Roof Valley Flashing Installation Details - Best Practices
ROOF FLASHING, ASPHALT SHINGLE VALLEYS - CONTENTS: Roof valley flashing specifications for asphalt shingle roofs. Open roof valley flashing installation details. Closed cut roof valley flashing installation details. Woven roof valley flashing installation details. Best practices for roofing material installation, flashing, ventilation, nailing, underlayment
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Valley Flashing for Roofs:
This article discusses details and specifications for roof valley flashing for best construction & roof leak resistance. We describe how to construct three common styles of roof valley: open valley, closed-cut valley, and woven roof valley, and we include roof valley underlayment and nailing specifications.
This article series discusses best practices in the selection and installation of residential roofing
Because valleys catch water rushing down two roof planes,
they are likely places for roof leaks. Leaks can be caused
by water rushing up the opposite side of the valley or from
wear and tear caused by the channeled water, snow and ice
buildup, or traffic on the roof. For that reason all valleys
should start with a leak proof underlayment system to back
up the shingle or metal valley detail.
Valley Underlayment Requirements for Asphalt Shingle Roofs
[Click to enlarge any image]
Start by cleaning any loose
nails or other debris and nailing down any sheathing nails
that are sticking up. If eaves flashing is used, it should
cross the valley centerline each way and be installed before
the valley underlayment (see “Eaves Flashing,” discussed in ROOFING UNDERLAYMENT BEST PRACTICES).
Next install a 36-inch-wide strip of self-adhering bituminous
membrane in 10- to 15-foot lengths up the valley.
Keep the membrane tight to the sheathing at the valley
center, since any hollow sections could be easily punctured.
Next install the 15-pound felt underlayment across
the roof, lapping over the valley flashing by at least
NRCA (Lile) recommends that the underlayment should always be centered in the valley, not what you might think that where a steep slope intersects with a more gently-sloped roof section - keep the underlayment centered rather than extending the underlayment further into the lower-sloped roof area.- Ed.
Roll roofing is also an acceptable underlayment
for asphalt shingle valleys, although it is more prone
to crack and is not self-healing around nails.
underlayment is complete, the valley can be completed in
any of the following ways (Table 2-3 above - click to enlarge the tables or illustrations in these articles).
Durability of Roll Roofing Used as Roof Valley Flashing
Our roof valley photo (above-left) shows roll roofing used as the exposed valley flashing for an open roof valley - an accepted practice.
In our OPINION -DF, while roll roofing (or peel and stick ice and water shield membrane) work well as roof valley underlayment, using roll roofing as the final surface in an open roof valley is not as durable as the metal lined valley options or closed-cut or woven valleys discussed below.
If you consider that a roof valley drains water from two or more intersecting roof planes you realize that more water flows down this roof area than anywhere else on most roofs, meaning that we want the most durable materials in this location. - Ed. In sum, the water volume and velocity running through roof valleys is often quite a bit more aggressive than other roof slope surfaces where the same material might have lasted longer.
NRCA (Lile) recommends that fasteners should not be located within 6-inches (152mm) of the valley center when installing any roofing material, including roll roofing and asphalt roof shingles - Ed.
Open Valley Details for Asphalt Shingle Roofs
With a heavy-gauge, noncorrosive metal
lining, open valleys are the most durable valley and
the most costly (see our discussion of alternative flashing materials found at FLASHING WALL DETAILS).
economical version uses two layers of roll roofing for the
lining, which should last as long as an asphalt roof.
NRCA (Lile) recommends that roof valley flashing metal for open roof valleys should be 26-gauge (0.45mm) galvanized steel or equivalent noncorrosive, nonstaining metal. Lile points out that valleys lined with roll roofing are not as durable. - Ed.
bottom layer of roll roofing goes on with the gravel facing
downward; the top layer with the gravel facing upward.
Nail along the edges every 12 to 18 inches, keeping the
material tight against the roof sheathing.
Metal valley linings should be 2 to 3 feet wide and no
more than 8 or 10 feet in length to prevent wrinkling from
Our photo (above left) illustrates the cross-valley wrinkle that occurs by repeated heating and cooling in long segments of metal roof valley flashing - this flexing eventually breaks the flashing leading to a roof valley leak.
Our roof valley photo (above-left) shows what happens if a single section of valley flashing is too long: it heats and buckles and eventually breaks and leaks.
The valley lining, whether asphalt or metal, should
have 6 inches open at the top (3 inches on either side of
the valley centerline) and increase by
1/8 inch for each
foot of valley length to accommodate the greater flow
further down the valley.
