How to Build a Roof Valley Where the Two Planes Have Different Pitches
The valley runs off the 45 degree line, it is flatter than either roof, water from the steeper side arrives fast, and the standard layout needs adjusting.
- Professional
- 3 to 5 hours per valley
- Updated
The short answer
When two roof planes meet at different pitches, the valley is not a 45 degree line in plan, it is flatter than either roof, and water from the steeper plane arrives fast enough to cross the valley. Snap the centreline on the actual joint, use a wider membrane, and choose an open valley with a W-rib diverter or a closed-cut valley with the lower-pitch plane run through first and the steeper plane cut. Keep nails 6 inches or more from the centre.
Most valley instructions assume two planes of equal pitch meeting at a symmetrical corner. Real houses rarely cooperate. An addition with a lower roof meeting a steeper main roof, a dormer, or a garage roof running into a two-storey gable all create valleys where the two planes have different slopes. The valley no longer splits the corner evenly, and the two planes behave differently.
The steeper plane sheds water faster and delivers it with more speed. That water can shoot straight across the valley and run up under the shingles on the lower plane, especially in heavy rain. This guide covers how to adjust the layout, which side to run through, and how to use metal and diverters to keep that water in the channel. The shared prep is in how to prep a roof valley.
Tools
- Framing square and pitch gauge
- Chalk line
- Tape measure
- Tin snips (straight cut and offset)
- Hook blade utility knife
- Roofing hammer or coil nailer
Materials
- Ice and water shield, 36 inch wide roll, more for a second layer
- W-valley metal, 24 inches wide or wider
- Roofing nails, 1-1/4 inch
- Asphalt roof cement
- Shingles approved for the valley method
Safety first
- A steep plane feeding a low plane means a steep fall into the valley. Work tied off and use roof jacks on the steep side. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Membrane on a steep plane is slippery, and the valley funnels you toward the eave. Keep the valley clear and do not stand in it.
- Plan shingle loading away from the valley, so bundles do not slide onto the membrane and tear it.
Step by Step
Step 1: Measure both pitches
Put a level and a tape on each plane and record the rise over 12 inches of run. A 3/12 plane meeting a 10/12 plane is a large difference and calls for special care. A 6/12 meeting an 8/12 is a mild difference and a standard detail may work.
Note any low-slope limits for your shingles. Most asphalt shingles are limited below 4/12 without special underlayment, and the lower plane of an unequal valley often sits near that limit. See how to shingle a low-slope roof.
Step 2: Find the true valley line
With unequal pitches the valley does not bisect the corner at 45 degrees in plan, so any layout that assumes a symmetrical V will be off. There is no guessing involved on a framed roof: the valley is wherever the two decks meet. Sight along that joint, snap the centreline on it, and measure membrane width, metal position and cutlines from it.
Check it after the deck is on and recheck if the framing is out of square. A valley rafter that wanders will show up as a curved joint, and the metal or cutline has to follow the joint, not a straight line snapped between the ends.
Step 3: Use a wider membrane
Unequal valleys deserve more protection. Use a 36 inch strip at minimum and consider a second layer centred over the first, or a wider sheet on the low plane where the steeper plane's water is likely to shoot across. Follow the manufacturer's requirements for low slopes and snow country.
Follow the rest of the membrane procedure in valley prep: pressed into the corner, lapped correctly, and no nails near the centre.
Step 4: Choose an open metal valley where the difference is large
The W-rib in open valley metal is a diverter. Water running fast down the steep plane hits the rib and is turned down the valley instead of riding up the lower plane. For large pitch differences, use metal 24 inches wide or wider, with a rib of 1 inch or more, and set the rib on the true valley line. See how to install an open metal valley.
Widen the exposed channel on the steep side. Cutlines that would be 3 inches on a symmetric valley may need more on the plane delivering the fast water, so the channel can carry it.
Step 5: Choose which plane runs through in a closed-cut valley
Common practice is to run the lower-pitch plane through the valley and lap the steeper plane's shingles over it. The steeper plane delivers faster water, so it should ride on the top layer where its cut edge directs water down the valley, rather than on the bottom layer where water could shoot under the cut edge of the plane above.
The cutline is 2 inches back from the true valley centre with the upper corner clipped, and the cut edge bedded in roof cement. See how to install a closed-cut valley. Confirm the method is approved for unequal pitches on the wrapper first.
Step 6: Keep fasteners well clear of the valley
Fast water pushes into every hole and every gap. A nail within the 6 inch zone that would be marginal on an equal valley becomes a leak when a steep plane feeds it. Set extra nails farther back up the shingle if it needs holding.
