Wood shingle roof, Key West Florida (C) Daniel Friedman Roof Valley Flashing Installation Details - Best Practices

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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

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Valley Flashing Requirements for Asphalt Shingle Roofs

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

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]

Table 2-3 (C) J Wiley, S Bliss

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 6 inches.

Roll roofing as roof valley (C) Daniel Friedman 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.

After the 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.

Continuing from from Best Practices Guide to Residential Construction:

Shingle Nailing Details Near Roof Valleys:

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

Roof valley flexing (C) Daniel Friedman

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).

An 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.

The 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.

Figure 2-15 Open Roof Valley detiails (C) J Wiley, S BlissMetal valley linings should be 2 to 3 feet wide and no more than 8 or 10 feet in length to prevent wrinkling from lengthwise expansion.

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.

Here(at left) is a photo of a buckled, cracked and leaking roof valley caused by just this problem.

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.

Roof valley intersection (C) Daniel FriedmanA 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.

Clips and nails should be the same metal as the valley or a compatible metal that will not cause galvanic corrosion (see GALVANIC SCALE & METAL CORROSION or see page 83 in the printed text Best Practices Guide.

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 roofing cement.

Closed-Cut Valley Installation Details for Asphalt Shingle Roofs

Figure 2-14 Closed cut roof valley details (C) J Wiley, S Bliss 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.

Continuing from from Best Practices Guide to Residential Construction:

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 plane.

Then continue roofing the smaller or lower slope roof plane, running each course at least 12 inches past the valley centerline.

Closed-cut valley (C) Daniel FriedmanPress 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 valley center.

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

Figure 2-13 (C) J Wiley, S Bliss 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.

Continuing from from Best Practices Guide to Residential Construction:

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.

Extend the end of each shingle at least 12 inches beyond the valley centerline and avoid placing any butt joints near the valley center.

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).

Woven valley (C) D Friedman F AlbertContinue 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.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Resources: Roofing Materials & Equipment Suppliers

-- Adapted with permission from Best Practices Guide to Residential Construction.


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