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With the underlayment wrapped around the frames, we next installed the flashing, starting with the bottom aprons. There's an integral rubber underglass gasket on the unit that must be pulled over the top of the apron's rim. A slide-on clip engages the aprons' upturned hems and makes a watertight connection between them. U-shaped aluminum channel flashing fits between units and overlaps the apron flashing by 6 inches. The channels are made for use with all models and, in our case, had to be cut to length. I used a plywood sled on the table saw, rotating the piece to cut one side at a time with a shallow-set 40-tooth ATB carbide blade. This made a clean, factory-grade cut.

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The head and apron flashings have an open hem that engages an interlocking clip, completing the water barrier.

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It's important to draw the rubber curb seal over the top of the apron flashing to prevent leaking.

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Cutting the flashing on the table saw prevents the distortion created using snips. The channel gets cut flush with the heads and is then overlapped 6 inches by the combination head-sill flashing.

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Tricky Access

We were able to install the bottom and middle rows by standing on the roof and on scaffolding set up inside. The vertical perimeter was easy to get to and we step-flashed and shingled as we went. But installing the field flashing was a bit of a challenge. To get access, we rigged up a sledlike scaffold that straddled the installed bottom row.

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Access to the middle row of skylights was provided by staging inside the house.

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On the outside, a pair of planks spanning the installed bottom row.

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The 2x8 planks were attached to the roof framing with 12-inch-long timber screws. Intermediate support runners, secured to the planks with 6-inch bolts, gave additional support.

First, we stood 2x8 "runners" on edge on either side of the general area, running from top to bottom. I had some 12-inch-long hex-head TimberLok bolts (FastenMaster, 800/518-3569, www.fastenmaster.com) left over from another job, which were perfect for anchoring the top ends into the roof framing. I figured the bolts would help prevent the boards from rolling over. For insurance we used two bolts at the top end, about a foot apart.

The runners supported two 18-foot 2x12 planks spanning the windows. Above the truss locations, on top of the channel flashing, we slid shorter lengths of clean, smooth 2x8 under the planks from below, taking care not to scratch the flashing's bronze finish or interfere with the head-flashing installation, which drops 6 inches below the heads. These intermediate supports broke the spans into sufficiently stiff 6-foot sections. We held them in place with 6-inch TimberLoks driven through the planks. The roof had a 6/12 slope, so it wasn't too awkward to work from the planks; any steeper, though, and I'd have added some angled blocks to level the work surface.

We lined up the first middle window above the bottom corner window and used the 4-inch plywood spacers to stack them. The rest of the windows more or less fell into place with the spacers guiding their placement. After finishing the underlayment, we replaced the cladding pieces and installed the combination head-apron flashings. These fit quite snugly and needed to be forced into place with firm pressure. The flashing also trapped the rubber gasket against the frame; it took some finessing to fish out the gasket and place it over the top of the flange where it belongs. The head flashings interconnect the same way as the aprons, with a slide-on clamp.

There was one slight catch in the installation process. On each side of every head flashing is a dimple, intended for a wood screw that holds it down and against the frame. With only a 3-inch space between units and two layers of aluminum to punch through, I couldn't think of any way to install those screws. Instead, I decided to hold the flashing down with 3/16-inch-by-1/2-inch aluminum rivets, drilled through a vertical fin that the flashing wraps over at the top of the unit. I knew the rivets wouldn't compromise the flashing — but I also didn't want to compromise the warranty, so I called the manufacturer's rep, described the problem and my solution, and got the go-ahead. Since the rivets are just visible from the ground, we treated the 6-inch-wide channels the same way.

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The combination head-sill flashings fit snugly in the 4-inch space between stacked units.

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Four small screws hold the finish cladding in place. A right-angle Phillips screwdriver (bottom left) would make good standard equipment in the installation kit, since the author had to run out and locate one mid-job.

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Note the stainless steel rivets — the author's solution for securing the head flashing because there wasn't enough room between units to insert a screw in the dimple along the side.

With the middle row completely flashed, we reset the scaffold for the top row. Initially, it seemed possible that we could install the windows by working from above, but a brief trial got rid of that notion — it was much too awkward and risky.

There were four venting units along the top row — one at each outside corner and two in the center truss bay. After installing four units using the plywood spacers as usual, we sighted a misalignment from down below, when we peered up the channels. This made no sense at first, but we found that, unlike the fixed windows, the venting units allow the frames to rack out of square. Good thing we used screws. We pulled three windows, realigned the first corner unit, and the rest went according to plan.

There was a masonry chimney to the left of the top row of windows, only about 12 inches away. I peeled back its lead flashing, wrapped the brick with underlayment just as we'd done with the roof windows, and tapped the lead back down. Roof shingles and 30-pound asphalt paper provide separation between the two flashings, in case aluminum and lead aren't compatible.

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The 18 roof windows are a dramatic improvement — in both appearance and performance — over the original translucent plastic.

Eight months and several cold, wet, windy storms later, the installation is leak-free (Figure 8). When the customer is ready, we'll trim out the interior. Given the precise alignment between units, that job should go like clockwork.

Dave Holbrook is a builder on Cape Cod, Mass., and a JLC associate editor.