# Framing details for traditional exterior window trim



## phil2 (May 4, 2014)

I'm having trouble finding details on how to construct/install traditional exterior window casings and sill in new construction. Specifically, traditional windows have a projecting sloped exterior sill that extends past the casing on either side, rather than just wrapping trim all the way around like a picture frame. (See picture below.)

How exactly are these constructed? Given that most windows have a flange or nailing fin all the way around the outside on the exterior, it seems that the sill would need to somehow be attached over the that. Can you purchase these sills as prefabricated moldings/stock, or do you generally cut your own? 

I've spent a while searching for a good tutorial, video, or other overview, but so far my Google-fu skills are failing me.

Thanks!


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## joecaption (Nov 30, 2011)

What's your plans for the siding on this home?
What do you plan on using for the trim around the window, wood, PVC ?
Installing vinyl or wooden windows?


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## phil2 (May 4, 2014)

Good questions Joe. Trim will be wood or Smart Trim. Siding will be Fiber Cement. Windows will be wood with aluminum exterior cladding.

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## Pittsville (Jan 8, 2011)

Look into block frame windows.


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## phil2 (May 4, 2014)

Thanks Joe. Block frame windows would seem to solve the problem of the bottom nailing fin getting in the way. What about framing details of the sloped sill? Everything I can find about new construction seems to assume you're just nailing the window in using the fins, and everything for replacement windows assumes that the frame and sill already exist.

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## phil2 (May 4, 2014)

I meant to say thanks Pittsville. ;-)


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## woodworkbykirk (Sep 25, 2011)

i do this detail all the time, actually just finishing up a house with it.

my first question that hasnt been asked is if you are using a rain screen, it will effect the process slightly but not by much

heres my method-

1) rain screen buildout around rough opening
first one piece of 1x4 installed flush with r.o sil, length of it is same as r.o width
2) 4 pieces of 1x4 cut the height of r.o plus 3 1/2" allowing for sil piece.. install 2 pieces per side of r.o keeping flush with the top of the rough opening

3) cut two pieces of 1x4 to fit across the top of the r.o and all 4 pieces of 1x4 on the sides, install one piece and keep the 2nd piece off for now

4) install window in r.o. with a proper sil pan flashing and then flash window with peel and stick membrane

trim process starts

1)measure side casing first, measure height of window along the brick mould, paint the cuts to seal the wood and install the side pieces so they are flush both top and bottom or 1/16" strong which will create a expansion gap allowing for movement.. if you desire to you can set the trim into a bead of caulking for an added seal

2) cut the sil stock to length via outside of casing to outside of casing, to install this piece i will use exterior trim head screws and screw up through the bottom of the sil into the side casing along with using a polybutyl type window and door caulking to adhere the trim to the brick mould

3) measure the top of the window outside of casing to outside of casing and add 1" to allow for a 1/2" reveal on both sides. . now set in place.. this is where that second piece of 1x4 furring comes into play.

with the header trim in place measure behind the casing down to the top of the first piece of 1x4 to the top of the trim, do so on both sides . add 1/2" to that measurement and rip a the 1x4 to the new measurement.. this allows for creating a slope on the metal cap flashing that will go on shortly. now nail on the ripped pc of 1x4 then install the header trim.

4) add the crown detail onto the header trim for a proper look the crown should be returned into the wall 

5) bend up and install a metal cap flashing


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## phil2 (May 4, 2014)

Thanks for the detailed description, Kirk. You do beautiful work! I think I get the gist, although there are a few things I'm having trouble envisioning completely. That said, a couple of questions:

It sounds like you're using a typical new construction window with the fins?
Do you make your own sill stock, or is it something you purchase? 
Do you cut the bottom of the side casing at a 10 or 12 degree angle to match the slope of the sill?


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## joecaption (Nov 30, 2011)

I'd also suggest using 5/4 PVC lumber not the other trims suggested.
Making that sill even with the trim on the sides of the window will make install the siding easier and will eliminate an area that may cause future moisture issues.
Another spot that can cause issues if it's not done right and there's not enough of an over hang on the roof is that header piece.
It forms a ledge for water to sit and work it's way in under the trim. 
Adding Z moulding over that header will help keep water out.


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## woodworkbykirk (Sep 25, 2011)

i mentioned the flashing in there joe, with such a larger header and the rain screen if the flashing were flat it creates a 3 1/4" shelf that water sits on, so i slope the flashing by 1/2"., not only that the window unit itself gets flashed with peel and stick before the trim goes on


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