6.4 Valley Construction Assemblies (Open Metal, Woven, Closed-Cut) & Ridge / Hip Cap Installations

Steep-Slope Valley Assemblies, Flashing Hydraulics & Hip/Ridge Caps

Roof valleys and ridge/hip intersections represent the highest hydraulic and aerodynamic stress zones on a steep-slope residential roof. Valleys collect and channel concentrated volumes of high-velocity stormwater runoff originating from two converging roof planes, creating turbulent hydraulic washing that can erode mineral granules and exploit the smallest flashing defect. Conversely, ridges and hips experience intense negative aerodynamic suction pressures (uplift vortices) during hurricane wind events. Mastering the engineering, flashing integration, and fastening mechanics of valleys and cap assemblies is essential for roofing contractors constructing hurricane-resilient assemblies in Louisiana.

---

1. Valley Hydrodynamics and Vulnerability Analysis

A roof valley is formed where two sloping roof planes intersect at an inside concave angle, creating an inclined drainage trough. Understanding the hydraulics of valley drainage explains why standard shingle application rules do not apply:

               ROOF PLANE A (e.g., 8:12 Pitch)           ROOF PLANE B (e.g., 4:12 Pitch)
               High-Velocity Runoff                     Slow-Velocity / High-Volume Runoff
                        ╲                                     ╱
                         ╲                                   ╱
                          ╲                                 ╱
                           ▼                               ▼
                          ═══════════════════════════════════
                          [ CONCENTRATED VALLEY CENTERLINE  ]
                          [ HIGH HYDRODYNAMIC FLOW DRAINAGE ]
                          ═══════════════════════════════════

The Physics of Valley Water Flow

1. Concentrated Flow Volume: A valley draining two 15-square roof planes handles the runoff of 3,000 square feet of roof surface. During an intense Louisiana downpour (exceeding 3 to 4 inches of rainfall per hour), gallons of water per second surge down the valley trough.
2. Differential Slope Hydraulics: When two roof planes of different pitches meet (e.g., an 8:12 dormer roof meeting a 4:12 main roof), water descending the steeper plane moves at higher velocity and can cross the valley centerline, forcing water beneath the shingles of the opposite, flatter plane.
3. Debris Damming and Granule Erosion: Valleys collect fallen pine needles, leaves, and moss, creating natural dams that cause water to pond and back up under shingle edges. Furthermore, high-velocity water strips mineral granules from shingle surfaces, exposing asphalt to rapid UV degradation.

---

2. Valley Linings (IBC 1507.2.8.2; IRC R905.2.8.2)

The code requires valley linings to be installed before shingles, per the manufacturer's instructions, and allows these lining types:

Most manufacturers add a full-width underlayment in the valley, commonly a 36-inch self-adhering membrane:

                                  ┌──────────────────────────────────────────────┐
                                  │         IRC R905.2.8.2 Valley Underlayment   │
                                  └──────────────────────┬───────────────────────┘
                                                         │
                                  ┌──────────────────────┴───────────────────────┐
                                  │                                              │
         ┌────────────────────────▼─────────────────────┐       ┌────────────────▼────────────────────────┐
         │  Common Practice: 36-in. D1970 Membrane      │       │  Code Option: Two Plies of Roll Roofing│
         │  • 36-inch wide self-adhering membrane       │       │  • Open valleys (D3909 or D6380)       │
         │  • Centered along valley centerline          │       │  • Bottom ply 18 in., top ply 36 in.   │
         │  • Also a code option for closed valleys     │       │  • Fastened only near outer edges      │
         └──────────────────────────────────────────────┘       └─────────────────────────────────────────┘

Installation Sequence:

1. Valley Lining First: Prior to installing any field underlayment, a dedicated valley lining measuring not less than 36 inches in width must be centered down the valley trough.
   • Self-Adhering Polymer-Modified Bitumen (ASTM D1970): The industry standard and preferred method. The 36-inch wide peel-and-stick sheet is scored down the back release paper, folded along the centerline, pressed into the valley junction, and rolled with a weighted hand roller to achieve 100% deck adhesion.
   • Roll-Roofing Alternative (code option for open valleys): two plies of mineral-surfaced roll roofing (ASTM D3909 or D6380), an 18-inch bottom ply centered in the valley and a 36-inch top ply over it.
2. Field Underlayment: lap the field underlayment over the valley lining as the manufacturer instructs, so water on the field underlayment flows onto the valley lining rather than under it.

