Window Installation 101- Step 4

Time for exterior trim. This window is on a 100 year old house & we wanted to keep the look somewhat consistent with what is already on the house. For a farm house in the 1920's, in the NorthWest, flat stock is traditional.

I recommend butt-joints whenever possible on the exterior of the home. In this region, we have pretty harsh conditions for wood to deal with. Hot-dry summers with long hours of exposure to UV light (from 5am to 10:30pm at the extremes), and cold, damp winters with constant moisture; along with the occasional fairly severe extreme cold snap with quick onset conditions of very low humidity. This keeps wood pretty confused, and makes keeping a mitered corner looking good in an exterior application nearly impossible.

We used JumboTec 60 minute paper for our vapor retarder. It is an excellent & cost-effective product. When applying, make sure to install it in a shingling fashion.

Like feathers on a bird, scales on a fish, or shingles on a roof; start at the bottom & work up, giving ample room for layering, and making sure to tuck it under the counter flashing at the bottom of the window. Then add another layer of Fortiflash membrane flashing, at the bottom, then the sides, last the top.

Measure your first piece flat stock for the window sill. Here we used 5/4x6" without a true sill. My personal preference for material for painted exteriors is pre-primed finger-joint cedar. It comes with all the traditional qualities of cedar (straight & weather resistive) and is much more affordable than true cedar. The other option in the budget category is pre-primed SPF. This has become an industry standard in our area, but I strongly recommend against it. It is Spruce/Pine/Fir & often finger jointed as well. Over time it tends (on wider stock in particular) to warp and twist. It is does not have the natural properties of cedar to protect against weather over time, either.

Back to measuring: first the bottom of the window, then the sides from the top of the window to the bottom of the piece of wood being used as the sill (window + 5 1/4" in this case), then the top outside trim to outside trim (window + 10 5/8" in this case). While installing, "blind-caulk" the bottom trim. This is a bead of calk on the back-side of the wood set against the wall. This is a secondary measure preventing water from getting behind the wood. Keep your nailing regular & just beyond flush, so there is a small depression to receive caulking afterwards. You can use a pneumatic gun, or hammer & nails. Either way, use hot-dipped galvanized 7 or 8 penny nails (2 3/8" X .113 for a gun). If you are working on the coast, upgrade to stainless steel nails for ALL siding applications! Install the sides, blind-caulking the top edge, and finally the header (lintel)- blind caulking it on the upper edge as well.

The original house had a 2x2" band around the exterior window trim, providing a thickened edge for the shingles to meet. We accomplished this with some 2x2 that we rabetted to cap the 5/4x6. These we mitered, & they will hold up over the years due to: its small & stable size, it is cedar, & glue. Get the rabet's done first: we took out 1" x 3/4". Then cut to size, remembering that a bit too long will be to your advantage (like 1/16" to 1/8"). Glue & nail the corners together & then fasten to the wall. Again, keep the nails regularly spaced. This looks more pleasing & also helps find the nails if remodeling is needed in the future.

We then installed the sidewall shakes. We used 24" pre-primed #1 shakes with a single course 12" reveal. Remember to double up the first course.

You must leave gaps, approximately 1/8" to 3/16" at the edges. Also, remember to layer, & try to keep the sizing random. The trickiest part is weaving the corners, but with some patience you can get the 100 year old look (no corner boards). The biggest pointer is to slightly over-cut & then rasp-plane down to flush with the other piece. You need to remember to keep things running level; snap lines with your chalk box on a small project like this. It's not the quickest way in a production setting, but it works well in tight small spaces.

Now caulking around the trim & window & you are ready for paint! The caulking should set up for almost a month, but at least give it a full 24 hours.