Apron Flashing that Abuts Bottom of Window Sill on Dormer

Hi guys,

I’ve got a dormer wall in which the bottoms of the window sills are only 1 inch above the adjoining roof. I have apron flashing installed where the roof and dormer meet, with 6 inches on the roof and the remainder on the wall. On the parts of the wall where no windows exist, this results in 4 inches of flashing on the wall and siding will be installed over the top three inches of it. However, as mentioned, where there are windows, there is only 1 inch of space between the bottom of the window sill and the roof. Given that this was a reroofing and siding job, and the windows were pre-existing, I had to cut out some of the wall flashing so that the flashing would fit under the window. Because there is only 1 inch of space, there won’t be any siding or house wrap below the window sills to serve as a type of counter-flashing.

So, I’m wondering if I should I caulk the flashing edge under the window sill to block any moisture that might seek to enter there?


I hope not. I can run 5"-8" of metal straight under the sill. I’ve only caulked a very few that had bad gaps deep under there. If you do caulk, try to get the bead 3" under the sill. Caulk doesn’t water or weatherproof anything, but it will act as a dam if any water gets blown under.

I clip a bit off the sill bottom so a step flashing will still have some side at the bottom.[attachment=2]Tight Dormer Corner (2).jpg[/attachment]

I run the apron 8" of more past the outer dormer corner so I can nail it without any exposed nails. This one has NO upturned sections.[attachment=1]Tight Dormer Corner (3).jpg[/attachment]

Here’s the bottom step-flashing before it goes in.
[attachment=0]Tight Dormer Corner (4).jpg[/attachment]

Here’s the installed step-flashing. Metal goes in the corner down the edge and over the step. No caulking necessary. It shouldn’t ever need to be used on a dormer.[attachment=2]Tight Dormer Corner (5).jpg[/attachment]

[attachment=1]Tight Dormer Corner (7).jpg[/attachment]

This is the other side, and it seems to be the preferred way to do these by ‘roofers’. As you can see, I operate a tad differently and not many agree with my ways. Your choice.[attachment=0]Tight Dormer Corner (8).jpg[/attachment]

Of course, you can ask yourself why I was making repairs if the other way really is better.

Thanks for the thorough, detailed reply tinner666. I believe I’m following you. But, if I am, my situation might be a little different. Now that I’m home, I’ve taken a picture that hopefully shows my situation. Please see the following picture. Under the housewrap (in between the two windows), I have four inches of flashing going up the wall of the dormer. However, below the window sill, I only have one inch of flashing, going up the wall (I had to cut it to fit it under the sill, as there wasn’t room for four inches going up the wall). And, because you are seeing the wall, under the sill (even if you are only seeing one inch of it), I’m unable to slide the flashing under the window.


So, I’m wondering if I should try to caulk that area where the cut edge of the flashing touches the underside of the sill? Hopefully this better describes my question.


Make the window pieces 8" longer than the sills. Notch it so 4" is up on each side, 1"± under the sill. Bend it a bit less than it needs for an exact fit and ‘pop’ it into place. You’ll be fine.

Since those are existing windows, I’d then use a roofer’s bar (Malco I-Beam) and get a gap behind the brickmold. Two choices for the next step. Cut/make the flashing for the area between the windows about 1/8"-1/4" shy of an exact fit. Set it in. I prefer nailing ‘wind-tabs’ to the roof 1"-2" above where the apron will sit. Then, place the apron and bend the tabs back over the face of the apron. No exposed nails.[attachment=0]Patterson and Tilden (4).jpg[/attachment]
Use a 4" by 8" piece of flashing behind the brickmold to to completely close the corners. No caulk needed.
Option 2; Make two pieces to go between each window, both 6" longer than 1/2 the distance. Slide one in behind the brickmold of one window, the other behind the brickmold of the other window give you about a 10" lap in the center. Again, no caulk needed.

Hope this helps.

My bad. I said “No caulk needed”, I should have so “No exposed caulk needed” When lapping, I’ll run a bead, or even two on top of the first piece, about an inch from where the overlapping piece will stop. Just keep it hidden. The caulk is only there to disrupt any capillary/wicking of the water. It isn’t necessary, but it never hurts to hedge your bets.
After the metal is in place, never caulk the visible lap. It just causes the water to run into the joint instead of keeping it out.

You might want to peruse my window/siding pages for more insights on properly flashing that wall and windows.
Window Flashing

[quote=“tinner666”]You might want to peruse my window/siding pages for more insights on properly flashing that wall and windows.
Window Flashing[/quote]

Geat lesson on how to properly “turn the corner” on dormers!

Tinner you have to love the clips,makes for such a nice clean install.

It looks much better than nailing the hell out of it and smearing goo on the nailheads.

Thanks tinner666, I think I’ll go that route of “popping” it in place! Plus I like that idea of “step-flashing the siding”. I may give that a go.