? The innie/outie article makes no mention of that, which is where you'd expect it if nail-fin are really a no-go for innies. In comment #15, Martin says that outies don't have nailing fins, implying that innies do. If nail-fin can't be innies, then they can't be innies *or* outies. Now I'm even more confused.

  7. User avatar
    Stu Turner | | #7

    My apologies, I stand corrected. You can in fact install nail-fin windows as "innies" with foam, and in fact that is exactly what the diagram in the article is depicting, upon closer inspection. If my reading of the section drawing is correct... the sill pan (not called out) is draining into the gap between the housewrap (building paper) and the foam. The window sill is the triangular part of the window with the four small notches. The area below it is nothing -- an air gap, the rectangle below that is a 2x4 that is the rough sill. The nail fin is the dark line directly behind the sill extension.

  8. Joe Suhrada | | #8

    Michael, I think innie windows with the foam on the outside and the window nailed to the sheathing through the nailing fins if the best set up, myself. That is my opinion. I think it is safer, better for the window, the wall, and the heating bill. Some might argue, but after assessing both ways and the various ways of doing it, I really conclude that for my own. I think the window is VERY securely fastened yo the building this way, you may be able to order a factory extension jamb in 2x6 configuration and reduce your trim work inside, and ultimately the window should stay drier, more shaded in summer, warmer and cleaner, and less likely to get broken from air bound tree branches, misguided frisbees or rotten tomatoes.

  9. ALAN Hart-McArthur | | #9

    The rough sill or plate is actually part of the framing on which the proper window sits, with or without a pan.

    Traditionally the window sill was integral to the window itself and sloped to shed water. But most manufactured wood windows now use a box frame with little or no slope, with a meager "sub-sill" attached to the underside of the box, creating the extension to the outside of the wall plane. All too often these under-sized sub-sills are poorly attached with insufficient drainage to manage the rain and become a point of failure. The exterior casing legs typically land on the sill or sub-sill.

    BTW - Interior horizontal surfaces would in my neck of the woods be called a stool, never a sill.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Michael,
    Windows with nailing flanges can be installed as innies or outies. Needless to say, no matter what approach you take, the construction sequence needs to be anticipated, and every step thought through carefully, before the details are finalized.

    It's also possible to install windows without nailing flanges as innies or outies. The same level of planning and anticipation is required for either type of window.

    For more information on these issues, see these two articles:

    ‘Innie’ Windows or ‘Outie’ Windows?

    Installing Windows In a Foam-Sheathed Wall

  11. User avatar GBA Editor
    Martin Holladay | | #11

    Michael,
    In my opinion, every window rough opening (except, perhaps, those for windows that face porches) needs a sill pan. The sill pan can either be a manufactured sill pan -- there are several brands out there -- or it can be a site-built sill pan, using some type of peel-and-stick flashing.

  12. User avatar GBA Editor
    Martin Holladay | | #12

    Michael,
    Q. "What is the piece under the window in this diagram?"

    A. The detail (reproduced below) has a label reading "Existing window unit." The line from those words leads to the glazing. Below the glazing is a piece of wood -- that's the sash frame. Below the sash frame is the window sill, which is part of the window frame. I don't see any indication that this detail includes any sill pan flashing; that said, sill pan flashing is always a good idea.

    Underneath the window sill (in this detail drawing) you can see the part labeled "1x sill extension" on the exterior, and you can also see the rough sill (an unlabeled 2x4).

    .

  13. User avatar
    Michael Maines | | #13

    "Sill" is the most over-used word in construction. It is used to mean any bottom plate of wall framing, the wooden member directly above the foundation, the bottom member of a rough opening, the integral bottom part of a window frame, an applied member to "beef up" the look or function of the bottom of a window, and also for the finished horizontal trim board inside the window. Wall panelizing software I've used also uses "sill" to mean any horizontal framing member. There are probably other uses as well. So don't feel bad for being confused.

