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Installing 2 Vinyl windows with no nail flange in a 6″ opening depth

Mike Tew | Posted in General Questions on

I’m turning our sunroom into a finished space. The current slider windows are 2 be replaced by 2 vinyl single hung units (73″ x 52″); each unit has a 2 single hungs and a vinyl joint between each to make it a solid unit.

The current sill is sloped 2×8. Based on my recommendations and manufactures install is to shim sill at 24″ and jambs at 16″ (only fastening the jambs).

They say if > 65″ wide consider fastening through the still; but from the advice I’ve been given from contractor and rona is to not fasten on the sill as it could compromise drainage.

My plan was to trim down the 2×8″ sill and make it flush with exterior 1/4″ sheathing and bring the house wrap overlapping the inner opening and install a self adhesive flashing over the sill jambs and header.

One recommendation I has was to use a j-mould over the sill. But I think self adhesive flashing would be better. If I go with self adhesive, what would be the best trim options to use? Fiber-cement siding is to go over the exterior.

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  1. Mike Tew | | #1

    Also, these are a vinyl-built windows. They recommend installing a drip cap at the header at least 2" up behind the building wrap.

  2. GBA Editor
    Martin Holladay | | #2

    Your description and questions aren't clear.

    For example, what is "the current sill"? Is that a finish sill from an existing window or a rough sill that is part of the rough opening?

    If you have never flashed a rough opening before, I hope that you have searched the GBA site for information on the issue. There are many ways to make mistakes with this job.

  3. Mike Tew | | #3

    Hi Martin.
    The sill is existing sloped sill is a true 2x8" that is apart of the rough opening.

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

    A part or apart... Flash the opening.

    Fibercement siding trim choices are many, fibercement trim, PVC board stock, preprimed fingerjoint western cedar. Visit your supplier for options and a chat.

  5. GBA Editor
    Martin Holladay | | #5

    You need a sill pan on top of the rough sill -- not housewrap.

    The sill pan can be a manufactured sill pan or a site-built sill pan. Most site-built sill pans are made with peel-and-stick rubberized asphalt flashing. The site-built sill pan needs to be sloped or else it needs an interior dam to prevent water from entering the house on the interior side.

    More information:

    Pan Flashing Choices for Windows and Doors

    Flashing the Jambs on Flanged Windows

    Flanged Window Video Series

  6. Mike Tew | | #6

    Hi Martin & AJ,
    I'll go to store and check out what options are available. The peel-and-stick option does seem the easiest / quickest to install.


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