The Short Answer
Roof valleys leak because they carry the water of two slopes at once, so any weakness is tested hard. The usual causes are nails driven too close to the valley centerline, a missing or undersized waterproof lining under the shingles, uncut or unclipped shingle corners that steer water sideways, debris that dams the channel, and ice that backs meltwater up the valley in winter. A lasting repair opens the valley, replaces the lining and rebuilds the shingles or metal to the manufacturer's rules.
Key Points
- A valley collects the runoff of two roof planes, so small installation errors leak there first.
- There are three valley styles: open metal, closed-cut and woven; each fails in its own way.
- Nails within 6 inches of the centerline and a missing valley lining are two of the most common causes.
- Michigan snow drifts and ice dams load valleys harder than any other part of the roof.
- A real repair opens the valley down to the lining; sealant on the surface only buys time.
Why Valleys Take the Most Water
Picture two slopes draining toward each other. Every square foot of both planes that drains toward the valley ends up running down a channel only a few inches wide. Water in a valley moves faster and deeper than water on the open field of the roof, and in a heavy rain it can run sideways under the edges of shingles that would never see water anywhere else.
The volume grows as the valley runs downhill. That is why GAF's open valley instructions call for the gap between the shingles to widen toward the eave: the bottom of a valley handles the water of the whole valley above it.
Open, Closed-Cut and Woven Valleys
Every shingle valley is built one of three ways. The style matters for a repair, because a repair should match how the valley was built or deliberately convert it to a better method. Under any of them there should be a waterproof lining. The 2015 Michigan Residential Code, Section R905.2.8.2, says valley linings shall be installed according to the manufacturer's instructions before the shingles go on.
Open metal valley
Metal exposed down the center
The shingles stop short of the centerline on both sides and a strip of corrosion-resistant metal carries the water. GAF's Timberline instructions call for metal at least 20 inches wide and 24 gauge, held by nails at its edges only, never through the metal, with the shingles overlapping it at least 4 inches and their ends set in asphalt plastic cement. The open gap starts at 6 inches at the top and widens 1/8 inch per foot toward the eave. Open valleys handle volume and debris well, but they depend on the metal and on the cut shingle edges staying sealed.
Valley metal and flashing repairClosed-cut valley
Shingles cover the center
Shingles from one slope run across the valley and at least 12 inches onto the other slope. Shingles from the second slope lap over them and are cut along a chalk line 2 inches back from the centerline, with the top corners clipped at 45 degrees and the cut edge sealed with roofing cement. It is widely used on architectural shingle roofs and looks clean, but it is unforgiving: a wrong cut line, an unclipped corner or one nail in the wrong spot can leak.
Woven valley
Shingles from both sides interlaced
Shingles from each slope alternate across the valley like a basket weave. It was common with older 3-tab roofs. GAF's current Timberline instructions show only open and closed-cut valleys, and NRCA describes woven valleys as acceptable only with 3-tab and low-profile architectural shingles on slopes steeper than 4:12, noting they collect more debris, snow and ice and are prone to bridging. If you have a woven valley on an older 3-tab roof, a repair usually means rebuilding it as closed-cut or open.
Repairing 3-tab roofsSnow, Ice and Valleys
Wind blows snow off ridges and drops it in the inside corners, so valleys often hold the deepest snow on the roof. When attic heat or sun melts the bottom of that snowpack, the water runs down the valley under the snow. At the bottom, where the valley crosses the cold overhang, it refreezes. That is an ice dam in the worst possible spot: the place where the roof concentrates the most meltwater.
Water backed up behind ice does not flow down the valley; it sits and rises. Shingles are designed to shed water running downhill, not to hold standing water, so it finds every nail hole and every lap near the centerline. A valley that never leaks in summer rain can leak every February thaw. If your leak only shows up in winter, the attic and the eave are as much a part of the diagnosis as the valley itself.
This is why the lining under the valley matters so much here. The 2015 Michigan Residential Code requires an ice barrier at the eaves of shingle roofs (two layers of underlayment cemented together or a self-adhering polymer modified bitumen sheet) running from the edge to at least 24 inches inside the exterior wall line. Manufacturers go further in the valley itself: GAF's instructions center a full-width roll of its self-adhered leak barrier down the length of the valley, with no fasteners within 6 inches of the centerline.
