A roof valley is the inward angle where two roof planes meet. It concentrates runoff from both planes, so a small defect in the valley can admit far more water than a similar defect in an ordinary field area.
Why roof valleys leak
The National Roofing Contractors Association identifies valley underlayment, flashing and construction as a critical asphalt-shingle detail. The vulnerable points include the upper termination, lower discharge, sidewall intersections and places where unequal roof planes meet.
Common causes include:
- Corroded, split or displaced metal valley flashing
- Underlayment laps that are open, reversed or punctured
- Shingles or tiles cut, fastened or sealed incorrectly near the centerline
- Debris that holds moisture or redirects water beneath the covering
- A prior patch that traps water instead of preserving drainage
- Deck movement or deterioration below the valley
- A transition where the valley empties against a wall, gutter or lower roof
GAF's valley overview distinguishes open and closed valley approaches. Neither label guarantees performance. The installed material, laps, fasteners, terminations and compatibility with the surrounding roof system determine whether the detail can shed water.
The interior stain may not be below the defect
Water can enter higher on the roof, travel along underlayment, decking, framing or a fastener, and appear inside several feet away. Before opening the valley, an inspection should look upslope at penetrations, wall flashing, ridge details and adjacent roof planes.
Useful diagnosis may include:
- Mapping the interior stain and attic moisture path
- Inspecting the valley from eave to upper termination
- Looking for debris lines, rust, displaced materials and prior sealant
- Checking surrounding shingles, tile, membrane and flashings
- Using controlled water testing only when conditions and access make it safe
Do not assume every leak near a valley is caused by the valley itself.
What a durable valley repair includes
The repair must expose enough of the assembly to find sound material on every side. Depending on the roof system and damage, that can mean:
- Removing surrounding shingles or tile without damaging reusable material
- Replacing wet or deteriorated deck sections
- Installing compatible valley underlayment or membrane with correct laps
- Replacing or correctly forming corrosion-resistant flashing
- Rebuilding upper and lower terminations so water has a continuous exit
- Reinstalling the roof covering with fasteners outside prohibited drainage areas
- Checking the adjacent gutter, wall or lower-roof discharge
A bead of roof cement may slow a leak temporarily, but it does not replace failed underlayment, missing laps or corroded metal. Heavy coating can also hide the route water still follows beneath the surface.
Shingle, tile and low-slope valleys are different
An asphalt-shingle valley may be open, closed-cut or woven when the selected products and instructions permit it. Tile valleys use tile-system underlayment, flashing, battens and cut-tile details. Low-slope intersections may function more like membrane sumps and require compatible reinforcement rather than steep-slope valley metal.
The repair detail should come from the installed roof system and current product instructions—not from a universal social-media patch.
Arizona conditions raise the stakes
Long dry periods can conceal a defect until concentrated monsoon rain arrives. Heat and ultraviolet exposure age sealants and exposed materials, while wind can load debris into the valley or shift loose tile. A pre-monsoon inspection should keep the drainage path clear and identify cracked coatings, rust, exposed underlayment and displaced covering before the first major storm.
Roof access is not a casual DIY task
OSHA's roof-repair fact sheet says roof integrity and fall hazards must be evaluated before repairs begin. Wet surfaces, steep slopes, brittle tile and damaged decking add risk precisely where a leak is already present.
From inside, protect people and property and document active water. Leave roof access, material removal and permanent valley reconstruction to workers with the right fall protection and system knowledge.
Questions to ask about a proposed repair
- What evidence identifies the valley as the entry point?
- How much roofing will be removed for inspection?
- Will damaged decking, underlayment and flashing be replaced?
- How are the upper and lower terminations handled?
- Are surrounding materials reusable and available if breakage occurs?
- What is excluded if concealed damage is discovered?
- How will the completed detail be documented?
America Roofing can inspect the complete drainage path and prepare an Arizona roof-repair estimate based on the actual assembly rather than sealing the first visible crack.
Primary references
Sources used for this guide
- In the Valley National Roofing Contractors Association
- What Is a Roof Valley? GAF
- Reducing Falls During Residential Construction — Roof Repair Occupational Safety and Health Administration