With open valleys, shingles overlap
into a flashing material channel that allows water to run through. The width of
the flashing material should be a minimum of 4-6 inches wide near the top of the
valley, and widen as it approaches the roof eve, or if you desire a consistent
open valley width the channel should be wider and consistent in width throughout
the entire length of the valley.
The reason for needing a wider
channel near the lower portion of the valley is that this area sees a higher
volume of water than near the top of the valley. If there is too much water
flowing through the channel there is the risk of water pushing up under the
shingles near the edge of the metal flashing. One reason for having a widening
valley is it is less prone for forming ice dams.
When shingling open valleys with
asphalt shingles, metal flashing or rolled roofing is used to form the open
valley. Metal flashing is typically 24 inches wide and rolled roofing is 36
When shingling an open valley with
metal flashing, an Ice and Water shield should first be applied to the valley.
Ice and water shields seal around nails that puncture them and thus prevent
The metal flashing should then be
installed over the Ice and Water shield. Metal flashing does expand and contract
with temperature so it is best to install the metal flashing in the roof valley
in sections that do not exceed 10 feet in length. The metal flashing sheets
should be installed so that they overlap each other by several inches, and with
the upper piece resting over the bottom piece. Roofing cement should be used to
seal the two sections of metal flashing together and nails should be installed
along the edges of the metal flashing. Again the Ice and Water shield will
prevent any water leaks.
When shingling an open valley using
rolled roofing, an 18” wide section of rolled roofing should first be installed
into the valley, followed by a 36” wide section. The 18 inch wide section should
be placed into the valley with its mineral surface laid face down, and the 36
inch wide section should be placed on top of it with its mineral side facing up.
In addition, and similar to metal flashing, the top piece should overlap the
lower piece and should be sealed with a roofing cement.
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Finally, after trimming the roofing
shingles near the valley to evenly expose the flashing, roofing cement should be
applied on the underside of the bottom edges of the shingles near the valley.
This will help prevent water from sliding up and underneath them.
Closed valleys have no channel or
flashing material exposed. Instead, shingling closed valleys involves the
weaving of shingles between one roof plane and another. This said an Ice and
Water shield is installed in the valley first.
The advantages of shingling a
closed valley with the woven technique are that no cutting is required, no
roof cement is required, and it is the most weather resistant. On the
downside, it is more tedious and complicated to install as both roof planes
need to be done at the same time.
Photo By Mark Donovan
Depending upon the types of shingles
being installed you may want to work your way into the valley or out of it. With
3-tab shingles it is best to work your way into the valley. With laminated
shingles you can work your way out of the valley. In the ensuing sections I am
referring to shingling with 3-Tab shingles.
Basically shingling a closed valley
involves shingling a roof plane through the valley and up over and into the
other roof plane by approximately 12 inches (a full tab with 3-Tab asphalt
shingles). As you work your way up the valley the shingles passing through the
valley are woven into place. This is accomplished by tying in and weaving into
the valley both roof planes simultaneously. To do this, one shingle of one roof
plane over laps the other roof plane, and then you move up a tier and alternate
the process. This creates the weaving pattern.
It is important that during the
weaving process, that no nails are installed within 6 inches of the valley and
that the shingles are pushed snuggly into the valley. If the shingles are not
properly pushed down into the valley bridging will occur such that an air pocket
is created underneath the valley. When someone steps on this air pocket the
shingles could be cracked or damaged. Ideally shingling closed valleys should be
done on warm days so that the asphalt shingles are more malleable.
For more help on Shingling Your Home's Roof, see HomeAdditionPlus.com’s
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