Hot Topics: This Window Leaks - A Lot

This window looks all right, but it leaks.

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The key quote from this thread is, “what is Z molding?” There are certain things you can figure out on your own, and there can be satisfaction in discovering a new way to accomplish an old task. But some things are done a specific way for a reason, and there’s no reason to reinvent the wheel. When you get your window install wrong, the Forum can tell you what you should have done, and what you need to do now.

Original Post: New Window Installation Screw Up, Need Advice

jhelp Member

Hey, first time poster. So here is the deal. I have a wall in my house with no windows, so I thought, hey how hard could it be to cut a hole and put in a new window? Piece of cake, right? Bought the window, cut the hole, installed the window per the manufacturer’s recommendations, lost the directions, then winged finishing off the exterior and closing it off. I installed the Left-Operable Vinyl Double Pane New Construction Sliding Window from Lowes about 6 weeks ago and everything has been great. No problems, looks good, lets in much-needed light into the kitchen, and the wife loves it.

However, this evening an infrequent to our area Easterly storm moved in and blew rain right at the window... leaks like a sieve. Seems that the rain is coming from above and leaking straight down. So here is what I did: Nailed a 2x4 to the header above the window, sprayed in the foam insulation stuff, put a bead of exterior silicone caulk around the window, replaced the cedar shingles, and then primed and painted the window. My question is what should have I done differently and how can I fix the leaking now? Can I just caulk the crap out of the top and then spray it with a hose to see if it fixed the problem, or should I remove a row of shingles and add some kind of aluminum window flashing? BTW I live up in the UP of Michigan and winter is coming soon. Thanks!

Window with a leak

Highlights from the Thread

joecaption1 Member

Going to need a lot more details as to exactly how you installed it. A properly installed window should have only needed caulking behind the nailing fin on the outside, no other caulking needed. However there should have been house wrap, window tape, and Z molding over the top of the window before the siding went on.

jhelp Member

What, you can't just tell me exactly what to do?!?! Regarding your questions, no to all the above. It did not have a nailing fin, it was like this window.

I just set it in, shimmed it, made sure it was plum, and screwed it in, then caulked around the edges to create a seal. The house wrap was the old wrap, house was built in 1903. No window tape, and what is Z molding?

chandler Forum Topic Moderator

Caulk, silicone, foam... none are your friends. Proper flashing of the window from above could have prevented the water leakage. At best, the window will need to be removed and flashing installed under the siding and tar paper (if any) and allow it to protrude slightly past the outside and both sides of the window. Basically, it will resemble a flat "gutter" for the water to run off.

czizzi Member

Sounds like a replacement window set in a hole. 1903 house, did we install a header above the window? Did any adjustments get made to the let in bracing?

jhelp Member

Yes, I put in a header to support the joists above the window. There was no let in brace where we cut in, it is a small window.

Chandler, thanks for your reply. Question: why will I need to remove the entire window to install flashing? Can't I just remove the row of shingles above the window and install flashing? Have it wrap down around the edges and replace the shingles?

XSleeper Member


Your first mistake was probably putting a replacement window in an opening where a "new construction" window should have been used. New construction windows have nailing fins, which you use to mount the window in the opening, and you also use them to seal the window to the wall, using sealant (behind) and the flashing tape (over the top).

Secondly, it looks like you have no flashing that goes behind your shake siding, and over the top of your top trim board. Caulk is not a reliable means of flashing something.

And then finally, when you have shake siding like that, you usually need to flash quite a ways above the window opening in order to ensure it is leak proof. Each layer of siding "weeps" onto the next, so it is hard telling how far up you would need to flash the window... it kind of depends on how the siding was installed and we can't see that.

Usually, each row of shingles will have a layer of #30 felt paper over them. The thing that prevents each course of shingles from leaking onto the next is the felt paper. The shingles are kind of just for looks. So what you might do is remove the shingles over your top 2x4, and install a metal Z-flashing over your 2x4. Go to Google images and search for Z-flashing if you need a picture. Then cover the Z-flashing with felt paper that tucks behind the rest of your felt paper.

At that point, you could shoot a hose at the window and water can't get behind the top 2x4. You would then replace your shingles, lapping felt paper over the top half of each course, working from the bottom up. All the felt paper you apply needs to be tucked behind the existing shingles and their felt paper in order to be waterproof. The top row of shingles will have to get tucked under the existing felt paper, which is hopefully still intact behind the existing siding.

jhelp Member

Thanks, XSleeper. I should have done better research before getting started. I explained my project to the person at Lowes and to another "handyman" I know, and they told me the window I found was fine. Also, there was nothing in the directions that said it was meant for existing windows only and not for new ones. I tried searching "installation of new windows" on Youtube, Google, etc. and nothing came up that was very helpful. (New project for someone out there?) It makes sense that there should be some sort of "fin" that goes around new windows that stops moisture from coming in, but hindsight is 20x20. Unless there is someone out there with another strong opinion I plan to follow what you said Xsleeper. Remove the shingles above the top 2X4, install #30 felt paper and Z-flashing.