lining was 30-lb. building paper, but peel-and-
wider is better than narrower flashing because it
stick WSU, although more expensive, is simpler
enables you to nail well back from the edge of the to install and generally more durable. Install
flashing—always desirable.
36-in.-wide WSU for valleys, centering a single
piece down the length of the valley and overlap-
VaLLey fLashing
ping the drip-edge flashing at the bottom.
There are basically two types of valleys: open,
If you line the valley with 30-lb. building
where the valley flashing is exposed, and closed,
paper, run a continuous piece of 36-in.-wide
where flashing is covered by shingles. Each has
paper down the valley or overlap pieces by at
advantages. Open valleys clear water well, are
least 6 in. Use tabbed roofing nails to secure the
easy to install, and work especially well beneath
paper, keeping nails 6 in. away from the center of
wood shingles, shakes, and laminated asphalt
the valley. The 15-lb. building paper underlay-
shingles, which are thicker and harder to bend
ment used elsewhere on the roof will overlap the
than standard three-tab shingles. In woven val-
outer edges of this heavier “valley paper.”
leys, shingles from both roof planes meet in the
install metal valley flashing that’s 18 in. to 24 in.
valley in alternating overlaps and are slower to
wide on most slopes so that each side of the val-
install but offer double-shingle protection and
ley is 9 in. to 12 in. wide. If the roof pitch is steep
are favored for low-slope roofs. And there are
Roofs
85
zzzzzz Closed Valleys
36-in.-wide felt paper
or peel-and-stick WSU membrane
Overlapping shingles extend
12 in. past valley center.
CLOSED-CUT
No nails within
VALLEY
6 in. of valley center
Shingle points snipped
at 45° angle
Cut shingles stop
1 in. to 2 in. short
of valley center.
WOVEN
VALLEY
Urethane caulk
under closed-cut
shingle edges
There are two kinds of closed valleys: woven
and closed-cut. Woven valleys offer better
protection from weather but take longer to
install. Closed-cut valleys are faster but are
only one-shingle-layer thick in the val ey center.
When shingling closed valleys, it’s sometimes
necessary to add a shortened shingle to the
course before you reach the valley, so that you
can run a full shingle across the valley.
or if you live in an area that gets a lot of rain,
galvanic action, use nails or clips of the same
have a metal shop fabricate valley flashing with
metal as the flashing.
an inverted V-crimp down the middle. This crimp
helps prevent heavy runoff from one side running
Vent-pipe fLashing
across the valley and up under shingles on the
Vent-pipe flashing (also called jack flashing) is
other side. Overlap lower sections of flashing 8 in. usually an integral unit with a neoprene collar
to 9 in. The heavier the metal, the more durable
atop a metal base flange. Some pros prefer all-
the valley: 26 gauge is standard for prefabricated
metal units because UV rays won’t degrade them
pieces, but the heavier 24 gauge is better.
and their taller collars are less likely to leak on
avoid driving nails through metal valley flash-
low-slope roofs.
ing. Instead, place nail shanks snugly against the
Neoprene combos are easier to install. In both
edge of the flashing and drive nails until the
cases, shingle up to the base of the vent pipe and
heads touch the flashing. Don’t dent the metal.
slide the unit down the pipe. Nail the top edge of
Space nails every 12 in. to 16 in. along both
the base, then overlap it with shingles above.
edges, or use clips that interlock seams along the
Neoprene collars slide easily over the pipes, but
edge. Not nailing through the metal allows it to
metal collars must be snipped and spread to
expand and contract freely and leaves no nail
receive the pipe, before being caulked with ure-
holes for water to penetrate. To prevent corrosive thane to prevent leaks. For either base, don’t nail
86
Chapter 5
the lower (exposed) edge; instead, apply a bead of
urethane caulk beneath the flange to seal it to the
shingles beneath.
Chimney fLashing
Chimneys must be counterflashed. The upper
pieces of counterflashing are usually tucked into
chimney mortar joints and made to overhang
various pieces of base flashing, which are nailed
to the roof deck. Counterflashing and base pieces
overlap but aren’t physically joined, so they can
move independently yet still repel water. (This
independence is necessary because houses and
1. As you roof along a chimney, alternate shingles and
2. This grinder is poised to grind out
chimneys settle at different rates, causing single-
L-shaped pieces of step-flashing. Counterflashing will
an old chimney mortar joint. Once
piece flashing to tear and leak.)
cover the tops of the step-flashing. For extra protection,
the abrasive wheel hits mortar, you’ll
replace counterflashing and base flashing
run a bead of urethane between the step-flashing and the see nothing but grit. Wear safety
when reroofing. To avoid damaging the chim-
chimney. Press the flashing into the urethane to achieve a glasses or goggles.
positive seal.
ney, use the gentlest possible method to remove
counterflashing. If mortar is weak and crum-
bling, you may be able to pull the flashing out by
hand; in that case, repoint the mortar after
replacing counterflashing. If the mortar is sound
and the counterflashing is firmly lodged, try
using a cold chisel or a carbide-tipped bit in a
pneumatic air chisel to cut out the flashing and
as little of the mortar as possible.
