Roof valleys concentrate runoff from two roof planes, so failures at valley details—flashing, underlayment, or shingle cuts—are a common cause of leaks on asphalt‑shingle roofs.[6] Typical failure points include corroded or split flashing seams, failed underlayment, uplifted or improperly cut shingles at the valley, and failed patching or sealants that can’t withstand concentrated flow.[6]
Three common valley types
Woven valley: shingles from both slopes are interlaced (a traditional look). Many manufacturers and the NRCA caution against—or prohibit—woven valleys with modern laminated/architectural shingles because thicker shingles may not lay flat and can fail to shed water properly.[1][2][6]
Closed‑cut valley: underlayment or flashing lies beneath both slopes, one roof plane is left continuous while the opposite plane’s shingles are cut back to overlap; it is an accepted method when installed per the shingle maker’s instructions and industry guidance.[1][4]
Open (exposed metal) valley: visible metal flashing forms the flow channel; the International Residential Code allows open metal valleys when the metal meets the code’s minimum width and corrosion‑resistance requirements.[5][1]
Initial inspection and emergency steps
Start by tracing the leak path from the attic ceiling stain toward the roof deck to identify where water is entering, then inspect the valley from the roof for flashing condition, shingle cuts, lift/uplift, debris, and gutter/backflow issues that might hide the source.[6][8]
If water is actively entering the interior, protect possessions. If a ceiling bulge threatens collapse, relieve the water into a bucket or make a small controlled hole to drain it (This Old House recommends these emergency interior steps).[8] On the roof, a properly anchored tarp or temporary patching tape/mastic can be used short‑term if it’s safe to access the roof.
Follow OSHA fall‑protection guidance for residential work: don’t work alone on steep roofs and use appropriate anchors, personal fall‑arrest systems, or hire a licensed contractor for work above simple, safe reachable areas.[7]
Diagnosing the specific valley problem
From the attic, follow stains to the highest point of water entry; that helps avoid being misled by downstream drips.[6]
From the roof, check for corroded or split valley flashing seams, missing or failed underlayment, improperly cut or uplifted shingles at the valley, and multiple prior patches that may have failed.[6]
Note the shingle type—if laminated/architectural shingles are present and the valley is woven, the detail may be unsuitable and may void some manufacturers’ warranties; check the shingle manufacturer’s valley instructions before repairing.[2][1]
Repair options and step‑by‑step approaches
1) Replace corroded or failed open (metal) valley flashing
When flashing is corroded or seams have failed, full replacement—rather than patching—is the recommended fix for a concentrated‑flow valley.[6][1]
Typical replacement steps (follow local code and manufacturer instructions exactly): remove adjacent shingles as required, remove existing flashing, install required underlayment (including self‑adhering ice & water barrier where the product requires it), install new metal valley lining sized per code/manufacturer (IRC minimum for open metal valleys is not less than 24 inches wide for the lining and must be corrosion‑resistant), fasten per manufacturer spacing, then reinstall or pattern shingles per the shingle maker’s method.[5][3][6]
2) Repair or replace a closed‑cut valley
For closed‑cut valleys with compromised underlayment or damaged shingles: remove and replace the valley underlayment if it’s failed, cut the downslope shingles carefully according to the shingle manufacturer’s method, and fasten and seal per the maker’s instructions so the overlap and seal points match the required detail.[4][3][6]
3) Convert woven or unsuitable closed‑cut valleys on laminated shingle roofs
Manufacturers and the NRCA generally favor converting failing woven or otherwise unsuitable valley details on modern laminated/architectural shingles to an exposed metal (open) valley or to whichever valley method the manufacturer permits for that product, because woven valleys may not perform and can jeopardize warranty coverage.[1][2][4]
4) Spot shingle repair near valleys
For isolated damaged shingles near a valley, remove and replace the damaged shingles with matching shingles, ensuring the correct overlap, sealant contact points, and nailing pattern per the manufacturer’s instructions—this is a commonly recommended minor repair.[8][3]
Manufacturer and code compliance: why exact instructions matter
Shingle manufacturers (GAF, Owens Corning, CertainTeed) require installers to follow the product‑specific valley instructions precisely to protect warranty coverage; their field guides and technical bulletins identify permissible valley methods and any restrictions (for example, avoid woven valleys with many laminated products). Always consult the specific installation document for fastener spacing, underlayment widths, and whether self‑adhering barriers are required in valleys.[2][3][4][11]
Also check the local authority having jurisdiction for any amendments to the IRC: the code specifies minimum valley lining dimensions and allowable materials (for open metal valleys, lining not less than 24 inches wide and corrosion‑resistant is one example), but local jurisdictions may have adopted changes.[5]
When to replace flashing and underlayment instead of patching
If flashing is corroded, seams have failed, or prior repairs are numerous and aged, replace the valley flashing and underlayment. Patched flashing in a concentrated‑flow valley commonly fails sooner than a full replacement and leaves the roof at continued risk of leaks.[6][1]
Costs and contractor guidance
Consumer cost guides can provide ballpark ranges for valley and flashing repairs, but actual cost varies with job scope, materials, roof pitch, region, and contractor. Use the consumer resources as a starting estimate and get multiple contractor bids for your actual job.[9][10]
Because details vary by product and code, consult the shingle manufacturer’s valley instructions and follow them exactly to preserve warranties and to meet code requirements. Manufacturer field guides and install instructions contain the fastening spacing, underlayment sizing, and leak‑barrier requirements you must follow—do not substitute general rules of thumb for these product documents.[3][2][11]
Practical checklist before you start work
Confirm the valley type and whether the existing detail is compatible with your shingle product; check the shingle manufacturer’s valley instructions.[2][4]
Inspect flashing and underlayment condition from the roof surface; if flashing is corroded or seams failed, plan for full replacement.[6]
Plan to install any specified self‑adhering ice & water barrier or other underlayment per the product instructions where required.[2][3]
Follow OSHA fall‑protection guidance; if you can’t meet safe work‑at‑height practices, hire a licensed contractor.[7]
Get at least two contractor bids for complex valley replacements and confirm contractors will follow the manufacturer method and local code.[9]
Abodivo Tool
Is your roof valley ready for repair?
Quick check
Practical checklist before you start work on a roof valley, as presented in the article.
No explicit guidance is given in the article for when none of these checklist items apply.
Confirm details with the shingle manufacturer’s installation instructions and a licensed contractor before performing repairs.