I design and build custom sunroom additions in Chittenden
County, Vt. (see
Sunrooms," 5/00). Many of these rooms include skylights as
a practical way of adding glazing overhead. I install around 20
skylights each year, so I need to be certain that I don't get
called back to fix leaks. So far, I've never had a leak, which
I attribute primarily to closely following the manufacturer's
Choosing a Skylight
When planning a sunroom, I usually work with the client to
determine the right location for each skylight. I'll make
recommendations about what would look best and how to avoid
glare within the room. In a sunroom that doubles as a TV room,
for example, I might recommend optional shades.
Most customers appreciate the extra sunlight on a winter day
but don't realize that skylights can overheat a room in the
summer. For that reason, I always steer clients toward a
ventilating skylight -- preferably manual rather than
motorized. I feel more comfortable with the simplicity of the
manual unit and like knowing I won't get a call in five years
to repair or replace a motor. Plus, electronic units cost twice
as much as manual units.
Nevertheless, some customers choose the motorized skylights
because they like the convenience. Motorized units can be
programmed to open and close on a schedule and will close
automatically when it rains.
I prefer Velux skylights (Greenwood, S.C.; 800/888-3589,
www.velux-america.com), simply because
they've never given me any trouble, I'm comfortable with their
flashing system, and I have the installation instructions
memorized. The VS model is my favorite; it provides good
ventilation and comes in a wide variety of sizes. Although I've
used other manufacturers with success in the past, the
distribution wasn't as extensive in my area, making their
products harder to get and increasing lead times.
Square Framing First
During framing, I make an extra effort to get the openings
perfectly sized and square. It makes setting the skylight --
not to mention the trim -- go more smoothly.
I leave the units boxed for transport and on the job site to
lessen the chance of damage and keep all the parts together. A
few days before the scheduled installation, we remove the
manufacturer labels and prefinish the frames and sash.
Prefinishing is neater and faster than working from a ladder
after the units are installed. I'm seldom asked to paint the
skylights, because the engineered beams that form the
structural frame of our sunrooms are naturally attractive, and
my customers usually want to continue the look of natural wood
to the trim and millwork. We typically use a couple of coats of
water-based polyurethane. It provides good protection from
humidity and condensation and cleans up easily.
Setting the Unit
In preparation for the skylight installation, the roof is
sheathed and covered with 30-pound felt. I hold the felt back
from the openings about 6 to 8 inches, so the waterproof
underlayment can adhere to the roof deck. Up to this point,
I've temporarily covered the openings with plywood to keep rain
out of the room. Before bringing the skylights out onto the
roof, I shingle up close to the openings. On hot days, I'll
stop shingling a little lower on the roof so the shingles don't
get scuffed while the skylights are being installed.
Removing the sash makes handling safer
and easier. Narrow strips of ceiling between skylights make
layout mistakes really obvious, so special attention is given
to accurate spacing during framing.
I start the skylight installation by first bending out the
L-shaped mounting brackets and removing the cladding and sash,
making the unit lighter and easier to handle. I pass the frame
through the roof to a helper and stand inside to center it in
the opening (see Figure 1). Centering ganged skylights is
especially important, because trim and narrow sections of
drywall will show up the discrepancy inside if the roof windows
aren't spaced equally.
Figure 2.An improvised tool helps center the unit
in the opening. The speed square rides along the rafter, and
the combination square indicates the space between the unit and
the rough opening. Note the layer of polystyrene insulation
installed beneath the roof sheathing.
I've improvised a simple tool to help with centering: A speed
square clamped to a combination square with a set of locking
pliers makes it easy to check the reveals quickly and get them
just right (Figure 2). I rest the units on a temporary cleat
nailed to the roof, which makes positioning easier and safer
(Figure 3). It also keeps multiple units in a straight line and
prevents expensive skylights from sliding to an untimely
Figure 3.Tacking a 2x4 cleat to the sheathing
keeps units in a straight line and prevents them from sliding
off the roof. Once the units are secured, the cleat will be
removed so shingling can continue.
When I'm certain the skylight is centered, we nail through the
mounting brackets in two opposite corners, using the heavy-duty
ring-shank nails included with the skylight (Figure 4). We try
to get them into the roof framing, but the annular rings resist
pull-out even if they're driven only into the roof sheathing.
Next I reinstall the sash to check operation and ensure that
the unit's frame is square and will remain so. Once the unit is
in its final position and the sash operates without binding, we
fasten through the other brackets, locking the skylight in
Figure 4.After the carpenters nail down two of the
four mounting brackets, they reinstall the sash to make sure
that the unit is square (left). Necessary adjustments are made;
then the other brackets are nailed off (right).