---

3. Construction of the Three Valley Assemblies

Building codes recognize three primary methods for shingling steep-slope valleys:

┌────────────────────────────────────────────────────────────────────────────────────────┐
│                            VALLEY CONSTRUCTION TYPOLOGIES                              │
├───────────────────────────┬────────────────────────────┬───────────────────────────────┤
│     OPEN METAL VALLEY     │     CLOSED-CUT VALLEY      │         WOVEN VALLEY          │
├───────────────────────────┼────────────────────────────┼───────────────────────────────┤
│ • Center metal exposed    │ • Shingles from one plane  │ • Shingles woven alternately  │
│ • Formed W-diverter rib   │   extend across valley     │   across centerline           │
│ • Shingles trimmed back   │ • Intersecting shingles    │ • Solid, continuous weave     │
│ • Lowest debris clogging  │   cut 2" from center       │ • STRICTLY PROHIBITED for     │
│ • Best for tree-lined lots│ • Clean cut line           │   laminated shingles          │
└───────────────────────────┴────────────────────────────┴───────────────────────────────┘

Method 1: Open Metal Valley (Highest Durability)

Open metal valleys represent the most durable and trouble-free valley assembly, particularly in heavily wooded areas prone to pine straw and debris accumulation.

1. Valley Metal Specifications: At least 24 inches wide and meeting IBC Table 1507.2.8.2. Examples are 0.0179-inch (26-gauge) G90 galvanized steel, 0.024-inch aluminum, and 16-ounce copper.
2. The W-Valley Center Splash Diverter Rib: The metal flashing should feature a 1-inch high crimped vertical inverted-V or inverted-W rib down its center. This rib acts as a hydrodynamic baffle, preventing high-velocity runoff from one slope from washing across the metal and pushing under the shingles on the opposing slope.
3. Fastening the Metal: Secure the metal flashing at the outer edges only using approved metal clips or nails driven within 1 inch of the outer metal flange. Never drive nails through the center drainage pan.
4. Chalk Line Snapping (Tapered Width): Snap two chalk lines, one on each side of the valley centerline. Manufacturers and ARMA place them 6 inches apart at the ridge (3 inches each side of center) and widen them 1/8 inch per foot of valley length toward the eave. An 8-foot valley is 7 inches wide at the eave; a 24-foot valley is 9 inches wide. The widening handles the growing water volume and lets debris wash out. Shingles should lap onto the metal at least 6 inches.
5. Trimming and Dog-Earing: Cut shingles cleanly along the chalk line. Cut a 45-degree angle (dog-ear) measuring roughly 1 inch off the top corner of each trimmed shingle. Dog-earing directs water runoff inward toward the center metal pan rather than along the top edge of the shingle course.
6. Embedding in Mastic: Embed the trimmed shingle ends in a continuous 3-inch wide ribbon of asphalt plastic cement (ASTM D4586) applied along the outer edge of the metal.
7. The 6-Inch Fastener Rule: Under no circumstances may a mechanical fastener be driven within 6 inches of the valley centerline.

Method 2: Closed-Cut Valley (The Contractor Standard for Laminated Shingles)

Closed-cut valleys are the most common valley assembly for laminated architectural shingles, offering a clean, uniform aesthetic without exposed metal.

1. Determine Application Order: Shingle the roof plane with the lower slope or smaller surface area first. If both planes have equal slope and area, shingle the plane that receives less direct visibility first.
2. Cross-Valley Extension: Apply shingles continuously across the valley and extend them at least 12 inches beyond the centerline onto the adjoining roof plane. Drive no fasteners within 6 inches of the valley center. Fasten the extended shingle with a nail placed at the upper corner at least 12 inches past the center.
3. Shingling the Opposing Plane: Apply shingles on the second (steeper or larger) roof plane, bringing them across the valley over the underlying shingles.
4. Snapping the Cut Line: Snap a chalk line 2 inches back from the valley centerline on the side of the second roof plane.
5. Trimming and Dog-Earing: Carefully cut the top shingles along the chalk line using a hook blade (inserting a sheet of protective metal or plywood beneath to avoid cutting into the underlying shingles or underlayment). Clip (dog-ear) the top corner of each cut shingle at a 45-degree angle.
6. Bedding in Asphalt Mastic: Bed the ends of the cut shingles in a continuous 3-inch wide band of asphalt roof cement.
7. Centerline Fastener Rule: Strictly maintain the 6-inch no-nail clearance from the valley centerline.