Ice dams are an attic problem showing up on the roof; the ice dam page linked below covers the full mechanism.
What the Michigan Residential Code Says About Valley Linings
Michigan's code goes further than the one-line rule most people quote. Section R905.2.8.2 of the 2015 Michigan Residential Code lists the valley linings that are permitted, with minimum sizes, and those minimums apply on top of whatever the shingle maker's instructions say. Where the two differ, the stricter number wins.
One example matters for open valleys. GAF's instructions call for valley metal at least 20 inches wide, but the code says metal lining for an open valley "shall be not less than 24 inches (610 mm) wide." Its Table R905.2.8.2 also sets minimum metals, including 26 gauge (0.0179 inch) G90 galvanized steel, 0.024 inch aluminum and 16 ounce cold-rolled copper. A thin strip of bent aluminum trim coil in a valley does not meet that table.
Michigan also answers the ice barrier question for the whole state. The ice barrier column of the state's Table R301.2(1) reads "Yes," so Section R905.1.2 applies to shingle roofs everywhere in Michigan: an ice barrier from the eave to at least 24 inches inside the exterior wall line, where the bottom of most valleys sits. Under Section R102.7.1, a rebuilt valley is new work and must meet these rules even when the rest of an older roof does not.
- Open valley, metal: lining at least 24 inches wide, from a metal listed in Table R905.2.8.2.
- Open valley, roll roofing: two plies of mineral-surfaced roll roofing, the bottom ply 18 inches wide and the top ply at least 36 inches.
- Closed valley: one ply of smooth roll roofing at least 36 inches wide, either open-valley lining above, or a self-adhering polymer modified bitumen underlayment meeting ASTM D1970.
- Old valley metal: when a roof is re-covered or replaced, Section R908.5 says rusted, damaged or deteriorated flashings shall be replaced, not reused.
Michigan Residential Code rules for roof repairs
The Chapter 9 sections that govern a repair, from flashing and nails to drip edge and reroofing, and what R102.7.1 means for older roofs.
Code rules for repairsThe Mistakes That Make Valleys Leak
Many valley leaks trace back to the day the roof went on, not to old age. The mistakes are small, invisible from the ground and can take years to show. These are the ones to know, each checked against the manufacturer's written instructions.
- Nails too close to the centerline. GAF's closed-cut and open valley instructions both say no fasteners within 6 inches of the valley centerline, because nails near the center sit in the deepest water. NRCA guidance recommends keeping them 8 inches back. A nail an inch or two from the center is a hole in the bottom of the channel.
- No valley lining, or the wrong one. Without a full-width waterproof membrane centered in the valley, every nail and lap above it relies on the shingles alone. Older roofs often have plain felt, or nothing extra at all.
- Upper corners not clipped. On the cut side of a closed-cut valley, each shingle's top corner should be clipped at 45 degrees. A square corner points into the valley and catches water running across the lap, steering it sideways under the shingle and onto the nail line.
- A cut line in the wrong place. The cut shingles should end 2 inches from the centerline along a straight chalk line. Cut too close and they sit in the flow; cut ragged or curving and some shingles end up with gaps or overhangs that channel water.
- Short crossover shingles. The shingles that run through a closed-cut valley should extend at least 12 inches onto the opposite slope. Short ones leave the cut side lapped over nothing but underlayment.
- Shingles bridging the valley. Laminated shingles are stiff. If they are not pressed down into the valley before nailing, they span the corner like a bridge, crack under snow load or foot traffic, and leave a hollow where water collects.
- Valley metal nailed through the middle. GAF warns that nailing through open valley metal can cause leaking and buckling as the metal moves. It should be held at the edges.
Debris, Wear and Where the Valley Ends
Not every valley leak is an installation error. Leaves, needles and seed pods settle in the valley and form small dams. Water backs up behind them, spreads sideways under the cut edges and soaks the shingle laps. A valley that stays damp under debris also grows moss that lifts shingle edges.
