How to Pre-assemble Exterior Window Trim

Installing Pre-Assembled Exterior Window Trim SurroundPre-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 MethodPiece-by-piece installation is inefficient because youҲe 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 sillWhen measuring horizontally, measure the furthest protruding part of the window. Take your measurement point to point to outermost point ֠and ADD 1/8Ԡplus the thickness of your trim X2 to that measurement. This will give you the total length of the top trim and windowsill. The 1/8Ԡmeasurement gives some adjustment room for fitting and sealant.Vertical Measurement ֠cutting legsWhen measuring vertically measure the furthest protruding part of the window. Take your measurement Point to point to the outermost point ֠and ADD 1/8Ԡto 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 ӒabbitԠChannel for Nailing FlangeYou 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. Itҳ 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 ???ider than the nailing flange and as 1/16-1/8Ԡdeeper 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 ConsiderationsExterior PVC sill can be bought or made on-site; we chose to use a pre-made sill for this project. Windowsills typically have a 10ֱ5-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 ProcessNote - 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 StepsDrill 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, theyҲe 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.SandingAfter assembly, sand off the glue, or any unevenness at the vertical leg and head trim joint. Attaching the WindowsillWith 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 TrimSlide the trim assembly over the window frame, evenly splitting your reveal gap. Install 2ԠCortex hidden fasteners every 16-20 inches apart. The Cortex fastener bit installs the screw and creates a perfect hole to insert a PVC plug.