Method 3: Woven Valley (Strictly Limited to 3-Tab Shingles)

In a woven valley, shingles from both roof planes are laid alternately across the valley centerline, overlapping each other in a basket-weave pattern.

• Absolute Material Limitation: Woven valleys are permitted exclusively with single-layer 3-tab strip shingles.
• Laminated Shingles: Manufacturers generally do not allow woven valleys with laminated architectural shingles. The code does not address it; the manufacturer's instructions govern. Laminated shingles are thick and multi-layered; weaving them through a valley creates massive hollow voids, bridging, and water dams that trap debris, channel water laterally, and cause severe roof leaks.

---

4. Hip and Ridge Cap Shingle Installation

Hips and ridges form the convex intersections where opposing roof planes meet at their upper boundaries. Cap shingles seal these junctions against water penetration while providing finished aesthetics and ridge vent weatherproofing.

                  [ Prevailing wind blows ◄════ from the right; caps started at the left (downwind) end ]

      Course 1                 Course 2                 Course 3 (Terminal Cap)
  ┌──────────────┐         ┌──────────────┐         ┌──────────────┐
  │ 5" Exposure  │ ◄───────│ 5" Exposure  │ ◄───────│ Blind Nailed │ ◄── Sealed Heads
  │ (Downwind)   │         │ Overlaps C1  │         │ & Mastic     │
  └──────────────┘         └──────────────┘         └──────────────┘
  Fasteners: 2 Nails per Cap • Driven 1" from edge & 5-1/2" to 6" from exposed edge

Cap Shingle Fabrication and Materials

1. Field-Cut 3-Tab Caps: Manufactured by cutting standard 3-tab shingles into three individual 12" × 12" (or metric equivalent) cap units. Cut a 1/2-inch to 1-inch taper on each side of the upper headlap to prevent the underlying corners from showing beneath overlapping courses.
2. Pre-Formed Perforated Ridge Caps: Specialized factory-manufactured cap shingles with scored perforations that tear cleanly into individual cap pieces, often featuring multi-layer profiles and high-wind adhesive strips.
3. Modified SBS High-Profile Caps: Heavyweight, pre-folded ridge caps designed to simulate heavy wood shakes or clay ridge rolls, offering Class 4 impact resistance.

Step-by-Step Installation Rules

1. Hip Installation Progression: Hip caps must be installed starting at the lowest point (the eave corner) and progressing upward toward the ridge apex. This ensures that each overlapping cap sheds water downward across the hip.
2. Ridge Installation Progression (Wind Direction Rule): Start ridge caps at the end of the ridge opposite the direction of the prevailing wind and work toward the windward end. Each cap's exposed edge then points downwind, so wind blows over the laps instead of into them. Some crews instead work from both ends toward the middle and finish with a saddle cap.
3. Exposure and Fastener Placement: Standard cap exposure is 5 inches (or 5-5/8 inches for metric). Each cap is fastened with two nails, driven exactly 1 inch from each lateral edge and positioned 5-1/2 inches to 6 inches above the exposed bottom edge (placing the nails in the covered headlap zone). Nails must never be driven through the exposed 5-inch weather portion of the cap.
4. Fastener Length: Because cap shingles wrap over multiple layers of field shingles, valley linings, and ridge vent plastic baffles, standard 1-1/4 inch shingle nails are too short. Contractors must use longer roofing nails measuring minimum 1-1/2 inches to 1-3/4 inches to guarantee full 3/4-inch penetration into structural deck sheathing.
5. Terminal Cap Finishing: The final cap shingle on a ridge cannot be covered by another overlapping unit. The contractor must face-nail the terminal cap with two nails driven through the exposed surface, then completely encapsulate the exposed fastener heads in a generous dollop of UV-resistant elastomeric or polyurethane roof sealant matching the shingle granule color.

---

Comparison Table: Steep-Slope Valley Assemblies