Sunday, March 6, 2011

Window Drip Caps

One of the things that building inspectors are fussy about is the flashing ... or drip cap...  that goes above the windows.  The purpose of the drip cap is to force water out and away from the top of the window; or more specifically, away from the joint where the top of the window meets the wall.  Here, the building code requires that the drip caps also have end dams to prevent water from running down the side of a window as well. 

The drip cap stock comes preformed in 10' or 12' lengths in either aluminum or vinyl.  I'm using vinyl because I could get it in the same color as my corner posts and soffits.  That's the easy part.  The hard part is making the end dams.

Here's the drip cap stock as it comes from the lumber yard.

I begin my making two cuts 3/4" from the end; one in the front lip, and one in the back.  I use tin snips to cut it.

Next, I clamp a small piece of thin plywood on each side of the flat part of the drip cap... with the edge of the plywood lined up with the cuts I made.  Now I heat up the end with my heat gun.

After 30 - 45 seconds with the heat gun, the vinyl becomes very pliable.  I bend the end upwards to almost a 90 degree angle, tucking the vertical flap around behind the back.  The edge of the plywood helps me get a nice clean bend.

This is looking at the bottom side.

The next step is to put some silicone sealant between the flap and the back of the drip cap, and then clamp them together.

Next, I put in a couple of rivets to hold the flap and the back together; and lastly I trim off the lip and round off the top corner on the end.  Now I have a finished end dam.  To finish the drip cap, I just cut it to length and then repeat on the process on the other end.

Here's the completed drip cap in place.  The front lip of the drip cap protrudes past the trim by about 3/8".  That protrusion forces water to drip away from the surface of the trim; thereby helping to prevent water from getting behind the trim by capillary action.  



  1. wow. I guess I can see the benefit of having end dams on the flashing, but it seems like that's gonna throw a major wrench into the gears when you start putting on the siding. I guess I'll just wait and see what you come up with. Good luck.

  2. Hi Reuben. Hey, thanks for the comment. It's all about planning (with perhaps a little bit of luck thrown in). The siding should work out so that the bottom of a row of siding sits just above the end dams.