Flashing Window Openings

In this article we will outline how to install metal flashing around the windows of your home. There are a lot of tricks of the trade for different situation you may encounter in doing such a job. There are also many variables that could be different, such as the type of window and what type of sill, siding or outside layer will be on the home after the flashing is installed. Flashing window openings can be simple or complex depending on the job and all its details.

For this job, we will assume a basic setup where the house will be finished with vinyl siding after the window flashing is complete. We must stress that because of all the variables possible in flashing window openings, that in many cases, you should probably hire a professional. If doing the installing yourself, read the window manufacturer’s guidelines carefully, or you risk voiding the warranty.

Basic Flashing Instructions

This article includes very basic instructions only and is not meant for every window flashing situation, so if you have any doubts as to how to properly flash in your windows to prevent any water from getting trapped under the siding or even inside your walls, consult a professional before continuing.

Okay, now that we have our words of warning about doing this yourself unless you have some experience in this area, let us continue with a basic set of steps for installing metal flashing around a window in your home. Of course, these steps assume that you are doing so before the siding is put up. If, in your case, this is not so, you will need to remove the siding around the window before you begin these steps.

Since there are four sides to a basic window opening in most homes, we will divide the steps for flashing the window opening into four steps, one for each side.

Four Steps to Installing Metal Flashing

As we mentioned above, we are going to divide this job into four very basic steps. Again, if you are not familiar with what you are doing here, consult someone that is because if you mess up the flashing it will affect the lifetime of both your windows and your walls. Here are the basic steps only:

1) The first thing to do is install the metal flashing along the bottom edge of the window. First cut the metal to the width of the window plus a few inches extra on each side. You have to do this one first because the sides have to overlap the bottom flashing so that water runs over it and not under it and into your wall. When it comes to flashing a window properly, it is all about how you overlap the flashing pieces so that no water gets in anywhere, so pay close attention to how this is done in these simple steps.

2) Now that you have the metal flashing in place on the bottom, you can go to the right side and start that flashing second. First you have to cut the metal flashing to size by starting at the top, but leaving room for the top piece to overlap the side piece in step four, and measuring down to the bottom of the bottom flashing. Then install the piece you cut making sure it overlaps the bottom piece correctly so that water runs over, not into it.

3) Now move to the left side of the window and cut and install the metal flashing on that side in the exact same manner as you did on the right side.

4) Cut the top flashing to the same length as the bottom was and install it over top of the two side flashing pieces from the previous two steps. That way, any water running over the top flashing goes down the sides, over the bottom and then is directed out of the siding after it is installed.


These are very basic instructions and are not appropriate for every situation, but hopefully will at least give you an idea of whether or not you want to do this job yourself or hire a professional window installer to do the job for you. If you have doubts, we recommend hiring a professional just because this is not a very easy do it yourself type job as are many we write about.

This is more of an intermediate to difficult job. Take your time installing your next window and no matter what, make sure it is done right as the integrity of your home is at stake.