In my article “Installing Engineered Horizontal Siding” (May/15), I wrote that preassembling exterior trim details can streamline the siding process. It can also dramatically improve the quality of the installations. We fabricate door and window trim, corner boards, and sometimes soffits and cornices safely on the ground or in the shop and then install them in one piece.
Window and door trim
Many flanged or block-frame windows have a minimalist “modern” look when installed, and we often dress them up with additional trim. I first measure all but the biggest windows that we’ll be trimming (larger windows are easier to trim on site), and I develop a cut list for each unit.
Because there are often many windows of a similar size on a job, I group them together and cut all the components at the same time. A trim package for a typical window consists of a sill nosing or extension, two vertical legs, a head, and an apron. If the window is plumb, level, and square, then the sill and the header trim are one length, and the two legs are another, so I need to calculate only two lengths for each window.
With a cut list in hand, I set the repetitive stop on the miter-saw stand to the appropriate position and cut all the components to length. To minimize confusion and error, I label each piece as it comes off the saw, before prepping the pieces for assembly.
Pocket-screw joinery
For preassembling trim packages for doors and windows, using pocket screws is the fastest and strongest way to join the pieces. Pocket-screwed joints also stay tight in almost any environment. There are several commercially available jigs for drilling the screw holes. We use a Kreg jig, which places the screws at a 15-degree angle to the face of the board. The jig clamps onto the back surface of the material being fastened and a specialized stepped drill bit with a stop collar creates the holes. We usually place one screw every 2 to 3 inches.
Before the trim is assembled, it’s important to seal any cut ends with quick-drying primer. We also seal the screw holes. Without sealer, exposed fibers can allow moisture to migrate into the material, which can lead to failure of the joint or of the material itself.
We place the pieces for each frame face down on an assembly table and put a bead of sealant in the joints. We clamp each corner firmly to the table to keep the outer faces flush and to prevent the pieces from moving while we drive the screws.
The screws themselves are designed specifically for pocket-screw joinery; they have washer heads and self-tapping threads to minimize splitting and to draw the pieces together. We use fine-thread screws with dense materials like hardwood, and coarse-thread screws with softer woods and most engineered materials used for trim. (For more about fastening PVC, see “Fastening and Finishing PVC Trim,” 09/14.)
We drive the screws using a long square-head driver bit. We’ve found that a cordless drill with clutch settings is the best tool for driving the screws; impact drivers can strip the threads out of the wood or overdrive the screws. We use corrosion-resistant screws for all exterior applications.
Corner boards
Corner boards can be built more neatly and efficiently on the ground. This is especially true for multistory buildings where the walls are not always in the same plane from one floor to the next. Instead of wrestling with individual pieces, trying to get the joints tight, we preassemble entire corners on our cutting table and install them as one piece. Full-length corner assemblies bridge any framing irregularities and allow us to make slight adjustments to the corners of a building, if necessary.
The most common method for building outside corners is a butt assembly, where the two corner boards are fastened to each other along one edge to form a right angle. To make the corners the same dimension on both sides, we first set the blade at a slight angle (2 or 3 degrees) to create a tighter-fitting joint; and then we rip down—by the thickness of the material we’re using—the board that will be less visible. For example, to make 5/4 by 5 1/2-inch corners, we rip the abutting piece to a width of 4 1/2 inches. When the corner is assembled and installed, it will read the same on both sides of the building.
Again, we seal all cut edges with primer before assembling the pieces. Then we run a bead of sealant along the long edge of the ripped piece and bring the two sides together, clamping them as needed. We drill holes for 8d stainless steel box nails, which we then drive by hand. We avoid pneumatic nailers when fastening corners; the dense, engineered material that we use can cause gun-driven nails to drift—and often to exit through the face of the trim.
Another corner detail we sometimes use consists of two boards of equal width. But rather than the two boards overlapping, they just touch each other at their back corners and a piece of 3/4-inch-diameter quarter-round molding fills in the space between the two boards. This type of corner can be installed piece-by-piece or preassembled and installed as a single unit. Preassembly is relatively straightforward, with the two boards joined by trim screws or finish nails driven through the quarter-round molding and into the mating corners. However, the completed corner tends to be a bit delicate.
Regardless of the corner detail we choose, we always steer clear of bevel rips, miters, and scarf joints in the boards. These joints are likely to open and provide a path for water to enter the assembly or to make its way behind the trim where it can become trapped. Instead, we opt for butt joints on all trim that isn’t profiled. Unlike solid wood, most engineered material has no end grain, so exposing the core of the material is fine, as long as it’s properly sealed.
Installation
Once the preassembly is done, we begin the installation, starting with corners and window trim. Because the fasteners are exposed, we always use 8d stainless-steel ring-shank nails. The stainless fasteners minimize the potential for rust streaks and bleeding under finishes.
We nail corner boards in place every 12 inches, staggering the nails between sides. Window trim is fastened with similar spacing, although we take care not to drive nails through the fins on flanged windows, which could restrict the window's movement (particularly on vinyl units) and even cause the frame to crack in colder weather.
For inside corners, we usually use square rips of 5/4 stock. We seat them tightly between the intersecting walls and nail them every 12 inches or so. As with siding, we always pay close attention to the depth the nails are driven. It’s better to underdrive nails and finish them off with a hammer than drive them too deep, which reduces their holding power.
