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Joe1. 08-13-2010 09:26 PM

Dormer flashing and roof trim
Hello guys,

We're building a set of gable dormers, and the sheathing is done and I'll be adding trim to the sides. It will be a boxed cornice with a crown around it basically, and the cornice with crown will dead into the roof. Now we're having open valleys (copper) and step flashing. My question is, when the crown trim deads into the main roof, how should that be handled. Should the valley copper be cut to overhang the trimmed area a little, so that the crown and cornice tuck under it. If that's the case, do I have to worry about leaving a space between the crown and roof? If it's under the copper valley a little, then it should be okay to run it directly to the roof shingles right.

I should say, the roof is being done by the roofers before I do the trim. I know it's probably best to do it before the roof, but I'm not going to be able, I don't think. Anyway, I'm allowing for the crown width, but I just want to make sure the roofers do what they need to do beforehand, so there's no issues with the timming afterwards.

As far as the rest of the siding goes on the dormer, I should cut the siding and pine corner posts so that they rest a half inch-3/4 off the surface of the shingle, right? Although it might look a little funny not to go directly to the shingle, I suppose it's more correct to do so.

Lastly, for the roof trim (the crown molding), what's a good drip cap/edge to put on, one that won't detract from the crown molding too much?

Thanks in advance,


Lulimet 08-13-2010 10:57 PM

Step flashings should be installed before the crown goes in. And yes, the copper valley should overhang a couple of inches. Your roofers should know that (assuming they actually know what the hell they're doing :laughing:)

Joe1. 08-14-2010 09:00 AM


Originally Posted by Lulimet (Post 485470)
Step flashings should be installed before the crown goes in. And yes, the copper valley should overhang a couple of inches. Your roofers should know that (assuming they actually know what the hell they're doing :laughing:)

Well, you know what happens when you assume, especially in construction....:)

Okay, so the step goes under the crown, and so before it, but how should it be treated when working around the last rafter tails, very close to the roof. The last rafter tails are only a few inches from the corner and an inch or so off the sheathing (enough for the shingle). Should the step flashing be bent to sit against the dormer wall and then turned up to go under the rafter tail? Or do they just cut the space around the rafter tail, like a notch (that doesn't sound right, though). My framer originally left no tail, just plumb cutting the rafter, but then there would have been no crown support for the last 18 inches or so, as well as no ply wood support for the roof.

Anyway, I'm sure they know what they're doing, but I don't like to take any chances.

Thanks for the reply,

Joe1. 08-14-2010 09:19 AM

Also regarding the drip cap on the dormers, is it okay if I have to shim out my crown, say .5", in order to maintain an even overhang. The plywood has already been cut to its current position, so moving it will create a slight gap between the sheathing and the crown (I'm running the crown so that it sits flush with the ply). Is this a problem if there is a drip cap?

I know ideally everything should be as tight and sealed as possible, but I may have to create a small gap. Should the drip cap be fine for protecting this?

Now, without a drip cap (say if I don't use one), would a shingle overhang be sufficient to protect it as well?


Lulimet 08-14-2010 09:49 PM

What is a drip cap? Drip edge metal?

Joe1. 08-14-2010 09:59 PM

Yeah, I guess the proper term is drip edge. But the type I want to use resembles the drip caps for doors/windows, since I do not want to detract from the crown too much. I just want a simple L-shaped edge.


Lulimet 08-15-2010 02:36 PM

You can buy pre-made L shaped edge metal. I believe it's called C3.

Joe1. 08-16-2010 11:37 PM

Hi Lulimet,

I'm wondering, can I install the drip edge afterwards, so as to avoid holding up the roofers while finalizing the trim I'm using? I read somewhere that it can be just slipped under with roofing cement. What do you think?



Lulimet 08-17-2010 10:24 AM

You could do that but I recommend lifting the first course of shingles and use nails instead of cement.

Joe1. 08-17-2010 11:45 AM

Thanks for the reply. Yeah. I could lift it and then nail it. That shouldn't be a problem.

Now, I'm trying to decided about the length of shingle overhang. I'm thinking that with the drip I could make it 3/4 sticking out (that is past the drip cap). Does that sound like too much. Perhaps I should just make it 1/2? I don't want to take away from the clean look of the dormer trim by making it overhang too much.

Lulimet 08-17-2010 11:48 AM

Most roofers say 1/2" - 1" so you should be good with 3/4".
I left 1" all around when I did my roof.

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