Base flashing should be removed because
you’ll strip the roofing and building paper at the
same time. Base flashing has several compo-
nents: a continuous sheet-metal apron across the
chimney’s downslope face, L-shaped step-flashing
3. Counterflashing is held in place by a folded lip jammed 4. Finally, run a bead of urethane
into the mortar joint. For good measure, the mason
caulk such as Vulkem® 116 to fill the
hammered masonry nails into the mortar and then used a
joint and seal out water. Once the
cold chisel to tap them in deeper.
caulk has set a bit, you can tool it
with your thumb.
running up both sides, and (when the chimney
sits below the roof ridge) a cricket (or saddle)
/>
running across the upslope face. A cricket is
P R O T I P
sloped like a tent roof to deflect water around the
chimney. Use a claw hammer, a flat bar, or a cat’s
Before inserting chimney
paw to pry up old base flashing from the roof
counterflashing, blow or brush
sheathing. Then hammer down any nails you
out any debris from the mortar
can’t pull.
joints. if your air-compressor
reattach base flashings first. As shingles butt
hose can reach the roof, use it.
against the chimney’s downslope face, place the
otherwise, insert a piece of
apron over them. The apron’s bottom flange
plastic tube in the joint and
just blow. Wear safety glasses—
should overlap shingles at least 4 in.; its upper
and when the tube is in place,
flange should run at least 12 in. up the face of the
don’t inhale.
chimney. Prefabricated aprons usually have
“ears” that wrap around chimney corners and are
nailed to the sheathing. As shingles ascend both
sides of the chimney, they overlap the bottoms of
Vent-pipe flashing overlaps shingles below and is
L-shaped pieces of step-flashing.
overlapped by shingles above. To keep water from entering
Keep nails as far back from the flashing
the snipped metal collar around the pipe, seal the joint
with urethane caulk.
crease as possible. Use a single nail to nail down
Roofs
87
each piece of step-flashing and the shingle cover-
with gasketed roofing nails spaced every 6 in.,
ing it. When shingle courses along both sides
down 2 in. from the top edge. Then cover the top
of the chimney reach the back (upslope) face of
edges of the cricket flange with a strip of peel-
the chimney, the lower flanges of the cricket
and-stick bituminous membrane, and overlap
overlap them.
that with shingles. Finally, caulk the top edges of
If a self-supporting cricket is fabricated from
apron, cricket, and step-flashings with urethane
heavy 20-gauge galvanized steel, predrill the nail
caulk to seal them to the chimney.
holes in the cricket’s lower flange. Nail it down
zzzzzz Chimney flashing
Urethane caulk seals
tops of step-flashing.
an alien on The rooF
you often see Tv antennas strapped to chimneys.
Terrible idea! Whipped by winds, an antenna
stresses the mortar joints and causes leaks. A
chimney is designed to be a freestanding unit
that safely carries hot gases out of the house.
don’t ask more of it.
BASE FLASHING
Step-flashing is
interwoven with shingles
(4-in. overlap minimum).
Apron flange
overlaps shingles below
by 4 in. minimum.
COUNTERFLASHING
Cricket flange
extends up under
shingles by 6 in.
minimum
Tops of apron, cricket, and
step-flashing are covered by
counterflashing.
The apron flashing along the downslope of the chimney wraps
The transition step-flashing that turns the corner on a
around the corners of the chimney and is itself overlapped by step-
dormer sidewall is complex and inclined to leak, so have it
flashing coming down each side. The cricket on the upslope side
prefabricated by a sheet-metal shop. Seal the top of the
step-flashing with 9-in.-wide peel-and-stick membrane,
is also complex; it wraps corners and overlaps step-flashing below.
then overlap that with housewrap before covering both
Have a sheet-metal shop solder all seams so they’ll be watertight.
with siding.
88
Chapter 5
There are several ways to install counter-
flashing. Counterflashing should overlap the
base flashing by 4 in. Traditionally, a mason used
zzzzzz flashing a shed Roof
that abuts a house Wall
a tuck-pointing chisel to remove chimney mortar
Siding
to a depth of 11⁄2 in. and then inserted a folded lip
of counterflashing into the mortar joint. The
Sheathing
joint was then packed with strips of lead to hold
the flashing in place, followed by fresh mortar
applied with a striking tool (also known as a slick).
