Backfill: Two Formed & Foamed Dormers
Faced with building two opposing eyebrow dormers across a
narrow, stress-skin foam panel gable roof, Narragansett, R.I.,
builder David Baud and collaborator Mike Rand came up with a
They cut dormer face profiles from 3/4-inch-thick
medium-density overlay (MDO) plywood and glue-nailed them over
2x lumber blocking to create two custom sandwich "headers."
Then they stood the headers parallel to each other, 12 feet
apart on the shop floor, and joined them by a series of flat
2x4 "purlins" spaced about 8 inches apart (see illustration,
below). Rand cross-laminated a double layer of 3/8-inch
bendable plywood over both sides of the purlins, filled the
cavities between purlins with expanding urethane foam, then cut
the structure into two matching, monolithic units.
Plywood patterns, which were later
installed as permanent structural bracing, served as
straightedge guides to establish the curved valley profile on
To establish a precise cut line for the curvy valley
intersection, Rand stood the dormer between a set of plywood
patterns cut to match the main roof pitch, then dragged a long,
marker-tipped straightedge down the slope. A similar setup
provided the skewed line of cut on the interior side.
Foam-packed, primed, and stacked, the
two dormers are ready for transport to the job
Roof panel sections and dormers were
assembled in sequence, working end to end. The dormers' weight
is largely supported by the bearing walls directly
They defined the roof opening by positioning the completed
dormer on the SIPS panels, laid on the shop floor in their
installed positions, and tracing the dormer's valley
On site, Baud used his Lull to assemble the components like a
big 3-D jigsaw puzzle. An injection of expanding foam sealed
the seam between dormer and SIPS.