  14. Malcolm Taylor | | #14

    A sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock.

  15. Michael Bluejay | | #15

    Martin, than you for clarifying that nail-fin windows can be used for both innies and outies. I did see (and study) the innie/outie article, and in fact it was your comment #15 there that made me think that nail-fins for outies were a no-go.

    About sill pans, I think my confusion is that you seem to use the term "sill pan" to refer to both rigid pans as well as to those made from mere peel-and-stick flashing. I had thought that something called a "pan" would be the rigid variety. The Journal of Light Construction says, "A sill pan is a three-sided pan installed across the bottom of the opening and integrated into the weather-resistive barrier (WRB)," and they seem to be talking about only the rigid kind. It seems to me that "sill flashing" would be either a "sill pan" (rigid piece) or peel-and-stick.

    Actually, re-reading your reply, "it can be a site-built sill pan, using some type of peel-and-stick flashing," perhaps you didn't mean that the "site-built sill pan" would be entirely comprised of peel-and-stick, but that the peel-and-stick would be an adjunct to a rigid piece made on site.

  16. Michael Bluejay | | #16

    Also, the innie/outie article says that for outies, "The bottom of the plywood box is flashed with peel-and-stick flashing, just like a conventional rough opening." It doesn't mention anything about using a sill pan, or attaching a piece of beveled clapboard to the bottom of the box to facilitate drainage. Is that not necessary for outies for some reason? I thought that either innie or outie, having a sloped piece under the window would be prudent.

  17. User avatar GBA Editor
    Martin Holladay | | #17

    Michael,
    Q. "Thank you for clarifying that nail-fin windows can be used for both innies and outies. I did see (and study) the innie/outie article, and in fact it was your comment #15 there that made me think that nail-fins for outies were a no-go."

    A. In that comment, I wrote, "Most outie windows are secured with metal masonry clips (masonry brackets)." Note that I wrote, "most," not "all."

    When the exterior rigid foam is very thick, masonry clips are recommended, to make sure that the windows are fastened securely to the framing. When the exterior rigid foam isn't quite so thick, it's possible to use flanged windows, with the flanges fastened to a "picture frame" of 1x4s. For more information on this issue, see Installing Windows In a Foam-Sheathed Wall.

    Q. "Re-reading your reply: ‘it can be a site-built sill pan, using some type of peel-and-stick flashing,’ perhaps you didn't mean that the ‘site-built sill pan’ would be entirely comprised of peel-and-stick, but that the peel-and-stick would be an adjunct to a rigid piece made on site."

    A. See my answer in Comment #11 on this page. A sill pan can be an manufactured product -- there are many brands available -- or you can make a site-built sill pan. Most site-built sill pans are made with peel-and-stick flashing. To prevent rain water from entering the interior of your house, you have two choices: you can slope the rough sill -- this is usually done by installing a piece of beveled siding on the rough sill -- or you can install a dam on the interior of the rough sill. The dam is usually only 1/4 inch or 1/2 inch high, and it is covered by peel-and-stick. Either approach works, although I favor a sloped sill.

    Q. "‘The bottom of the plywood box is flashed with peel-and-stick flashing, just like a conventional rough opening.’ It doesn't mention anything about using a sill pan, or attaching a piece of beveled clapboard to the bottom of the box to facilitate drainage. Is that not necessary for outies for some reason? I thought that either innie or outie, having a sloped piece under the window would be prudent."

    A. When I wrote that the bottom of the plywood box is flashed "just like a conventional rough opening," I meant that you need a sill pan -- either a manufactured sill pan or a site-built flashing. That has become the conventional way to flash a rough opening. I didn't mention all the steps needed to flash a rough sill, but there are videos on GBA that show this process, step by step. As I wrote in my previous answer, you can choose to install a sloped shim under the flashing, so that the rough sill drains outward, or you can install an interior dam. Either approach works, and the choice of which method to use is up to you.

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