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Flashing on window sill: To be or not to be? That is the question

Elizabeth Croes | Posted in Building Code Questions on

New construction – builder installed Marvin windows w/o flashing the sill, although Marvin calls for it. Is this acceptable?

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  1. Steve Knapp CZ 3A Georgia | | #1

    I insisted my builder flash the window openings even though it delayed the installation. Most builders in our community simply tape the flange and go on there merry way. I think that is a mistake, but no doubt the tape is good enough to get the builders through their one-year warranty period.

  2. Elizabeth Croes | | #2

    Thanks Steven - that's how I feel ... Plus Marvin Warranty calls for it...thanks!

  3. GBA Editor
    Martin Holladay | | #3

    All building codes require materials to be installed according to manufacturers' instructions. Failing to follow the building code is illegal. You are under no obligation to pay a contractor for work that violates the code or that fails to follow the manufacturer's instructions.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    What do the plans call for? Does your contract call for following all manufactures installation instructions?

    What you want done should be down in writing.

    Did you do your do diligence and go check a project being built by your builder?

    Go to your builder explain your needs and add an addendum to your contract with a new cost for added work which may be nothing or possibly you may need a new builder.

    I have a feeling more issues are coming..... Next time get the builder that builds what you want. Get a more detailed set of plans and scope of work. Make sure they tell you they do windows with pans not you telling them to. And the cost will be higher.

  5. Elizabeth Croes | | #5

    Martin thank you. I live in VT is there a site w building

  6. GBA Editor
    Martin Holladay | | #6

    According to section R612.1 of the 2009 International Residential Code, "Windows and doors shall be installed and flashed in accordance with the fenestration manufacturer's written installation instructions."

    Most residential construction in Vermont is (unfortunately) not governed by any building codes. Nevertheless, if there is ever a construction dispute that is worth bringing to court, it's reasonable to assume that professional builders who charge money for their work should follow the expected practices of their chosen profession. That means that they shouldn't engage in practices that are illegal in most states.

  7. David Meiland | | #7

    I don't think that "flashing" adequately describes what should be done at a rough window sill--I like the term "panning" a lot more. In most situations, we use a piece of cedar bevel siding nailed to the sill to create slope, then layer the sill and wall with self-adhesive flashings, using Dupont FlexWrap on the sill. After the window goes in, we seal it to the rough opening with either backer rod or canned foam. Backer rod is easy to use on window installs because you have consistent width gaps to fill. In some cases I have sheet metal pans made, soldered at the seams and with built-in back dam and side dams.

    Elizabeth, I don't know what type of windows you have, but we mostly install wood/clad units. These have corner joints that rely on sealant. In rare cases there will be a joint that leaks on day 1, as soon as the first rain hits it. If you do not have a sloped sill with a membrane or pan, your framing gets wet. I think vinyl units are slightly less worrisome because the corners are "welded", but there is certainly a chance those welds will open over time.

    Anyway, I think windows without sill pans are a serious mistake. When a leak happens it can stay hidden behind the trim for a LONG time as the framing, sheathing, drywall, and insulation gradually get wet. Eventually the owner becomes aware of the problem, once it's big enough. Part of my business is repairing problems like this, and by the time I get a call there is major damage that costs a lot to fix, all because a few minutes and a few dollars were not put into installing a pan.

    I should say that a sill pan creates another potential hidden condition. A window I install could start leaking and I'll never know it. If water drips through and gets on the sill pan, it will fall to the outside. A window could sit there leaking and rotting for a long time... but at least the structure would not be getting wet.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Pan away.... But a little my area I have never ever seen one window using panning. 99.9% are nailfin nailed up and taped to housewrap done. Some now use special window materials. No pans no sloped cedar.... Nice ideas.... Around here window frame areas are not rotting out in mass.

    There's bad poor OK good and tops A plus for every part and assembly in home building.

    I shoot for A and settle for plus or minus.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Martin, it's wrong to ask a contractor for A level workmanship after a contract is signed for less than such. What is in the contract does indeed matter to me at least.

    U want pans, spec and pay for pans.

  10. David Meiland | | #10

    She should withhold payment until windows are installed per manufacturer's instructions, unless she signed a contract that calls for less. I'd like the to see the written scope of work and the plans.

  11. Elizabeth Croes | | #11

    David where are you out of?
    Windows are clad//wood Marvin.

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