How to Install a Metal Roof Valley: Pan, Cleats and Panel Cuts
A lined valley, a formed valley pan held by cleats, and panel ends cut and hemmed so no screw sits in the water path.
- Professional
- 3 to 6 hours per valley
- Updated
The short answer
Line the valley with a 36 inch strip of self-adhered membrane, then lay a formed valley pan or W-valley over it, fastened only along its outer edges with cleats or screws outside the water path. Cut panels to the valley at an angle, hem or fold the cut edges, leave a gap of about 1 to 2 inches each side of centre and seal the panel edge with butyl tape.
A valley takes the water from two roof planes and runs it down a single channel. On a metal roof that channel collects a lot of water quickly, moves snow and ice as a stream, and can be filled by pine needles, leaves and moss. Valleys are where most of the leaks on metal roofs begin, usually because of screws in the water path, cut edges left unhemmed or a pan that is too narrow for the load.
The rule for a valley is simple: fastened at the edges, never in the middle, with the panels lapping over the pan. The sequence is to line the valley, lay the pan, cut the panels to an angle and form the cut edges. On standing seam, the seam legs also have to be notched and closed where they meet the valley.
Read this alongside underlayment for metal roofs and how to cut metal roof panels. For shingle valleys, see the roof valleys category.
Tools
- Sheet metal brake
- Aviation snips, offset snips and hand seamer
- Chalk line and straight edge
- Tape measure and sliding bevel
- Impact driver
- Caulk gun
- Fall protection equipment
Materials
- Valley flashing or pan, pre-formed or bent from coil, typically 16 to 24 inches wide
- Self-adhered membrane, 36 inch width
- Metal cleats or continuous cleat strip
- Butyl sealant tape
- Trim screws with washers
- Closure strips for the valley edge if required
- Touch-up paint
Safety first
- Valleys are where two slopes meet and debris, frost and moisture collect. They are slippery. Tie off and keep feet off the valley pan.
- Cut edges of pre-painted steel are razor sharp. Wear cut-resistant gloves.
- WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury.
Step by Step
Step 1: Prepare the valley deck and line it with membrane
Replace damaged sheathing along the valley. Make sure the deck planes meet cleanly with no gaps. Any bump will telegraph through the pan and create a hold-up for water.
Centre a strip of high-temperature self-adhered membrane on the valley line, pressing it into the trough without bridging. A 36 inch strip gives 18 inches each side. Membrane at the valley seals around fasteners and gives a backup if the metal leaks. See ice and water shield. Then lap the main underlayment over the membrane edges.
Step 2: Choose the valley pan and set the width
A W-shaped valley with a centre rib keeps water on each side of the valley from crossing over at high flow. Pans are commonly made of the same gauge steel as the panels, or heavier, in colour to match. A long valley with large catchment areas needs a wider pan and deeper edge hems.
Pre-formed valleys come in 10 foot lengths. You can bend your own on a sheet metal brake. Whichever you use, the pan should be in one piece per length with laps at joints.
Tip: Size the valley to the roof. A large roof plane draining onto a small pan can overshoot the edge in a downpour.
Step 3: Lay the pan from the eave up and lap sections downslope
Set the pan in the valley trough, with its centre rib along the valley line. The eave end is cut to match the eave trim and extends past the fascia so water flows into the gutter. Lap sections so water passes over the joint, with two rows of butyl tape in the lap. Do not fasten the lap with a screw in the water path. Use rivets or sealant in the outside edges if the manual permits.
The upper end of the valley tucks under the ridge cap or hip cap. See ridge and hip caps.
Step 4: Fasten the pan at the edges only, using cleats if possible
A cleat is a metal strip fastened to the deck that hooks over a hem on the pan edge, holding it down without a screw through the pan. Use a continuous cleat or short cleats at about 12 to 24 inches. If screws are used, they go in the outer flange that the panels will cover, never in the channel.
The pan must be allowed to move slightly as it heats and cools. A valley fastened rigidly along the whole length will buckle. See thermal movement in metal roofs.