On metal, fasten the outer edges only, or use cleats.
Step 7: Check the lower end and any transitions
Unequal-pitch valleys often end at an addition wall or a lower roof, where all the valley water lands in one place. See how to fix a dead valley for ends that die into a wall or flat, and how to flash a pitch change where the roof slope changes abruptly.
Test with a hose from the ridge on both planes, one at a time, and watch the valley from above and from the attic.
How the Valley Runs When Pitches Differ
Along a valley the two planes are always at the same height, so for every foot of rise the low-pitch plane needs more horizontal run than the steep one. Take a 12/12 main roof meeting a 6/12 addition with the eaves at the same height. The valley travels 2 feet across the addition's slope direction for every 1 foot it climbs the main roof, so in plan it runs at about 27 degrees to the main roof's eave line instead of 45, and it reaches the main roof lower than a 12/12 addition of the same width's valley would.
The valley itself is also flatter than either roof. Two 6/12 planes produce a valley of roughly 4-1/4 in 12. The 12/12 and 6/12 pair above produces a valley of a little under 5-1/2 in 12. Because the valley is always shallower than the lower plane, water in it moves slower than water on either roof, and a low plane that is already near the shingle's minimum pitch has a valley below that minimum. That is a reason to use full membrane and metal on these valleys, not just a closed shingle valley.
None of this changes where you snap the line on the roof: the valley is where the two decks meet. It explains why the corner looks lopsided from the ground, why the valley rafter is not at 45 degrees, and why the valley drains slower than the planes feeding it.
Fraser Valley Notes
In heavy rain, which can run over 100 mm in a day in parts of the valley, a steep plane feeding a low one can overshoot a standard valley. Wider metal and a full-height rib are cheap insurance. Where a steep plane feeds a low plane under conifers, needles slow the flow and back up water, so pair the layout with a yearly clean out.
Common Mistakes
- Laying out from an assumed 45 degree line. The valley on an unequal roof does not bisect the corner, so membrane and metal centred on a guessed line leave the real joint under-protected.
- No diverter on a big pitch difference. Water from the steep plane jumps the valley and runs under the shingles on the far side.
- Running the steep plane through first. The fast water can shoot under the cut edge of the lower plane's shingles. Industry practice (CRCA and ARMA) is to start on the plane with the lower slope and cut the steeper one.
- Nails too near the centre. Fast water finds every nail hole.
- Ignoring low-slope limits. The lower plane may be under the shingle's minimum pitch, and needs a low-slope detail.
- No second layer of membrane. Large unequal valleys deserve more protection than a single strip.
When to Call a Professional
Valleys with a large pitch difference, a steep plane feeding a low one, or a low plane near the shingle's minimum pitch are not first-time jobs. A roofer will check low-slope limits, choose the valley method, and usually specify wider metal and extra membrane.
If an existing unequal valley is leaking, it is often a diverter problem rather than a flashing failure, and a roofer can rebuild it with the right metal.
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 Handle Valleys With Unequal Pitches: Questions
What happens when two roof planes have different pitches at a valley?
The valley no longer splits the corner at 45 degrees, it is flatter than either roof, and the steeper plane sheds water faster. That water can shoot across the valley and run up under the shingles on the lower plane. The fix is wider membrane, wider open valley metal with a raised W-rib, or a closed-cut valley built with the lower plane running through first and the steeper plane cut back along the centreline.
Which side runs through first in a closed valley with unequal pitches?
Industry practice, including the Canadian Roofing Contractors Association manual and ARMA, is to start on the plane with the lower slope, run its shingles at least 12 inches onto the steeper plane, and cut the steeper plane's shingles 2 inches back from the centreline. The faster water from the steep plane then rides on the top layer and runs down its cut edge instead of shooting under it. Confirm the method on the shingle wrapper.
What is a diverter in a valley?
A diverter is the raised rib running down the centre of W-valley metal, typically about 1 inch high. It stops fast-moving water from the steeper plane from shooting across the valley and up under the shingles on the far side. On valleys with a big pitch difference, a taller rib and wider metal give more protection.
Do valleys with different pitches need special membrane?
They deserve more. A common approach is a 36 inch strip of ice and water shield at minimum, often with a second layer centred over the first or extra width on the lower plane. Also check the low-slope requirements for the lower plane, because it may be close to the shingle's minimum pitch. Follow the manufacturer's instructions.
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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