Valleys collect runoff from two roof slopes, so a small installation error can expose the roof deck to substantial water. Ice and water shield—a self-adhering waterproofing membrane—adds protection beneath shingles or other roofing materials. Correct ice and water shield valley installation depends on the roof design, roofing manufacturer’s instructions, local building code, and the membrane’s specifications. The steps below explain common methods, preparation, and inspection points for U.S. homes.
Meta description: Learn how to install ice and water shield in a roof valley, including preparation, membrane placement, overlaps, flashing details, and common mistakes.
What Ice And Water Shield Does In A Roof Valley
Ice and water shield is a generic term for a self-adhering roof underlayment designed to resist water penetration. Many products use a rubberized asphalt or other polymer-modified adhesive layer with a protective facing. When installed on a clean, sound roof deck, the membrane adheres to the surface and helps seal around fasteners driven through it.
In a valley, the membrane forms a secondary water-shedding layer beneath the visible roofing. It can help protect the deck if wind-driven rain, snowmelt, or water backing up beneath shingles reaches the valley. It is not a substitute for properly installed shingles, valley flashing, or sound roof construction.
Valleys concentrate water from adjoining roof planes. They may also collect leaves, pine needles, and other debris, which can slow drainage. Because of this exposure, contractors commonly install a wider strip of membrane in the valley than they use along other roof areas. The required width and method depend on the product, roofing system, and local requirements.
Check The Roof Design, Product Instructions, And Code
Before work begins, identify the valley type and the roofing material. Common asphalt-shingle valley details include open valleys, where metal flashing remains exposed, and closed valleys, where shingles cover the valley. Some closed valleys use shingles woven across the valley; others use a cut-valley method. Not every shingle or roofing product permits every method.
Read the instructions for both the membrane and the roof covering. They may specify acceptable deck materials, minimum installation temperatures, primer requirements, side and end laps, valley width, and whether the membrane may be exposed temporarily. Follow the more restrictive applicable instructions, and check for local code amendments.
Building-code requirements vary by location. For example, the International Residential Code’s ice-barrier provisions address areas where ice dams are likely and specify protection near eaves; the exact requirements and adoption status depend on the jurisdiction. That provision should not be treated as a universal instruction for valley width. Local officials, project plans, and roofing manufacturers may require additional valley protection.
For background, consult the International Residential Code adopted by the relevant jurisdiction and the membrane and roofing manufacturers’ current installation guides. Product instructions take priority over generic installation advice where they differ.
Tools And Materials
Typical materials include the specified ice and water shield, compatible valley metal if the roofing design calls for it, and the approved roof covering. Common tools include a tape measure, chalk line, utility knife, straightedge, roofing shovel for tear-off, broom or brush, and a roller if required by the membrane manufacturer. A primer may be necessary on certain surfaces or in cold conditions, but it should be used only when the product instructions call for it.
Roof work carries serious fall and weather risks. Use appropriate fall protection, secure ladders, and avoid working on wet, icy, windy, or otherwise unsafe surfaces. Roofing materials can become slippery, and a partially installed membrane may be difficult to walk on.
Prepare The Roof Deck
- Inspect the deck. Replace rotted, delaminated, or damaged sheathing. Fasten loose panels and correct protruding nails or other sharp objects that could puncture the membrane.
- Remove debris. Sweep away dust, sawdust, old roofing fragments, and loose material. The deck should be clean and dry unless the product specifically permits another condition.
- Check the surface. Confirm that the deck is suitable for the membrane and that its temperature and the ambient conditions meet the product’s application limits.
- Plan the layout. Mark the valley centerline and determine the membrane width, seams, and any valley-metal arrangement before removing the release film.
Adhesion can fail when a membrane is installed over damp wood, frost, dust, or incompatible coatings. If the deck cannot be dried or prepared as required, installation should wait rather than rely on the membrane to bond over a poor surface.
How To Install Ice And Water Shield In A Valley
Choose The Valley Membrane Layout
Many installations use a continuous membrane strip centered in the valley, extending onto both roof planes. A commonly used width is 36 inches, but this is not a universal requirement. The specified width may be greater or smaller depending on roof design, product directions, local rules, and the roofing system. Measure the coverage on both sides of the valley, not just the roll width.
Whenever possible, use a piece long enough to cover the valley without a crosswise seam. If the valley length requires more than one piece, arrange the upper piece to overlap the lower piece so water flows over the lap rather than into it. Use the manufacturer’s stated end-lap dimension; do not assume that a particular overlap is suitable for every product.
