How to Install an Open Metal Valley on a Shingle Roof
Membrane, then metal, then shingles trimmed back to a chalk line. The fastest-draining valley and the easiest to keep clean.
- Advanced
- 2 to 4 hours per valley
- Updated
The short answer
An open metal valley lays W-profile or V-profile valley metal, commonly 24 inches wide with a rib about 1 inch high, over a 36 inch strip of ice and water shield. Start at the eave, lap sections 6 to 8 inches upper over lower, fasten only along the outer edges or with cleats, then snap shingle cutlines starting about 3 inches from the centre at the top and widening roughly 1/8 inch per foot toward the eave.
An open valley is the workhorse of wet climates. The W-valley metal sheds water fast, the raised centre rib stops a rush of water from one roof plane from jumping across onto the other, and the open channel lets needles, leaves and moss wash or sweep out. Nothing sits against a cut shingle edge, so there is less for debris to trap water against.
The price is appearance, because the metal shows, and the need for care: the metal has to be lapped correctly, fastened only at its edges, and kept free of nails. Everything underneath is covered in how to prep a roof valley. If you are not sure an open valley is the right choice, see open, closed-cut and woven valleys explained. This guide is for shingle roofs. On a metal panel roof the panels run into a valley pan instead, see how to install a metal roof valley.
Tools
- Tin snips (straight cut and offset)
- Tape measure
- Chalk line
- Hook blade utility knife
- Roofing hammer or coil nailer
- Caulking gun
- Hand seamer
- Cut resistant gloves
Materials
- W-valley metal, prefinished or galvanized steel, 24 inches wide with a 1 inch centre rib
- Ice and water shield, 36 inch wide roll
- Metal valley cleats or roofing nails, 1-1/4 inch
- Asphalt roof cement or polyurethane sealant for the lap joints and cut edges
- Shingles rated for open valleys
Safety first
- Work tied off. Valley metal is slick, and wet needles make it slicker. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Never stand or walk in the valley channel. It dents the rib and is the most likely place to slip.
- Cut metal edges are razor sharp. Wear cut resistant gloves and break the sharp edge with a hand seamer or fold a hem.
Step by Step
Step 1: Prep the valley and lay the membrane
The metal is the drainage channel, but the membrane underneath is the real barrier. Follow every step of valley prep: sound deck, membrane centred, pressed into the corner, field underlayment lapped over its edges, no nails near the centre.
Install the metal only after the membrane is in, and before the shingles on either side reach the valley.
Step 2: Choose and measure the valley metal
Common valley metal is 24 inches wide with a rib about 1 inch tall down the centre. The rib is called the W because in cross-section the channel looks like a W. On a long valley with a lot of roof draining in, wider or deeper metal is used, and some manufacturers call for a minimum of 24 inches. Always follow the shingle manufacturer's minimum width.
Steel, prefinished steel, aluminum and copper are all used. Use galvanized or prefinished steel where it will touch mortar or concrete, and avoid mixing copper with aluminum or galvanized steel, because dissimilar metals corrode each other. If the valley is under 8 feet long, a single piece is best and avoids a lap altogether.
Step 3: Start at the eave and set the first section
The metal should sit tight into the valley corner with its centre rib on the centreline. Cut the bottom end so it hangs slightly over the eave and is trimmed to shed water into the gutter or over the drip edge. See how to install a drip edge for the eave detail.
Bend the metal gently into the valley as you place it. Do not force a stiff piece down into the corner, because it will spring and buckle, and the buckle becomes a low spot where water sits.
Step 4: Fasten the edges and lap the next sections
Nails go in the outer 1 inch of the metal, well away from the channel and the centre rib, or use metal cleats that hold the edge without a hole. A nail near the rib is a hole in the highest-flow part of the valley. On long valleys, a cleat that lets the metal slide slightly is best, because the metal expands and contracts with temperature.
Work up the valley. Each upper section overlaps the lower one by 6 to 8 inches, and the lap should be sealed with a bead of roof cement or sealant, following the metal manufacturer's instructions. Lapped the wrong way, the joint is a dam.
Watch out: Never nail in the centre of the metal or through the rib. A single nail there leaks in every rain.
Step 5: Snap the shingle cutlines
A common layout puts the lines about 3 inches from the centre at the top of the valley and widens them about 1/8 inch per foot as they run toward the eave. The open channel gets wider where more water flows. Some manufacturers give different numbers, so read the wrapper.
Measure from the centre rib on both sides so the valley looks even. A crooked valley shows the metal wandering from line to line, which is the detail most easily seen from the street.
Step 6: Run the shingles to the cutlines and trim
Cut shingles with a hook blade knife, cutting from the back over a scrap. Clip the upper corner of each trimmed shingle end so water running down the valley cannot get driven under the corner. See how to cut asphalt shingles for clean cuts.
