How To Shingle A Roof Valley: The Definitive Professional Installation Guide
Shingling a roof valley requires precise integration of water-shedding underlayment and meticulous shingle cutting to direct high-volume runoff safely away from vulnerable seams. Utilizing an open metal valley or a closed-cut weave method with high-tensile ice and water shield ensures long-term leak prevention and compliance with international residential building codes.
Pre-Operation & Equipment Checklist
A roof valley functions as the primary drainage chute for converging roof planes, handling exponentially more water than standard field shingles. Executing this installation requires strict adherence to safety protocols and precise measurement to prevent structural water intrusion.
Essential Gear, Tools, and Materials:
- Heavy-duty roofing hammer or pneumatic coil roofing nailer
- Utility knife with hook blades for clean shingle cuts
- Chalk line box with blue or red chalk
- Tape measure and heavy-duty tin snips
- Self-adhering polymer-modified bitumen sheet (ice and water shield)
- W-profile or roll valley metal flashing (minimum 24-gauge galvanized steel or aluminum, if using an open valley method)
- Architectural or 3-tab asphalt shingles matching the existing roof
- Roofing nails (galvanized steel, 11-gauge, minimum 1-inch length) and flashing cement (asphalt plastic cement meeting ASTM D4586)
Mandatory Prerequisite Knowledge and Standards:
- Installers must understand slope differentials between intersecting roof planes to determine appropriate underlayment overlaps.
- Work must comply with local building codes, which typically require a minimum 36-inch wide strip of ice and water barrier centered along the entire length of all valleys.
- Harnesses, roof anchors, and OSHA-compliant fall protection are mandatory for sloped surfaces.
Estimated Budget and Duration Benchmarks:
- Material costs range from $150 to $400 depending on linear footage and whether open metal or closed-cut methods are chosen.
- A standard two-person crew can complete preparation, underlayment, and shingling for a 20-foot valley in approximately 2 to 4 hours.
Step-by-Step Roof Valley Installation Workflow
Step 1: Strip, Inspect, and Apply the Ice and Water Barrier
Remove all existing shingles, underlayment, and flashing down to the wood deck across the entire valley and 2 feet out to each side. Inspect the exposed plywood or OSB sheathing for rot, warping, or fastener protrusion, replacing damaged panels immediately. Center a minimum 36-inch-wide roll of self-adhering ice and water shield directly down the center of the valley, running continuously from the eave up to the ridge. Peel away the plastic backing progressively while smoothing the membrane firmly with a heavy roller or hand pressure to eliminate air bubbles and wrinkles.
Pro-Tip: If the valley run exceeds the length of a single membrane strip, lap the lower sheets underneath the upper sheets by at least 6 inches to ensure continuous gravity-fed shedding.
Step 2: Install Metal Valley Flashing (For Open Valleys)
If you are constructing an open metal valley, snap two chalk lines parallel to the center of the valley, starting 6 inches apart at the ridge and widening by 1/8 inch per foot as they run down toward the eave. This splay accommodates the increased volume of water moving downward. Position your pre-bent W-metal or roll valley flashing down the center, overlapping sections by a minimum of 12 inches in the direction of water flow, and seal the overlap with a heavy bead of polyurethane roofing sealant. Secure the metal by nailing through the outer edges approximately 1 inch from the outside perimeter, spacing nails every 8 to 10 inches.
Warning: Never drive fasteners through the center 4 inches of the valley metal or within 2 inches of the center crease, as exposed fastener holes will inevitably leak under moving water.
Step 3: Run and Trim Field Shingles Into the Valley
Install your standard field shingles across the main roof planes, bringing them horizontally toward the valley until they overhang the designated valley boundary line. If installing a closed-cut valley, extend the shingles from the roof plane with the lesser slope continuously across the valley and at least 12 inches onto the intersecting roof plane. If installing an open metal valley, snap a straight chalk line parallel to the center metal strip, maintaining your predetermined width, and cut the shingle ends cleanly along this line.