Install The Membrane
- Center the first strip. Align the membrane with the valley centerline and confirm that it covers the intended area on both roof slopes. Keep it straight and avoid stretching it.
- Peel and position gradually. Remove the release liner in manageable sections. Press the membrane onto the deck as it is laid, working from the center outward to limit trapped air and wrinkles.
- Roll or press the surface. Use the pressure method specified by the manufacturer. Pay particular attention to edges and laps, where poor contact can allow water to travel underneath.
- Install additional lengths in water-shedding order. At a crosswise joint, the upper piece should lap over the lower piece. Seal or roll the lap as directed by the product instructions.
- Protect the valley ends. Extend and terminate the membrane at the eave, ridge, and valley intersections according to the roof plan and product details. Avoid leaving gaps where adjoining roof details meet.
Some systems use separate membrane pieces on each slope or a membrane-and-metal combination. In those cases, follow the roofing manufacturer’s detail for the specific valley construction. Avoid improvising a seam or cutting pattern that could direct water beneath the membrane.
Coordinate The Membrane With Valley Flashing
Open valleys commonly use metal flashing that remains exposed between the shingles. Closed valleys use shingles over the valley center, with the precise method determined by the roofing product. A membrane may be installed beneath the flashing or shingles as part of either design, but the layers must be arranged to shed water outward and down the roof.
Where metal valley flashing is specified, install it in the sequence and manner required by the roofing-system instructions. Confirm that the metal is compatible with the surrounding materials, properly secured, and free of sharp edges that could damage the membrane. Do not place fasteners in locations prohibited by the flashing or shingle instructions.
For a closed valley, follow the approved shingle layout and cutting details. A common cut-valley approach keeps shingles from both slopes out of the central drainage area, but the exact cutting line and fastening locations vary. For an open valley, shingles should terminate at the designated distance from the valley centerline, leaving the planned metal channel exposed.
Valley metal, membrane, and shingles work together as a system. A membrane that is too narrow, poorly bonded, or interrupted at a roof intersection cannot compensate for incorrectly installed flashing or shingles.
Common Installation Mistakes To Avoid
- Guessing at the width or overlap. Product instructions and local requirements determine dimensions. A familiar measurement is not automatically correct for a particular roof.
- Installing over a dirty or wet deck. Poor surface preparation can prevent adhesion and create pathways for water.
- Leaving wrinkles, bubbles, or fishmouths. Raised areas can interfere with roofing materials and create channels. Smooth the membrane as it is installed, following the manufacturer’s method.
- Reversing a lap. A crosswise lap that faces into the direction of runoff can funnel water beneath the membrane.
- Stretching or pulling the sheet tight. Tension can cause the membrane to shift or wrinkle as it warms and cools.
- Using unapproved fasteners or sealants. Certain adhesives and roofing products may be incompatible. Use only materials permitted by the system instructions.
- Leaving the membrane exposed beyond its stated limit. Some products have restrictions on ultraviolet exposure or temporary weather exposure. Install the roof covering within the stated period.
- Ignoring the full valley detail. Membrane coverage must connect properly with eaves, ridges, sidewalls, and other transitions to avoid gaps.
Inspection Checklist Before Installing Shingles
Before covering the membrane, inspect the valley from both roof slopes. Confirm that it is centered, bonded, and free of open edges, tears, punctures, trapped debris, and significant wrinkles. Check all seams for the specified overlap and firm contact. Verify that the membrane extends to the planned boundaries and connects with adjoining waterproofing details.
Also confirm that flashing is installed in the correct sequence and that the valley’s drainage path remains clear. Photographing the completed underlayment before shingles cover it can help document the work. If a defect is found, repair it using a method and materials approved by the membrane manufacturer; do not assume that roofing nails or general-purpose caulk are an adequate repair.
When Professional Installation Makes Sense
Valley work can be challenging on steep roofs, complex roof intersections, or projects involving multiple roof coverings. A qualified roofing contractor can coordinate the membrane with the flashing and shingle details, identify damaged sheathing, and follow local code requirements. Homeowners should confirm licensing and insurance where applicable, request product-specific installation details, and ensure the contractor follows the roofing manufacturer’s instructions.
Good ice and water shield valley installation starts with a sound, dry deck and a layout tailored to the roofing system. Correctly placed membrane, properly layered seams, and compatible valley flashing create a continuous drainage path beneath the roof covering. Because product requirements and building codes differ, the project’s written instructions and local rules—not a one-size-fits-all measurement—should guide the final installation.