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Assembling and Installing Exterior Trim

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Flashing and Trimming a Window

Assembling and fastening the trim

Flashing and Trimming a Window

Assembling and fastening the trim

  • Figure 8. The author uses a track saw to create exterior trim of any width from 34-inch PVC sheet stock.

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    Figure 8. The author uses a track saw to create exterior trim of any width from 34-inch PVC sheet stock.

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    Emanuel Silva

    The author uses a track saw to create exterior trim of any width from 3/4-inch PVC sheet stock.

  • Figure 9. The exterior sill is built up from two thicknesses of material fastened with cement and screws; the screws are spaced back from the edges to avoid contact with the saw blade when the stock is ripped to the required angle (A, B). The countersunk screw holes are filled with manufactured plugs and cut flush (C).

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    Figure 9. The exterior sill is built up from two thicknesses of material fastened with cement and screws; the screws are spaced back from the edges to avoid contact with the saw blade when the stock is ripped to the required angle (A, B). The countersunk screw holes are filled with manufactured plugs and cut flush (C).

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    Emanuel Silva

    The author fashions the exterior sill from two thicknesses of PVC material fastened with cement and screws; the screws are spaced back from the edges to avoid contact with the saw blade when the stock is ripped to the required angle.

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    Emanuel Silva

    The author rips the sill stock at an angle to promote water run-off.

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    Emanuel Silva

    The countersunk screw holes in the built-up sill are filled with manufactured plugs and cut flush.

  • Figure 10. The backs of the prepared sill and casings are drilled for pocket screws (A). Facing surfaces are then coated with PVC cement and temporarily clamped together while the screws are driven (B). Next, the completed trim assembly is drilled for the deck screws that will fasten it to the framing, and caulked on the back before being lifted into place (C, D).

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    Figure 10. The backs of the prepared sill and casings are drilled for pocket screws (A). Facing surfaces are then coated with PVC cement and temporarily clamped together while the screws are driven (B). Next, the completed trim assembly is drilled for the deck screws that will fasten it to the framing, and caulked on the back before being lifted into place (C, D).

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    Emanuel Silva

    The backs of the prepared sill and casings are drilled for pocket screws.

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    Emanuel Silva

    Mating surfaces are then coated with PVC cement and temporarily clamped together while the screws are driven.

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    Emanuel Silva

    The completed trim assembly is drilled for the deck screws that will fasten it to the framing.

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    Emanuel Silva

    Beads of caulk are applied to the back of the trim assembly before it's lifted into place.

  • Figure 11. Once the trim unit has been tacked in place and leveled, the screws are driven home, starting at the bottom (A, B). A PVC trim band covers the outer row of screws (C, D); the remaining holes are plugged.

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    Figure 11. Once the trim unit has been tacked in place and leveled, the screws are driven home, starting at the bottom (A, B). A PVC trim band covers the outer row of screws (C, D); the remaining holes are plugged.

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    Emanuel Silva

    The trim unit is leveled and  tacked in place.

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    Emanuel Silva

    After all adjustments are complete, the screws are driven home starting from the bottom.

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    Emanuel Silva

    An outer trim band is applied to add detail and cover the screws along the outer edges of the casing.

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    Emanuel Silva

    Countersunk screw heads not covered by trim will be plugged.

I cut the sill, head casing, and side casings from PVC sheet stock, preassemble them, and install them as a unit. Starting with sheet stock (rather than dimensional material) gives me the option of cutting my trim boards to any width I need without being limited to nominal sizes. Sheet stock is easier to transport between jobs because it comes in bigger pieces.

It’s not easy to push full sheets of material through a table saw, especially when I’m working by myself, so I make the initial cuts with my TrueTrac track saw, which is easy to set up and break down. Once I have my stock cut to width, I pull the length measurements directly from the installed window to make sure they’re accurate. I often jot them down on the back of my tape-measure blade as I go, then rub them off with my thumb after I’ve made the cuts.

I build up the sill from two pieces of 5/4 stock fastened together with glue and screws. After ripping the edges to the required 10-degree angle and cutting a shallow dado along the underside to act as a drip edge, I clamp the side casings, sill, and head casing together and predrill them for pocket screws. Before assembling the unit, I put a screw in each hole to speed things along. Glue-up is fast once all the pieces are ready — I coat the mating surfaces with PVC cement and use a pocket-screw clamp to hold each joint together while I drive the screws home. The side casings are fastened to the sill first, then the head casing.

To fasten the trim assembly to the house, I use 2-inch deck screws, arranged in two rows about 8 inches apart within each row. I drill and countersink the screw holes and insert the screws, then turn the unit around and apply a good-quality caulk to the back just before installation. The head and side casings each get a double bead, while the narrower sill gets a single bead.

Once I’ve tilted the unit into place, I tack it there temporarily and check it for plumb and level. When I’m satisfied, I start driving the screws from the bottom and work my way up. I then lay the level flat against each casing to make sure it’s not bowed in or out by a localized dip or hump in the sheathing. If it is, I back off on the screws to allow the unit to shift position as needed.

The outer row of screw holes is covered by a separate band molding, which I assemble from three pieces of PVC stock that are mitered, glued, and pinned together at the corners with stainless-steel finish nails. I use more cement and the same nails to fasten the molding to the casings. The last step before paint is to cement PVC plugs into the remaining screw holes and cut them flush with a sharp 18-point handsaw.

Emanuel Silva owns Silva Lightning Builders in Andover, Mass.