How To Flash A Window

Lots of people have problems where the wood has rotted out around their window or their drywall is stained. Most of the traditional methods of flashing a window have failed. If you want to make sure your window doesn’t leak, use a flexible flashing tape to make sure no water ever gets behind the window.

One reason why housewrap is not a perfect water barrier is because, if you put siding on your home, you will put a couple hundred holes in the housewrap when you install your siding and other exterior cladding products.

You want to keep water on the sheathing. If it stays on the sheeting, the sun comes out, it drys up, and everything is fine.


Remove Housewrap From The Rough Opening Of The Window


Cut ALL the house wrap out of the rough opening of the window. DON’T let the housewrap wrap back inside on to the wood, and don’t let the housewrap fold over the sill.

Remove 4″ Of Housewrap Off The Jamb Sides Of Window


Cut the housewarp back 4″ on the jamb sides of the window (left and right sides) so 2″ of substrate will be visible once you install your window.

Trim Off Upper Corners Of The Housewrap


(1) Make a 45 degree angle cut that is about 8″ long from the corner of the window frame.
(2) Trim off the bottom piece so it lines up with 6″ vertical cut we made on the jam sides of the window.

Stick Headlap Of Housewrap Up With A Small Piece Of Flashing Tape


Prepare Your Flashing Tape For Installation

It is a great idea to cut all of your flashing tape before you get to your jobsite. This will alleviate the hastles of trying to cut on the job, dropping material in water, wasting time, etc.

You are going to use:
– 6″ wide tape for the top
– 6″ wide tape for the sides
– 9″ wide tape at the bottom

This is a modified ARMA spec. We don’t think you really need 9″ tape anywhere but the window sill. The spec was originally writen years ago to accomidate felt and metal, and it needs to be changed to accomidate flexible flashing membranes.

Figure Out The Length Of Flashing Tape You Need For The Sides

To figure out what lenght of tape you need to cut for the sides you can use this equation:
Length of Tape At The Sides = 5″ at the Top + Rough Opening Height + 6″ at the Bottom

In the example in the video we have a 36″ tall rough opening. These are the lengths of flashing tape we used:
-Lenght of Tape At The Sides = 47″ [because we have 5″ + 36″ + 6″]

Prime The Area Where The Flashing Tapes On The Jamb Side Will Be Installed


Priming is optional BUT IT IS HIGHLY RECOMMENDED. Allow the primer to flash off for 2 to 3 minutes before sticking down your flashing tape. Our experience has shown that, whether it’s warm or cold, you’re using waxed or smooth substrate, you should always use primer.

Stick The Flashing Tape To The Sill


Use a 9″ piece of flashing tape for the sill. This allows you to stick 3″ on to the sill and let 6″ hang. Remove 1/2 of the flashing tape liner using the split release liner. If you don’t have a split release liner, try to cut just far enough through the liner so it doesn’t puncture the tape. Stick the half of the flashing tape without the liner to the window sill.

Why did we leave the release liner on the bottom half of the flashing tape? We do this so that any water that comes down over the jamb side of the window will be directed over the top of flashing tape and on top of the siding.

As you are installing your siding, the top edge of the last piece of siding (before we reach the flashing tape area) will go underneath the flashing tape. The tape will get stuck to the siding when the siding is installed.

To figure out the length of tape you need to for the sill, use this equation:
Length of Tape For Sill = Height of Flashing On Left + Rough Opening Width + Height of Flashing On Right

In the example in the video, our flashing reached 6″ up our window jamb.

Roll Flashing Tape Up The Jamb


Make sure you roll the flashing tape really tight into the jamb. If you don’t, your window might not sit correctly and the drywall installers will have to cut the tape in order to fit their drywall into the corner.

Cut Flashing Tape Away From The Jamb And Stick Bottom Part Of The Tape To The Wall


Roll The Flashing Tape Into Place


Put A Butterfly On The Corner


Install Your Window


Stick Flashing Tape To Both Sides Of The Window Jamb


Remove the the first few inches of the flashing tape’s release liner, and stick the tape 5″ above the rough opening. Work your way down the rest of the tape, removing the liner and sticking the tape to your substrate and the housewrap. Make sure you roll the flashing tape in place.

The Areas Around The Window That Have Been Sealed By The Flashing Tape Are…


1) Window Flange
2) 2″ Of Substrate (in this case OSB)
3) Housewrap

Install The Tape Across The Top Of The Window


To figure out what lenght of tape you need across the top of the window, use this equation:
Length of Tape For Top = 7″ + Rough Opening Width + 7″

Why are we using 7″ on each side? 7″ will allow us to overhang the flashing tape on the side of by 1″. And since we started the side flashing tape up 5″ from the rough opening, we have also overlapped the top of the side flashing tape by 1″.

Make sure you roll your flashing tape into place after you stick it down.

Lower Headlap of Housewrap and Place 6″ Pieces Of Tape On Headlap Edges


Install Nails At The Top Of The Side Flashing Tapes


These are only used as a precaution to make sure the flashing tape doesn’t unstick at the top. Flashing tapes are self healing, so they will seal around the nail.

Final Flashed Window


We’ve flashed 6″ (which is sufficient) on the side and top of the window and 9″ on the bottom (which allows us to have a 6″ sill projecting out). We now have a window that won’t leak and let any water or air around into it.

Things to remember:
– Make sure you prime your substrate before trying to stick your flashing tape.
– Roll your flashing tape after you stick it.
– Precut your material before you get to the job.
– Use the split release tape if you have it.

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