How To Pre-assemble Exterior Window Trim

Pre-Assembled, Exterior, Window Trim Surround

Pre-assembling trim is faster, more efficient, neater, and produces near-perfect joints every time. This approach does require a completely different way of working, but the advantages are worth it.

Pre-Assembly vs Piece Meal Trim Method
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Piece-by-piece installation is inefficient because you’re always running back and forth between your miter saw and wall, or up and down a ladder or scaffold. Plus, any imperfections, on the nailing flange or unevenness in the wall, translate into uneven/open trim joints.

Measuring and Fitment

Measuring pre-assembled trim should always be done off the window itself. You can accomplish this on the ground, prior to installation, or after the window is installed. When feasible, I prefer the former method.

With multiple, same size, windows, this method can become an efficient, fast, best practice way to finish a project's exterior window trim.

Horizontal Measurement - cutting head trim and sill

When measuring horizontally, measure the furthest protruding part of the window. Take your measurement point to point to outermost point – and ADD 1/8” plus the thickness of your trim X2 to that measurement. This will give you the total length of the top trim and windowsill.

The 1/8” measurement gives some adjustment room for fitting and sealant.

Vertical Measurement – cutting legs

When measuring vertically measure the furthest protruding part of the window. Take your measurement Point to point to the outermost point – and ADD 1/8” to that measurement, this will give you some adjustment room for fitting and sealant. This measurement is the length of your window trim.

Cutting Sill Bevel and “Rabbit” Channel for Nailing Flange

You will need to determine the bevel cut of your windowsill and cut the bottom of each leg trim to that level.

Pro Tip: Prior to cutting your rabbit channel mark the back of the leg parts to avoid errors. It’s easy to flip a board around and cut a bevel or a rabbit channel on the wrong end or edge. I mark the back the trim bevel end and rabbit edge to avoid these errors.

Cut a rabbit channel on the back of the trim by making two cuts on a table saw. The rabbit width cut is usually ¼” wider than the nailing flange and as 1/16-1/8” deeper than the thickest part.

This rabbit creates a space for the nailing flange so the trim can sit tight to the wall sheathing, without rocking or interference.

Windowsill Considerations

Exterior PVC sill can be bought or made on-site; we chose to use a pre-made sill for this project.

Windowsills typically have a 10–15-degree bevel to drain water out and away from the window. The sill should project out away from the window, leg trim, and siding for water management, off and away, from the house.

The bottom edge of the sill should have a drip groove installed to break the surface tension of the water and prevent the water from running back to the house siding.

A rear groove under the backside of the sill allows for house siding to tuck up and under the sill for overlapping weather coverage.

Pre-Assembly Process

Note - You should always use PVC cement adhesive to secure the joints and prevent separation or visible gaps between the boards. The best practice to keep your joints tight, and boards secure is to use a combination of glue and mechanical fasteners.

Drilling And Fastening Steps

Drill two pocket holes into the tops of each window leg casing.

Apply PVC cement to both parts being fastened and allow the glue to become tacky. The glue melts the PVC creating a permanent, new bond to each other.

Align the vertical leg to the top casing and clamp so the joint is flush. Once your pieces are aligned, install the pocket hole screws. Repeat the process on the other end. The screws hold the joint tight, without the need for clamps.

Note - Use pocket hole screws are rated for exterior, they’re easy to spot as they are blue in color.

Pro Tip: if you have a pocket screw blow out because you drove it too far, leave it in place until the PVC glue has dried. Once dry, back out or remove that screw and fix it on the sanding stage.

Sanding

After assembly, sand off the glue, or any unevenness at the vertical leg and head trim joint.

Attaching the Windowsill

With the window trim assembly back side facing up, line up the back of the windowsill with the pre-cut nailing flange channels on the vertical legs. Clampdown the leg casing and drill 2 pilot holes through the sill into the leg casing. Use exterior screws, long enough to penetrate the leg casing by an inch, preferably more.

Installing The Pre-Assembled Trim

Slide the trim assembly over the window frame, evenly splitting your reveal gap. Install 2” Cortex hidden fasteners every 16-20 inches apart. The Cortex fastener bit installs the screw and creates a perfect hole to insert a PVC plug.