When we have all the skylights secured in their final
position, I remove the cleat that helped with placement and we
cut 10-inch-wide strips of self-adhering eaves membrane for a
secondary water barrier. This is the last defense against water
infiltration, and the manufacturer requires it. The skylight
manufacturer makes its own membrane, but we generally use Grace
Ice and Water Shield (Grace Construction Products, Cambridge,
because it's readily available, less expensive, and it sticks
better than other products I've tried.
We start at the bottom, attaching the membrane to the skylight
and smoothing it onto the roof. The sides are next, with the
areas between adjacent skylights the biggest challenge. With
all hands on deck, we take positions at top, bottom, and middle
and try to keep the membrane from sticking to itself and making
a mess. The piece is cut so that it extends about 6 inches
beyond the skylight, top and bottom. We adhere the membrane to
one skylight and smooth it down toward the roof and back up
onto the other skylight (Figure 5).
Figure 5.To reduce the chance of leaks, the author
uses a single piece of membrane in the space between skylights.
Folding the membrane in half helps to keep it from sticking to
itself (top). Aligning the membrane's edge on the first unit
and smoothing toward the roof deck and back up the second unit
minimizes wrinkles (center). Cutting the corner allows the
vertical leg to wrap around the top corner
Finally, we install the top piece of membrane. It laps over
the side pieces and is stuck to the roof sheathing under the
felt (Figure 6). We use a hammer stapler to better secure the
membrane to the skylight, stapling as close to the membrane's
top edge as possible. This isn't as critical in warm
temperatures, but I had one occasion where the membrane
separated from the skylight and had to be redone. Since then, I
always staple the membrane as extra insurance.
Figure 6.The top piece of membrane laps over the
sides (left), and a slit cut outward forms a flap that folds
around the corner (right). The membrane's leading edge is
adhered to the roof deck under the felt.
After the membrane is installed, we continue shingling up the
roof to the bottom edge of the skylight. We know we have
shingled far enough when the sill flashing, the bottom piece of
flashing, covers up some of the shingles' 5-inch exposure
(Figure 7). Special nails are provided for attaching the
flashing: They're 3/4 inch long so they won't poke through the
frame, and they're noncorrosive to the aluminum.
Figure 7.When the shingling reaches the skylight,
the sill flashing is slid up from the bottom and fastened at
the top corner with the manufacturer's special nails (left).
The first piece of step flashing overlaps the sill flashing 3
1/2 inches. Where adjacent sill flashings overlap, urethane
sealant provides extra insurance against leaks
We shingle up the sides, weaving the step flashing into the
courses. The manufacturer specifies at least a 3 1/2-inch
overlap on every piece, and the pieces are nailed near the top
on the downhill edge. The last piece of step flashing is slit
so it can bend around the top corner (Figure 8).
Figure 8.Cutting the last piece of step flashing
at the fold allows it to be bent around the skylight's top and
nailed like the others to the side of the frame.
The small section of roofing between ganged units gets a
U-shaped piece of flashing rather than step flashing. We bend
our own in the shop out of high-quality bronze aluminum trim
coil; if you don't have a brake, the manufacturer offers a
pre-bent U-shaped flashing as well. We make our own flashing
because I like to have flexibility in spacing the units. The
U-shaped metal is held in place by the cladding when it's
reinstalled. Next, we install the head flashing over the step
flashing and shingles, but we don't install the screws (Figure
9). We put a strip of eaves membrane over the head flashing and
under the felt.
Figure 9.The leading edge of the head flashing
(top left) gets covered with a second strip of eaves membrane
(top right). Note that the felt is lifted so the membrane can
be adhered to the roof deck. Notching the shingles around the
skylights maintains a 4-inch exposure (bottom).
Shingling is easier now because we no longer have to cut and
fit around the skylights, but it's important to leave a 2 3/8-
to 4-inch space between the top of the skylight and the
shingles. That space prevents roofing nails from punching holes
in the head flashing and allows water and debris to get around
the skylight unimpeded.
Figure 10.Reinstalling the skylight's cladding
completes the process. The bottom goes on first (top left).
Slots in the bottom receive the side pieces (top right).
Plastic inserts in the sides receive the screws inserted
through the head flashing (bottom left). Finally, the top
cladding piece is slipped over the head flashing and secured
with two screws (bottom right).
Next we reinstall the cladding that was taken off when the
sash was removed (Figure 10). Those pieces, held in place by
screws, give the skylight its finished look and act as
counter-flashing for the U-flashing and step flashing. We start
at the bottom, continue up the sides, and finish with the top
piece.Dennis Batesis owner of Vermont Sun Structures in