Valley Construction in Metal Roofing: Open vs Closed
A valley carries the runoff of two roof planes through one narrow channel and fills with ice every winter. Here is how a metal valley is actually built.
A valley is the highest-stress water detail on a pitched roof. Two planes drain into one channel, so the flow rate through it is far higher than anywhere else on the roof, it moves fast, and in an Ontario winter it packs with ice and stays full of it for weeks. Every valley on a metal roof is an open valley, and understanding why explains most of how one is built.
Why Metal Valleys Are Always Open
On an asphalt roof there are three choices: a closed-cut valley where shingles from one plane run through and shingles from the other are cut back over them, a woven valley where courses interlace, and an open valley with exposed metal in the channel. Metal panels can only do the last one. A standing seam or ribbed panel is a rigid formed sheet — it cannot be woven, and it cannot be laid over a fold in the roof plane. So the valley gets a formed metal pan and the panels stop short of it on both sides.
The Valley Pan
The pan is formed sheet, generally the same gauge and finish as the roof, folded to the valley angle and hemmed along both outer edges. Those hems do two jobs: they stiffen a long, unsupported piece of metal so it does not oil can or flutter, and they give a clean edge for the cleats to hook.
- ✓Overall pan width is generous — commonly in the range of twenty to twenty-four inches, which puts eight to ten inches of metal under the panels on each side.
- ✓Wider is better in snow country. A valley that fills with ice needs the water backing up behind that ice to still be sitting on metal, not on underlayment.
- ✓Self-adhered membrane runs the full length of the valley, centred on it and wide, under the pan. This is a location where membrane is not optional.
- ✓Field underlayment from both roof planes laps over the valley membrane so the drainage direction is respected.
- ✓The pan is held by cleats hooked into its hemmed edges. There are no fasteners through the face of a valley pan, ever.
The W Rib and Cross-Flow
A plain V-shaped pan has one weakness: on a valley where the two planes are unequal — different areas, different pitches, or one plane much larger than the other — the water coming off the big plane arrives with real velocity and shoots across the valley and up under the panels on the small side. Heavy rain and rapid melt both do it. The answer is a W-valley: the pan is formed with a raised rib running down its centre, which acts as a divider. Water from each side is held on its own half of the channel and cannot cross.
The centre rib also stiffens the pan considerably, which matters on long valleys, and it gives a definite line to work to when cutting the panels. On any valley of consequence — long, unequal planes, low pitch, or heavy expected flow — the W profile is the right call.
How the Panels Meet the Valley
The panels are cut on the valley angle, and this is where the work shows. The cut edge of each panel is hemmed — folded under — so that no raw cut edge sits in the water channel. A raw edge wicks water up under the panel by capillary action, and on a steel panel it will also rust and streak orange down the valley within a couple of years.
The panels stop short of the valley centreline, leaving an open channel. A common approach is to keep the panel edges a few inches back from centre at the top and to widen that opening progressively toward the eave, because the volume of water in the channel increases as it descends. A valley that is the same width top to bottom is narrowest, relative to its flow, exactly where the flow is greatest.
No fasteners through a valley pan. Not one. Every screw through the face of a valley is a hole sitting in the fastest-moving, highest-volume, most ice-prone water on the roof, and it will be under standing water every time an ice dam forms in that valley. The pan is held with concealed cleats hooked into its hemmed edges.
Terminations
The two ends of a valley are as important as the middle. At the eave, the pan runs over the eave trim and is cut and folded so the water it is carrying is directed out over the trim and into the eavestrough rather than being allowed to spill sideways behind it. A valley that simply stops at the fascia dumps a concentrated stream at one point, and in winter that point builds an ice column.
At the top, a valley either runs up to a ridge, dies into a headwall, or meets another valley. Each of those junctions needs a formed transition, membrane run generously in every direction, and no reliance on sealant to make the connection.
Dead Valleys
A dead valley is a valley with little or no fall — where a roof plane runs into a wall or another roof and there is nowhere for water to go quickly. They are common where an addition meets an original house, and they are the single most reliable source of leaks on a complicated roof. The correct answer is usually to build a cricket that gives the water somewhere to go, or to build a fully sealed pan — soldered or welded rather than lapped — because the detail will hold standing water and ice by design. A dead valley detailed as though water will run through it will fail.
Maintenance
Valleys collect. Leaves, pine needles, shingle grit from a neighbouring roof, seed pods — all of it funnels into the valley and then packs at the eave end. A dammed valley holds water where the assembly was designed to shed it. Clearing valleys, particularly on properties with mature trees, is one of the few genuinely useful maintenance tasks on a metal roof, and it is best done in late autumn before things freeze in place.
Frequently Asked Questions
How wide should a metal valley be?
Wide enough that the pan extends well under the panels on both sides — the twenty to twenty-four inch range is common for the overall pan. In a snow climate, err wide, because the effective channel narrows dramatically once ice builds in it.
Should I specify a W-valley or a plain V?
A W is the safer choice on almost any residential valley and is strongly indicated where the two planes are unequal in size or pitch, where the valley is long, or where the pitch is shallow. The centre rib prevents cross-flow and stiffens the pan.
Why does my valley only leak during heavy rain?
That pattern usually points to cross-flow or to the panels being cut too close to the centreline. In ordinary rain the channel handles the volume; in a downpour the water depth exceeds the effective channel and runs sideways under the panel edge. A W rib and a wider opening address it.
Do valleys need ice and water shield if there is a metal pan?
Yes. The pan sheds running water. The membrane is what protects the deck when the valley is full of ice and water is standing in it, which happens most winters somewhere in Southern Ontario.
Can a closed valley be done in metal?
No. Closed-cut and woven valleys depend on overlapping small courses of flexible material. A metal panel is rigid and formed, so a metal roof valley is always an open valley with a formed pan.
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