Watch out: A screw through the valley floor is a leak waiting to happen. Water runs directly over it every time it rains.
Step 5: Cut the panels to the valley line
The gap between the two panel edges at the valley is typically 2 to 4 inches total, or about 1 to 2 inches each side of centre, and wider on a high-flow valley. Snap a line on each plane and scribe each panel to it. Panels get cut at a different length and angle each time, so work from the eave up and mark each panel before cutting.
Cut with shears or a nibbler, not an abrasive wheel, which damages paint and leaves rust-forming particles. See cutting metal roof panels without damaging the finish. On standing seam, notch and close the seam leg at the cut.
Step 6: Hem or fold the cut edge of each panel
Form a hem of about 1/2 to 1 inch along the cut edge with a hand seamer, or a bent drip edge that turns down toward the pan. This stiffens the panel edge, hides the raw steel and prevents capillary flow under the panel. See how to hem, fold and bend panel ends.
On exposed-fastener panels, add a foam closure or butyl tape under the cut edge so wind-driven rain cannot blow up the ribs.
Step 7: Fasten panels away from the water path and seal the edge
Keep every screw a minimum distance from the valley line, typically 6 inches or more. Screws near the valley sit where water flows hardest in a downpour and where snow melts first. Seal the edge with a bead of butyl tape under the hem so water cannot get under the panel.
Check the valley with a hose from the top, looking for any water that leaves the pan. Clean shavings off the pan, which rust and leave brown stains.
Open Valley Versus Closed Valley on Metal
Nearly all metal roof valleys are open: the pan is visible with a gap between the panels. A closed valley, where panels meet and overlap, is rare in metal because the joint is difficult to seal and water flow is high. Open valleys are also easy to clean and inspect.
Debris and Snow in Valleys
In the Fraser Valley, needles and leaves pack into valleys under cedars and firs, and heavy wet snow can sit in a valley and form an ice dam. A wider pan and a steeper valley slope help water escape. Do not block the gap at the valley with closures or sealant. Clear needles at least once a year.
Common Mistakes
- Screws through the valley floor, in the water path.
- A pan too narrow for the roof area, so water overshoots in heavy rain.
- Unhemmed cut edges, which let water wick under the panels.
- Fastening the pan rigidly along its length, which buckles with temperature change.
- Omitting the self-adhered membrane under the valley.
- Closing the gap down the valley centre so debris and ice block it.
- Cutting with an abrasive wheel, leaving rust stains on the pan.
When to Call a Professional
Valleys are the highest-risk detail on a metal roof and an area where most leaks start. On a roof with valleys, particularly ones with multiple planes, dormers or low pitch, a roofer who works in metal regularly is worth the cost.
If an existing valley is leaking, rusting or packed with debris, a roofer can check whether it is a pan, screw or cut-edge problem.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Metal Roofing service or the towns we cover.
How to Install a Metal Roof Valley: Questions
How do you flash a valley on a metal roof?
Line the valley with self-adhered membrane, then lay a formed metal valley pan over it, lapped downslope with sealant tape between joints. Fasten the pan only along its outer edges, cut the panels back from the centre, hem the cut edges and keep screws away from the water path. The panels sit over the pan edges.
How wide should a metal roof valley be?
Valley pans are commonly 16 to 24 inches wide overall, with a gap of roughly 2 to 4 inches between the panel edges at the centre. Larger roof areas and lower slopes need wider pans. Follow the panel manufacturer's detail and size the valley to the roof area it drains.
Do you put screws in a metal roof valley?
Not in the water path. The pan is held with cleats or with screws in its outer edges under the panels, and never through the valley floor. Panels are fastened several inches back from the valley line. A screw in the valley floor leaks every time the roof drains heavily.
Why do metal roof valleys leak?
Common causes are screws in the valley floor, cut panel edges that were not hemmed, pans too narrow for the water, missing self-adhered membrane, and debris or ice that backs water up under the panels. A leak at the valley is often a trim or panel cut problem, not the pan itself.
Related Guides
All Metal Roofing 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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