So a 16-foot valley would have
6 inches open at the top and 8 inches at the bottom (see Figure 2-15).
Overlap metal roof valley sections by 12 inches,
and seal the lap with a flexible sealant, such as polyurethane
or butyl, on roofs shallower than 5:12.
Where two roof valleys
meet, for example above a gable dormer, a soldered joint
is likely to break from the movement. You can see in our roof valley photo (left) that there has been a history of patching at the valley intersection on this roof.
A lead cap overlaid
6 inches onto each valley is an effective way to seal the top.
Where the roof slopes are uneven or one roof is larger
than the other, a 1- to 1 1/2 -inch-high V crimp in the middle
of the metal valley will prevent the uneven flow from
running up one side of the valley.
The crimp also stiffens
the valley. A hem is also desirable, both to stop any overflow
water and to provide a place to attach nailing clips,
which hold the flashing securely while allowing movement.
Nails wedged against the edge of the flashing and
driven lightly against the flashing may also be used.
Shingles should overlap the valley lining by at least
6 inches. With a roll roofing valley, keep the nails at least
6 inches from the valley centerline. With a metal liner, nail
1/2 inch outside the liner. Seal each shingle to the liner and
overlapping shingle with a 3-inch-wide bead of plastic
Closed-Cut Valley Installation Details for Asphalt Shingle Roofs
NRCA (Lile) explains that closed-cut valleys combine some of the advantages of other valley types: their partial-open design improves roof drainage down the valley (compared with a woven or irregularly-shaped valley), and they are relatively abuse resistant.
A closed cut roof valley will have at least four layers of roof materials: one layer each of felt underlayment, mineral-surfaced roll roofing, and two layers of shingles. Closed-cut shingle valleys work fine with strip shingles, and laminated asphalt shingles.
But, Lile continues, for double-layer T-lock or double-layer individual lock-down shingles, the minimum valley slope should be 5" in 12". Finally, he warns that single-layer (3-tab) shingles can't be used in a closed-cut valley because nails may be needed to hold tabs at or near the valley center. - Ed.
A closed-cut asphalt shingle roof valley starts the same way as a woven
valley, with the first course of shingles run across the valley
from both roof planes, lapping the shingle from the larger
or steeper roof plane over the shingle from the smaller/shallower
Then continue roofing the smaller or lower slope
roof plane, running each course at least 12 inches past
the valley centerline.
Press the shingles tight into the valley
and nail in place, locating no fasteners within 6 inches of
the valley center and adding an extra nail at the end of each
shingle that crosses the valley (see Figure 2-14). Do not
allow any butt joints to fall in the valley.
Next, snap a chalk line 2 inches out from the valley
centerline on the opposite slope and shingle up the other
side of the valley, holding nails back 6 inches from the
Our photo (left) of a closed-cut valley on a group home in upstate New York shows that the roofer had a different idea about where the cut-line should go - s/he kept the cut line high out of the valley.
Trim each shingle to the guide line as you
go, or run them long and trim them later. In either case,
clip about 1 inch off the uphill corner of each shingle to
help direct rushing water into the valley.
Finally seal each
shingle to the valley and to the overlapping shingle with a
3-inch-wide bead of plastic roofing cement.
Closed valleys go up quickly and provide a clean appearance
with either standard or laminated shingles. If
sealed well, they provide adequate protection.
Woven Shingle Valley Installation Details for Asphalt Shingle Roofs
NRCA (Lile) points out that woven roof valleys are limited to cutout-style 3-tab asphalt roof shingles (as there are no openings to weave solid or architectural or dimensional or laminate style asphalt shingles together), and he adds that the valley's slope should be at least 4" in 12" or steeper, installed typically over a layer of mineral granule faced roll roofing.
Lile also warns that woven valleys have some drawbacks in areas where moss is likely to grow between the shingle cutouts, hindering roof drainage. - Ed.
On the first course across the valley, the
shingle from the larger or steeper roof plane overlaps
the shingle from the smaller or shallower plane.
the end of each shingle at least 12 inches beyond the valley
centerline and avoid placing any butt joints near the
Press the shingles tight into the valley when
nailing and place no fasteners within 6 inches of the valley
center. Add an extra nail at the end of each shingle that
crosses the valley (see Figure 2-13 at above left).
Continue to the top of the valley. Done correctly, woven
valleys are very weather-resistant and best for high wind
regions, but they are somewhat slow to install.
Woven valleys work
better with three-tab shingles than with heavy laminated
shingles, which do not conform well to a crisp valley line.
Our woven valley photo (above-left) courtesy of Frank Albert, shows a valley constructed using laminated asphalt shingles.
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