Step 4: Seal, Clip, and Secure Valley Shingle Edges
Apply a 3-inch wide band of asphalt plastic cement along the cut edges of the shingles that interface with the valley to prevent wind-driven rain from lifting the tabs. Clip the top corner of each shingle entering the valley at a 45-degree angle (approximately 2 inches off the corner) to prevent water from catching on square corners and backing up under the shingle course. Drive a single roofing nail at least 12 inches back from the valley centerline to anchor the cut shingle securely; never place a nail closer to the valley center.
Roof Shingle Installation
Technical Comparison of Valley Construction Methods
| Parameter | Open Metal Valley | Closed-Cut Valley | Woven Valley |
|---|---|---|---|
| Aesthetic Impact | Highly visible metal strip running down the architectural lines. | Clean, uniform shingle appearance with a neat diagonal line. | Completely covered by interwoven shingles for a seamless look. |
| Durability & Lifespan | Exceptional (metal outlasts asphalt shingles by decades). | Moderate to High (relies heavily on shingle wear resistance). | Moderate (high friction and granule wear points). |
| Debris Clearance | High (pine needles and leaves wash away easily over smooth metal). | Moderate (debris can catch along the diagonal cut line). | Low (debris easily traps within the overlapping folds). |
| Skill & Labor Intensity | Moderate (requires precise metal cutting and bending). | Low to Moderate (standard cutting techniques). | High (tedious weaving of every shingle course). |
Common Site Failures and Field Fixes
Root Cause: Fasteners driven too close to the valley centerline puncture the underlying ice and water shield or metal flashing.
- Actionable Fix: Lift the surrounding shingles, pull the errant nail, and apply a generous patch of polymer sealant over the hole. Install a new piece of flashing or membrane over the compromised zone if the damage is extensive.
Root Cause: Debris accumulation and ice damming backing water up underneath the upper shingle courses.
- Actionable Fix: Clear all organic debris from the valley biannually. If ice dams persist, ensure proper attic insulation and soffit ventilation to stabilize roof deck temperatures.
Root Cause: Failure to clip the top corner of the shingle entering the valley, causing water diversion.
- Actionable Fix: Carefully lift the overlapping shingle tab, use a utility hook blade to slice off the top 45-degree corner, apply roofing cement beneath the tab, and press it back down firmly.
Root Cause: Insufficient underlayment overlap allowing wind-driven moisture to penetrate the deck seams.
- Actionable Fix: Strip back the field shingles adjacent to the failure point, clean the deck, and install a secondary layer of self-adhering membrane that extends at least 2 feet past the leak source.
Frequently Asked Questions
How wide should an open roof valley be at the eave?
An open valley should typically start at 6 inches wide at the top (ridge) and widen downward at a rate of 1/8 inch per foot of run. This means a 20-foot valley will measure approximately 8.5 inches wide at the bottom (eave) to safely accommodate accelerated water volume.
Can I install new valley shingles over old ones during a re-roof?
Installing new shingles over old valley systems is strictly discouraged by industry standards and building codes. Always strip the valley down to the bare wood deck to inspect for rot, remove trapped moisture, and install a fresh ice and water barrier.
Do I need ice and water shield for roof valleys in warm climates?
Even in warm climates without freezing temperatures, valleys handle concentrated volumes of wind-driven rain and standing water. Building codes and manufacturer specifications universally mandate self-adhering waterproofing membrane in valleys regardless of regional climate.
What is the best tool for cutting shingles cleanly inside a valley?
A heavy-duty utility knife equipped with a hook blade is the optimal tool for cutting asphalt shingles. The curved hook allows you to slice smoothly through the granule surface and asphalt core from the top or underside with maximum precision.
How do I prevent water from backing up under closed-cut valley shingles?
You must apply a continuous 3-inch wide bed of asphalt plastic cement under the edge of every shingle terminating in the valley, and ensure all top corners are clipped at a 45-degree angle to direct water downward rather than sideways.
Protect your home's structural integrity by utilizing professional-grade underlayment materials and precise cutting techniques for every valley installation. Consult with a licensed roofing contractor if your roof geometry involves complex multi-plane intersections or steep pitches.