Underlayment
4 in.
This method works well, but you need to be care-
minimum
ful not to damage the surrounding bricks. Caulk
Flashing
with urethane caulk once the mortar has set.
Alternatively, you can use an abrasive wheel in
a cordless grinder to cut narrow slots in the mor-
1-in. clearance
tar joints; then insert counterflashing with its lip
above roof
folded back so sharply that it resembles the barb
of a fishing hook. This barbed lip friction-fits
tightly into the slot, so mortar is unnecessary.
Instead, fill the slot with urethane caulk, which
adheres well and seals out water.
fLashing adjoining stRuCtuRes
Leaks are common where a shed roof abuts a
wall, if the joint isn’t flashed. At the very least,
you’ll need to remove enough siding nails so you
Flashing seated in
roofing cement
can slide the upper leg of L-shaped flashing up at
or urethane caulk
least 4 in. under both underlayment and siding—
8 in. in snow country. Because the bottom of the
siding must clear roof shingles by at least 1 in.,
however, wise builders will strip all siding above
the roof and reinstall it so courses are evenly
skyLight and Ridge fLashing
spaced above the required 1-in. clearance.
(Cutting 1 in. off the bottom course would be
Skylight flashing is shown in chapter 6. As a gen-
faster but won’t look as good.)
eral rule, follow the instructions in the flashing
The lower leg of the L-shaped flashing goes
kit provided by the skylight manufacturer.
over the uppermost course of shingles on the
Traditionally, ridges were flashed with a con-
roof. Because that leg will be exposed, caulk
tinuous strip of 12-in.-wide, 30-lb. building paper
under it and use gasketed roofing nails to secure
folded lengthwise, which straddled the ridge and
it, placing the nails at least 1 in. above its
overlapped the top courses of shingles. The build-
lower edge.
ing paper was then covered with a shingle saddle
Where a gable-end addition abuts a vertical
or overlapped ridgeboards. Metal flashing was
sidewall or a sloping roof meets a dormer wall,
sometimes used instead of building paper.
install step-flashing. Fashion L-shaped step-
These days, ridges are as often covered with
flashing by folding in half 5-in. by 10-in. pieces
ridge vents th
at allow hot air to escape, as shown
of sheet metal; alternate pieces of flashing and
on pp. 90–91.
shingles, much as you would install chimney
base-flashing. Again, you may need to pull siding Asphalt Shingles
nails or remove courses of siding to fit the upper
Until recently, most asphalt shingles were three-
legs of the flashing up under both underlayment
tab shingles with two slots dividing the exposed
and siding. Place nails as far as possible from the part of the shingle into thirds. But today, lami-
flashing folds. Use two nails to attach each piece
nated shingles (also called architectural and
of step-flashing: one nail (into the sidewall) 1 in.
dimensional shingles) are gaining in popularity.
from the top edge of the upper leg and the other
Consisting of two bonded layers, laminated shin-
nail through the bottom leg and the shingle over-
gles are thicker, more wind-resistant, and some-
lapping it. Apply urethane caulk under any flash-
what easier to install because they have a random
ing legs or shingle edges that don’t lie flat.
Roofs
89
Venting a Roof
Roof vents allow air to flow beneath the roof deck, thus moderating
attic temperatures, extending the life of roofing materials, reducing
ambient moisture in the house—a big source of mold—and preventing
ice dams from forming along the eaves in cold climates.
zzzzzz Roof Venting
typically, in a house with passive ventilation, air enters through
Baffled ridge vent
soffit vents, flows up through an unfinished attic or over insulation
between rafters, and exits through either gable-end vents or a ridge
vent. passive ventilation is desirable because it consumes no energy,
but intake and exit vents must be balanced and large enough to allow
adequate flow, and channels must be continuous to the exit vent.
minimum venting requirements are summarized below.
it can be difficult to retrofit existing roofs to add ventilation. some
houses lack soffits or have eave trim too shallow to add intake vents;
blown-in insulation may fill the eaves and block intake air. in those
cases, adding rooftop intakes may be a viable way to draw in cool air.
Rooftop intakes are typically installed a few feet up from the eave;
they include the eyebrow ventilator shown on the facing page and
slotted intake vents such as that shown below. Both admit air, but the
slotted vent will admit a greater volume and more evenly distribute
flow of intake air—provided that there are no impediments between
Renovation 4th Edition Page 21
