Roof Trim · Metal building trim guide
Metal Roof Valley Flashing: The W Valley for R-Panel and PBR
A W valley lines the inside corner where two roof slopes meet, carrying heavy water flow safely down to the eave.
Also called: valley flashing, W valley flashing, roof valley trim, valley pan, open valley
Metal roof valley flashing lines the inside corner where two roof slopes come together, such as where a wing meets the main building on an L-shaped shop. The most common style on metal buildings is the W valley, named for the raised W-shaped rib down its center.
A valley carries more water than any other spot on the roof, so it has to be done right. This guide covers what a W valley does, how to figure valley length with a real example, how many pieces to order, and how to seal R-panel and PBR panels into it.
What Is W Valley Metal Roof Valley Flashing?
A W valley is a wide sheet of metal bent into a shallow V with a raised rib down the middle. The two sides lie under the roof panels of each slope. The center rib keeps a rush of water from one slope from shooting across the valley and up under the panels on the other slope.
It does three main jobs: it gives water a clear channel from the ridges down to the eave, it keeps that water from getting under the cut ends of the panels, and it covers the framing at the valley so the roof looks finished.
Where a W Valley Goes on a Metal Building
You only need a valley where two roofs meet at an inside corner. That happens on L-shaped and T-shaped buildings, cross gables, dormers, and some porch roofs tied into a main roof.
Most simple carports, barns and box shops have no valleys at all. A lean-to tied to the side of a building does not use a valley either; it uses roof-to-wall flashing. If your plan has a valley, it needs solid support under it, such as decking or framing between the purlins, because the panel ends must be fastened along it.
Metal Roof Valley Flashing Sizes, Gauge and Colors
Valley flashing is typically 26 gauge to match R-panel and PBR, in Galvalume or painted colors. It is commonly about 18 to 24 inches across, but widths vary, so ask us. Wider valleys handle more water and give more room to seal the panel ends.
Pieces are typically 10 ft long. The W valley is the usual pick for exposed-fastener panels. A plain open valley without the center rib exists too. Custom bends are available if you are matching an older roof.
How to Measure Valley Length and How Many Pieces to Order
Formula: when two roofs of the same pitch meet at a square corner, valley length = the horizontal run of the smaller roof (half its width) x the valley factor. Valley factors: 2:12 = 1.42, 3:12 = 1.44, 4:12 = 1.45, 6:12 = 1.50. Then add 10% and round UP. Valley laps are longer than most trim, about 6 to 8 inches, because so much water runs through them.
Worked example: our 30 x 40 gable shop with 12 ft walls and a 3:12 pitch gets a 20 ft wide wing at the same pitch, making an L. The wing's half-width is 10 ft. 10 x 1.44 = 14.4 ft per valley, and the L makes two valleys, so 28.8 ft total. Add 10% = 31.7 ft = 4 pieces of 10 ft W valley.
Check it by the run: each 14.4 ft valley takes one full piece plus about 5 ft of a second piece with the lap. Three pieces will just cover both valleys if every cut is perfect, so the fourth piece is cheap insurance for the angled cut at the eave.
How to Install a W Valley on a Metal Roof
The valley goes in before the roof panels on both slopes. Exact details vary by panel maker, so use their valley detail if you have one.
- Make sure there is solid, straight support under the whole valley.
- Start at the eave. Set the first piece in the valley, trim the bottom to line up with the eave, and fasten only along the outer edges of the flashing, never through the center channel.
- Lay the next piece above it so the upper piece laps over the lower one 6 to 8 inches, with two beads of sealant or butyl in the lap. Work up to the ridge.
- On each side of the valley, run a strip of butyl tape a few inches out from the center. Cut inside foam closures to the valley angle and set them on the tape, then add another strip of tape on top.
- Cut the roof panels on the valley angle so they stop a few inches short of the center rib, leaving a clean open channel. Do not let panels touch the W.
- Fasten the panel ends through the closure and valley into the support with roofing screws, keeping every screw out of the water channel.
- Where the ridges meet at the top of the valley, seal the joint with flashing cut to fit and plenty of sealant; this is the spot most likely to leak.
Common Metal Roof Valley Flashing Mistakes
- Screwing through the center of the valley where water flows.
- Lapping pieces backward, lower over upper, so water runs into the joint.
- Using short 2 in. laps on a valley; it needs longer laps and sealant.
- Running panels all the way to the center so leaves and dirt dam up the channel.
- Leaving the cut panel ends open with no closures or tape.
- Installing a valley over purlins with no support under it.
What to Order With a W Valley
A valley is a small piece with a lot of parts around it. Order these together:
- Inside foam closures matched to your panel, extra for angle cuts.
- Butyl tape and a good roof sealant.
- Color-matched roofing screws.
- Ridge cap for both roofs and flat stock for the joint where the ridges meet.
- Eave trim or gutter for where the valley drains.
Get Metal Roof Valley Flashing in Houston From Ironrod Steel
Ironrod Steel Co. rolls its own R-panel and PBR roofing and supplies W valley, matching trim, closures, butyl tape and screws. Send us your building plan, roof pitch and photos and we will help you measure the valleys and build the full trim list. Pick up at 8122 Hillsboro St in Houston or ask about delivery across Houston and Southeast Texas. Call or text (281) 902-7619.
W Valley FAQ
Why is it called a W valley?
Looking at the end of the piece, the two sloped sides plus the raised center rib make a shape like the letter W. The center rib stops water from one slope from washing across and under the panels on the other side.
How do I figure the length of a roof valley?
For two roofs of the same pitch meeting at a square corner, multiply the horizontal run of the smaller roof by the valley factor. At 3:12 the factor is about 1.44, so a 10 ft run makes a valley about 14.4 ft long.
Can you put screws in a metal roof valley?
Only along the outer edges and through the panel ends away from the water. Never put a screw through the center channel, because that is where the most water runs.
How much should valley flashing overlap?
Most installers lap valley pieces about 6 to 8 inches, upper piece over lower, with sealant in the joint. That is more than other trim because a valley carries so much water.