Valley shingles also wear out first. Constant flow strips granules from the channel, and the exposed asphalt dries and cracks sooner than the field. On a roof in its later years, the valleys are usually the first place you can see bare, dark shingle.
Finally, look at where the valley ends. A valley that dumps onto a lower roof, against a sidewall or into an inside corner sends a concentrated stream onto a spot that may not be built for it. If that bottom detail is wrong, the stain shows up near the valley even though the valley itself is fine.
How to Tell if Your Leak Is the Valley
Water travels along the deck and framing before it drips, so a stain near a valley does not prove the valley is the source. A chimney, a pipe boot or a dormer sidewall uphill can send water to the same spot. Still, a few patterns point strongly at a valley.
- From inside: a stain along the line of an inside roof corner, or at the low end of it near an exterior wall.
- In the attic (only where you can stand on framing safely): darkened wood or water tracks along the valley rafter, or rusty nail tips near the center of the valley.
- From the ground with binoculars: debris piled in the valley, dark bare shingles in the channel, lifted cut edges, or exposed nail heads near the centerline.
- In winter: a leak that starts during a thaw, with a thick ridge of ice at the bottom of the valley.
What a Proper Valley Repair Includes
A valley is a system: deck, lining, underlayment laps, shingles or metal, and the details at the top and bottom. Fixing one visible spot rarely works because the water that found that spot came down the whole channel. A real repair opens the valley and rebuilds it to the manufacturer's instructions.
- Step 1
Open the Valley
Remove the shingles on both sides of the valley for its full length, or at least well past the failed area, without destroying the field shingles that will be woven back in.
- Step 2
Check the Deck
Look for soft, delaminated or stained sheathing along the valley. Rotted wood gets cut out and replaced before anything goes back on.
- Step 3
Install a Full-Width Lining
Center a self-adhering leak barrier down the valley, with laps per the manufacturer, and run the field underlayment over it (GAF calls for at least 6 inches).
- Step 4
Rebuild the Valley
Closed-cut: crossover shingles at least 12 inches past center, cut line 2 inches back, corners clipped at 45 degrees, cut edge sealed. Open: metal centered and held at its edges, shingle edges set in cement. No nails within 6 inches of center either way.
- Step 5
Finish the Ends
Tie the top of the valley into the ridge or hip properly and make sure the bottom delivers water onto the shingles or into the gutter, not behind a wall or fascia.
- Step 6
Clean Up and Document
Clear the debris, photograph the deck, lining and finished valley, and note what was found so you have a record if the stain ever reappears.
At Stellar, a valley rebuild typically falls in our moderate repair tier, about $1,500 to $4,000 and usually a full day, depending on length, pitch and any deck damage found.
Patch, Rebuild or Bigger Conversation
Roofing cement smeared over a lifted cut edge or a nail hole in the valley can stop a drip for a season. It will not fix a missing lining, a nail line in the wrong place or a valley full of worn shingles, and the cement itself cracks with freeze-thaw.
If the rest of the roof is sound, rebuilding a valley is a normal, contained repair. If the valleys are failing because the whole roof is near the end of its life, the honest conversation is about how much you want to spend on a roof you may replace soon.
Spot patch vs section rebuild
When a small repair is honest and when it is money spent twice.
Compare repair scopesMatching the new shingles
A rebuilt valley sits in plain view. What to expect when the original color is discontinued or weathered.
Shingle matchingValley Types Compared
| Open metal | Closed-cut | Woven | |
|---|---|---|---|
| What carries the water | Exposed metal down the center | Shingles from one slope, cut shingles over them | Interlaced shingles from both slopes |
| Used with | Most shingles; common on older and custom roofs | Most strip and architectural shingles | Mainly 3-tab; not in GAF's Timberline instructions |
| Strengths | High water volume, sheds debris, easy to inspect | Clean look, shingles double up in the channel | Double shingle coverage, no exposed cut edge |
| Typical failure points | Cut shingle edges unsealed, nails through metal, corroded or undersized metal | Nails near center, unclipped corners, bad cut line, short crossover | Bridging, debris and ice buildup, cracked shingles at the weave |
| Manufacturer nail rule | Nail metal at edges only; no nails within 6 in. of center | No nails within 6 in. of center | Follow the shingle maker's instructions |
Dimensions from GAF Timberline installation instructions (RESTL622, 4/23). Other manufacturers' rules differ; the shingle maker's instructions for your product govern.
