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New construction window question

10 years ago

Hi all,

I've been perusing these forums off and on for several years - the info and experience here is amazing. I only recently joined to ask a couple of specific questions regarding new construction window installation. I plan to install the windows myself. I'll try to provide as much detail as I can regarding my circumstance.

I have a town home built in 1971 that still has the original aluminum framed slider windows. They are installed with a nailing fin directly against the stud on the outside of the rough opening, with 3/8" plywood sheathing over the nailing fin, 2x3" wood trim over the sheathing, and wood clapboard siding abutting the sides, top and bottom of the 2x3 trim. The trim is caulked to the window on one side and to the siding on the other. I'm not convinced there is any other sealing inside of that. I am anticipating some water damage to the rough opening.

Inside the home, there is drywall inside the rough opening all the way to the aluminum frame on the sides and top. There is a 1x6 stool on the bottom (butts up against the inside of the frame - does not go under it), caulked to the window, and .5" x 1.5" trim tacked around the sides and top, which is also caulked to the window.

My plan for the interior is to cut the bead of caulk around the trim and stool then remove them both so I can cut the drywall back, since I'm counting on the jamb depth of the new window being greater than that of the existing window. For the exterior, I'll cut the caulk, remove the trim, then use a circular saw to plunge-cut into the sheathing just outside the nailing fin so I'm able to pull those nails and get the window out.

Here's my question. I read conflicting opinions on whether new construction windows should be installed over or under sheathing. Should I cut strips of 3/8" plywood to replace the sheathing I have to cut back, and screw it in place so I have a surface flush with the old sheathing and install sill pan over that, or should I just put in the sill pan, making sure the flex tape adheres to the outside edge of the stud then overlaps the old sheathing?

Next up, when re-installing the exterior trim, I'll be using PVC brick mould rather than wood. Is it safe to assume that caulking to the side of the window and back to the siding is the extent of the sealing I'll need to do?

Comments (6)

PRO
Windows on Washington Ltd
10 years ago
You got a picture?

That is certainly a weird way of doing it. Before you cut the jamb back, measure the jamb to fin distance on the new window to be sure.

I like the idea of filling the gap and have the new fin flush with the sheathing.

Seal it up and put flashing tape and sill pan in the new opening.

See if you can incorporate a drip cap as well.
PRO
East Bay 10
10 years ago
What is causing some of your confusion is the term "sheathing". Here in California, sheathing is installed by code because of earthquakes and siding, stucco, or other exterior finish material is installed on the sheathing (there is more to it but I am simplifying it). What you are calling "sheathing" I would call siding and the window should always be installed, flashed etc. under the sheathing/siding if you are using a finned window.

What you are planning to do sounds fine but you didn't mention what type of window you what to use.
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kansasguy
Original Author
10 years ago
windowsonwashington: "That is certainly a weird way of doing it."

Are you referring to the original installation or my plan? :-) Nothing about this house has EVER been how I expected it to be when tearing it apart for replacement of anything, and this was certainly not an exception.

I don't plan on cutting the jamb back until I have the old window out and the new one ready to go in. I'm also leaning towards filling in the gap, but am open to opinions from folks more experienced than I am.

eastbay: As for the sheathing question, maybe I am confused. There are two layers on top of the framing of the house: a 3/8" plywood, which is what I'm calling sheathing, and then wood clapboard siding over that. I'll be installing vinyl slider windows for all but my smallest window - for that one I'm going with a double hung since it is so narrow.

I'll see if I can get a picture tonight when I'm home from work.
PRO
Windows on Washington Ltd
10 years ago
I was referring to how the windows were put in originally. Your install plan is great and well thought out.

You are correct. The plywood is the sheathing and the siding is the clapboard.
kansasguy
Original Author
10 years ago
I finally had a day nice enough to get out and pull back a piece of trim during the daylight hours. I've attached a picture of the setup - it was hard to really show the nail fin, but you can sort of see the flash reflection coming off of it between the window and sheathing.
PRO
Windows on Washington Ltd
10 years ago
Back up and get another picture. When they are that tight in...its tough to get a good look.

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