Keep every shingle nail at least 6 inches from the valley centreline, or farther if the manufacturer says so, and keep nails off the metal entirely. The shingles hold the metal down by lapping it, so the valley metal needs no extra fasteners in the exposed channel.
Step 7: Seal the shingle edges along the metal
A thin band of cement under the trimmed edge, not on top of it, stops wind from lifting the end and keeps water from being driven under it. Keep the cement within about 1 inch of the cut edge and never put it in the channel. Too much cement traps debris and makes the valley a dirty mess.
Press the edge down and wipe off the squeeze-out. Do not cover the metal with cement.
Tip: Hold the cement back from the centre rib so the channel stays clean and clear for water to run.
Step 8: Check the top, the bottom and the finished channel
At the top of a valley that starts on a hip or ridge, the metal should run up to the ridge and be covered by the cap shingles, so no edge shows. See how to install hip and ridge cap for the cap detail.
Sweep the channel clean, check that no stray nail or cement glob sits in it, and watch a hose test if you can: water should run straight down the metal and out with no pooling.
Metal Choices and Corrosion
Prefinished steel is the common choice because it is strong, easy to work and holds colour. Galvanized steel is cheaper and dulls over time. Aluminum is soft and dents under a ladder. Copper lasts longest but costs the most and should not sit under or beside galvanized steel where runoff from copper would corrode it. Wherever runoff from one metal flows over another, check that the pair will not corrode.
Avoid bare aluminum in contact with wet mortar, and never use lead-coated or unfinished metal where the water runs onto a different finish.
Fraser Valley Notes
An open valley under a fir or cedar will collect needles, and moss will begin growing in the wet months. The metal makes it easy to sweep the valley with a soft broom once or twice a year. Do not use a pressure washer, which drives water under the shingle edges. See clearing debris from the roof for safe methods.
Common Mistakes
- Nailing in the channel. A nail in the centre of the valley metal leaks in every rain, and is the most common open valley failure.
- Lapping sections upside down. The lower piece must sit under the upper one so water flows over the joint, not into it.
- Cutlines that do not widen. A valley the same width from top to bottom chokes where the water volume is highest, and debris gets stuck.
- Cement in the channel. Globs of roof cement dam the flow and trap needles. Keep cement under the shingle edge only.
- Metal too narrow. A 12 inch strip leaves almost no room for the cutlines and lets water jump across the valley. Use at least 24 inches.
- Forcing stiff metal into the corner. It springs and buckles and leaves a low spot where water pools.
- Mixing metals. Copper with galvanized or aluminum corrodes the lesser metal quickly.
When to Call a Professional
If the valley runs onto a low-slope roof, meets a wall, carries the runoff of a very large plane or sits on a roof with a steep pitch, the valley deserves a roofer. The same goes for any valley where you cannot safely work tied off.
If an existing open valley is leaking, the problem is often rust at the lap, debris damming or a nail in the metal. A roofer can diagnose it from the roof and either repair the metal or rebuild the valley. See how to repair a leaking valley.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Install an Open Metal Valley: Questions
How wide should open valley metal be?
Open valley metal is commonly 24 inches wide, with a rib about 1 inch high running down the centre. That leaves room for the shingle cutlines on each side and enough metal to lap under the shingle edges. Some manufacturers specify a different minimum, and a large volume of water may call for wider metal, so always check the shingle instructions.
How wide should an open valley be at the top and bottom?
A common layout leaves about 6 inches of exposed metal at the top, cutlines about 3 inches from the centre on each side, and widens the gap about 1/8 inch for every foot of valley length as it runs toward the eave. The widening channel handles the growing volume of water. Follow the shingle manufacturer's numbers if they differ.
What is a W-valley?
A W-valley is valley flashing with a raised rib running down the centre, which gives the metal a W shape in cross-section. The rib acts as a diverter: when a lot of water runs down one roof plane it cannot shoot across the valley and up under the shingles on the opposite side. The metal is typically 24 inches wide.
Do you nail valley flashing?
Only at the outer edges, never in the centre or through the rib. Nails go within about 1 inch of the outside edge, spaced every 12 to 24 inches, or metal cleats hold the edge without a hole. The shingle edges also lap the metal and hold it down. A nail in the valley channel is a leak.
Should you put sealant under valley shingles?
Yes, a narrow band of asphalt roof cement under the trimmed shingle edge, about 3 inches wide and kept within an inch of the edge, holds the ends down and stops wind-driven rain from being pushed under. Keep it off the metal channel, because cement in the channel traps debris. Some manufacturers give specific sealing instructions, so check the wrapper.
Related Guides
All Roof Valleys guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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