Leak Near an Inside Corner of the Roof?
We trace valley leaks from the attic and the roof, show you photos of what we find, and lay out the repair options with a firm written price. The inspection is free.
Related Guides
Why Roof Leaks Show Up Far From the Source
How water runs along the deck and rafters before it drips, and how to trace it.
Learn MoreRoof Patch vs Section Repair
When a small fix is honest and when rebuilding a section saves money.
Learn MoreFlashing Repair
How we rebuild valleys, chimneys, walls and pipe boots with layered metal.
Learn MoreIce Dams
Why ice builds at the eaves and in valleys, and how to stop it coming back.
Learn MoreQuestions We Hear
Which roof valley type is best for Michigan?
There is no single winner. A properly built closed-cut or open metal valley over a full-width self-adhered membrane both hold up well here. Open metal valleys shed leaves better under heavy tree cover; closed-cut suits most architectural shingle roofs. Woven valleys are mostly a 3-tab method and collect more snow and debris. The lining under the valley matters more than the style on top.
Can I just put roofing cement or caulk in the valley?
It can slow a drip from a lifted cut edge for a while, but it cannot fix a missing lining, nails placed too close to the center or worn valley shingles. Cement also cracks with freeze-thaw and can trap water once it lets go. Use it only as a stopgap until the valley is opened and rebuilt.
How often should roof valleys be cleaned?
Check them from the ground every spring and fall, and after big storms. Valleys under oaks, maples or pines may need clearing once or twice a year. Use a soft method from a safe position, or hire it out; walking a valley and scraping it with a shovel damages the shingles.
Does a valley repair mean replacing the whole roof?
Usually not. A valley can be opened, relined and rebuilt while the rest of the roof stays in place, with the new shingles woven into the existing ones. It becomes a bigger conversation when the shingles around the valley are brittle and near the end of their life, because new work tied into failing shingles does not last as long as the repair itself.
Why does my valley only leak when snow melts?
Snow drifts deepest in valleys. When heat from the attic or sun melts the bottom of the snowpack, the water runs down the valley and refreezes at the cold eave, forming ice that backs meltwater up the channel. Standing water then finds nail holes and laps that rain alone never reaches. The cure is usually a better valley lining plus attic air sealing and ventilation.
What This Page Cites
- Timberline HDZ, UHDZ, ASII, CS, HDZ RS and RS+ Installation Instructions (RESTL622, 4/23), GAF (https://images.thdstatic.com/catalog/pdfImages/ce/ce744b21-6c3d-4c1e-9396-919c6ffa7fe8.pdf), accessed October 2026
- In the valley, Tom Bollnow, Professional Roofing, National Roofing Contractors Association, March 2013 (https://professionalroofing.net/articles/article/2236), accessed October 2026
- Roofing handout, Michigan Residential Code 2015 (R905.2.7.1 ice barrier, R905.2.8.2 valleys, R907.5), Muskegon Charter Township Building Department (https://muskegontwpmi.gov/wp-content/uploads/2025/09/Roofing-Handout-Fillable.pdf), accessed October 2026
- 2015 Michigan Residential Code, Chapter 9 Roof Assemblies (R905.1.2, R905.2.8.2 and Table R905.2.8.2, R908.5), International Code Council (https://codes.iccsafe.org/content/MIRC2015P2/chapter-9-roof-assemblies), accessed October 2026
- 2015 Michigan Residential Code, Chapter 3 Building Planning, Table R301.2(1) (R 408.30516), International Code Council (https://codes.iccsafe.org/content/MIRC2015P2/chapter-3-building-planning), accessed October 2026
- 2015 Michigan Residential Code, Chapter 1 Scope and Administration, Section R102.7.1, International Code Council (https://codes.iccsafe.org/content/MIRC2015P2/chapter-1-scope-and-administration), accessed October 2026