[cornpop]

This is one of the most precise and well-done videos I've seen on ANY subject.

Great job!

[davidmurphy]

I’m not a carpenter but it’s so interesting to see the craftsmanship in building a quality home.

[redneckcreations]

Looks like a project on my summer to do list!!!

[jackjohnson]

Time for a new hat. :-)
Go DAWGS!

Thanks for continuing to provide quality and useful content.

[garyg]

Great attention to detail.I enjoy your series..Best wishes in 2022

[thecubanflyer]

Hi! This comment is not relevant to this video but I honestly think that this company/channel is great. I’ve managed to upgrade a lot of my own tools and learnt a lot of different techniques for when I’m working! Keep up the great work! 👍🏽

[poman]

Wow Rob thanks, great techniques and as you said huge time saver and very precise!!!

[ConcordCarpenter]

Guys,
please check out the new shirts we made. There at the bottom of the video. Thanks ROB

[GSchrades]

Love pre-assembly. I used a few different products for Azek but the concepts are pretty much the same. Saves a lot of time and its easier to install and caulk

[jncn]

As a framer, we have done both pre assembly and at the window installation. Definitely not faster pre assembling. What is faster is cutting all the parts at the mill and making kits, stool, sides and top and a quick wrap of tape around that window kit.
We don’t use that glue as it does melt the trim and needs set up time. We use “ White Hot” to assemble pvc ( azek) window parts or exterior trim, it gives you 20 sec to get the fit right before setting. For quicker set time use “2P10” and the accelerator.
We only use stainless trim head screws for installation on the sides and top and 3” stainless thru the stool. Multiple painters and builders have said the plugs don’t stay or set well, it’s better to putty and paint all exterior trim. PVC or azek trim can yellow over time so painting is still needed for best results not plugs.

[irishlordgaming]

Great for standard shaped windows but does the same principal apply to irregular shaped windows? Im replacing 45 year old rotted wood trim with PVC and I found your video while seaching for tips on an octagonal shaped windows (22.5° cuts, what a joy). The trim has an inside edge length of only 8½" (so lots of little pieces). The PVC I found for window trim at Lowes was limited so I've found an edge 11/16 x 2 3/8 to replace a thicker 1.25 x 2. With the limited thickness, I don't think think the pocket screws will work but those Cortex screws are absolute GENIUS!

[davetaylor]

Lovely system. Looks good and would definitely save time. Shame we have nothing like it in Australia. Mostly aluminium windows. They say one style suits all which it doesn't. Just poor designed windows mostly.

[bobbytwitchell]

Recently installed PVC trim the same way and it worked great! I had two questions when I attached the pvc trim and wonder if you could help? First do you apply z head flashing to the window and trim? I have seen it both ways. Second, you mentioned adding sealant… do you add sealant to all the gaps between trim and window? Just vertical gaps? Vertical and head and leave the sill open for water to escape? With all my research and checking through what the manufacturers recommend i wasn’t able to get a straight answer and wonder what you find the best practice to be!? Thanks for the help!

[Jonnyvids]

Nice work.

Does anyone know how Cortex screws compare with just shooting 15 ga. finish nails with a nail gun? Seems like finish nails would hold just fine, then fill holes with white caulk?

Trying to decide if screws are overkill to hold casing.

[prototypea]

What do you recommend if you can't use PVC. Trim on our house is painted very dark and with the climate here, we can't use it as the PVC would get very hot and expand too much.

[persistentone]

To make sure I understand, this approach requires you to ship your windows to your trim carpenter so that the exterior trim can be fitted and the flashing done in accordance with that manufacturer's requirements? Then the windows and trim get installed on site together?

[ozziestrom]

I have no problem using hot glue, the only peeve I have is the sill only comes comes 7ft long and I have to splice together to extend for double or longer windows with mullions. What would you do?

[jaylopes]

Can you install the trim on a window that is installed with the vinyl siding already installed?

[dannymeil]

What do you think about the Dewalt dcb208 8.0 ah battery

[jackofmosttrades]

What length pocket hole screws are you using?