Roof Flashing Guide

Flashing is metal or another compatible waterproof material that moves water safely across joints where the roof changes direction or meets another surface. Many leaks blamed on “the shingles” actually begin at flashing around walls, chimneys, valleys, skylights or pipes.

Common flashing locations

Location	Typical detail	Common concern
Sidewall	Individual step flashing woven with roofing	Pieces reused, omitted or trapped behind cladding
Headwall	Continuous flashing behind wall finish	Poor laps or insufficient height
Chimney	Step flashing plus counterflashing	Sealant-only repairs and failed masonry joints
Valley	Open metal or approved closed system	Fasteners or joints in the main water channel
Pipe	Boot or field-formed flashing	Cracked rubber, corrosion or incorrect integration

Step flashing

At a sloping roof-to-wall intersection, separate pieces overlap each course of roofing. This creates a layered path that drains even if one joint admits water. A single continuous piece smeared with sealant is usually not an equivalent substitute.

Counterflashing

Counterflashing covers the upper edge of base or step flashing and directs wall water onto the roof. At masonry, it may be set into mortar joints. The roof and wall components should be separable enough for future repair without relying on exposed caulk as the primary defense.

Valleys

Valleys collect water from two slopes, so layout, width and fastening are critical. Open metal valleys leave the lining visible; closed systems cover it with roofing according to approved methods. Debris buildup or poorly placed nails can turn a minor defect into a significant leak.

Material compatibility

Aluminum, galvanized or coated steel, copper, lead-coated copper and other metals behave differently. Contact between incompatible metals or runoff from one metal onto another can accelerate corrosion. Fasteners, sealants and treated wood also need compatibility checks.

Reuse or replace?

A replacement proposal should state which flashings will be replaced. Reusing old step flashing may save labor but can conceal corrosion, holes or poor dimensions. Chimney counterflashing and specialty metal sometimes remain serviceable, but the decision should follow inspection rather than assumption.

Warning signs

• Heavy roofing cement spread over joints
• Rust, pinholes or lifted metal
• Loose counterflashing or open mortar joints
• Water stains below valleys, chimneys or roof-to-wall junctions
• Pipe boots that are split, distorted or pulled away

Questions for a contractor

• Which flashings will be replaced and which reused?
• How will wall cladding or masonry be integrated?
• What metals and fasteners are specified?
• Will photos be provided before details are covered?
• Who handles chimney masonry or siding repairs outside the roofing scope?

Flashing should create a physical drainage path. Sealant remains useful at selected joints, but exposed sealant needs maintenance and should not be the entire design.

Related guidance

Flashing details should be coordinated with the selected roofing material and the complete underlayment system. If a leak has already developed, compare likely roof repair cost factors.