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Last summer, a homeowner here on Cape Cod called to ask if I'd look at a large skylight he wanted to replace. The house — which he'd bought earlier that year — was an earth-sheltered, passive-solar structure dating from the late 1970s. On its south-facing roof was a site-built 12-by-16-foot skylight glazed with three layers of translucent, fiber-reinforced plastic sheets separated by stacked 2x4 framing. It leaked, and the sheets had become too yellowed to see through. I was surprised it had lasted so long without blowing away.

My client, however, loved the dramatic size and the interior daylight. He wanted to replace the entire skylight area with manufactured roof windows, which would be both transparent and presumably more energy-efficient. I liked the idea and agreed to help design and install the replacement.

Layout and Framing

The unusual 70-inch-on-center truss spacing created three basic bays that I would need to subdivide into smaller mulled openings. The trusses measured 5 1/2 inches wide, leaving a 5-foot-4 1/2-inch space between them. I found that two Velux 306 roof windows ( — with a unit size of 30 9/16 inches wide by 46 1/4 inches high — would fit side-by-side between the trusses, with just enough room for a double rafter between them. In fact, the two units' combined width of 61 1/8 inches left only 3 3/8 inches — not quite enough space for the framing and the recommended 1/4-inch rough-opening clearances. But because the units would be deck-mounted, not inserted into the openings, I figured I didn't really need any wiggle room provided the framing was meticulously square.

Referring to installation information on the Velux Web site, I drew a layout schematic. The final design contained a total of 18 windows — six across stacked three high, with alternating 3-inch and 6-inch spacing in between. Fortunately, the side-to-side flashing options allow for spacing from 2 1/2 to 12 inches wide in 1/2-inch increments. Head-to-sill stacked spacing is restricted to 4 inches. Along the top and bottom, about 12 inches and 6 inches, respectively, are needed to accommodate the head and sill flashings.

Before providing a quote, my vendor faxed the drawing to the Velux distributor to verify my assumptions. In response we received a confirmed list of all the proprietary flashing components needed. In all, we'd be using 14 fixed units, four venting units, and a slew of proprietary flashing components, at a cost of just under $7,000. On delivery, we were buried in cardboard boxes.

After tearing out the old skylight, we found that the existing opening was within 1/8 inch of square — a great start. To ensure that the new framing would be accurate, I made story sticks showing the unit dimensions and the flashing spaces between them, one for the horizontal layout and one for the vertical layout.


The original rough opening was almost perfectly square.


A lucky start on a project that demanded accurate framing.

We used 2x8s for the openings, to match the existing built-up insulated roof. The framing rested securely on the trusses and the exterior bearing wall below. After a couple of days of prep and framing, we were ready to install the skylights.

Installing the Units

On the ground, we prepped the units by removing the aluminum cladding as instructed and folding the installation brackets out from their shipping position. The bracket locations were already offset from one side to the other to avoid conflict when combining units. We snapped a chalk line square and parallel to the opening's bottom edge and screwed the story stick down alongside it. The stick supported the first row of units and guided its placement. We started arbitrarily at the lower right corner and worked to the left.


The author laid out the units and the flashing spaces between them on a 1x3 story stick that he nailed to the roof to guide framing. Here, the stick is screwed to a chalk line below the completed rough opening to support and position the bottom row of windows. Plywood gauges ensure accurate spacing between the window frames and prevent dimensional creep.

The window package includes 1 1/4-inch annular-ring installation nails, but we used 2-inch-long #8 Fastap Plus screws instead (800/847-4714, Screws are easier to remove when adjustments are needed, and they're plenty strong. I cut plywood spacers to precisely control the flashing gap between units — 3 inches wide and 6 inches wide for the side-by-sides, and a set 4 inches wide for stacking the units vertically.

The installation manual requires roofing underlayment around the window frame, folded up from the roof onto the sides and wrapped in sequence from bottom to top around the four corners. According to the manufacturer's rep (who was attentive and helpful throughout our unusual installation), this helps prevent problems from moist interior air condensing on the underside of the aluminum flashing. I don't quite agree with that premise; that's what minimal-expanding foam is for. But providing a carefully installed secondary barrier against wind-driven water and ice dams does make sense.

We used self-adhesive membrane, beginning with Vycor (Grace Construction Products, in 6- and 9-inch-wide rolls. Between units, we had lots of three-way U-shaped folds to make. We scored the release paper on the back so we could bond first to the roof deck and then to the frames. But in the August heat, Vycor's release paper tore away in annoying, uncontrolled strips. Picking at it only made the problem worse, rendering the product useless. Vycor is undoubtedly great when laid flat around a vertical window — the problem was in the folding.


U-shaped and L-shaped pieces of self-sticking roofing membrane seal the transition between roof and frame.


The author cut strips from a 3-foot-wide roll of ArmourGard Ice & Water Protector, then scored the release sheet so it could be peeled away a section at a time — first the middle.


We switched to 3-foot-wide ArmourGard Ice & Water Protector (IKO, 888/456-7663,, which has a silicone-plastic backing that reliably peeled away in one piece. We cut it into the widths needed and scored the backing with a razor knife. Although thinner than Vycor and a little trickier to score without slitting all the way through, it worked like a charm, even in 90°F weather.