Flashing is the unsung hero of roof systems. This weatherproofing layer seals vulnerable joints and intersections against water damage. Improperly installed or missing flashing is the number one cause of preventable roof leaks. Following best practices for flashing installation protects roof integrity.
Flashing comes in sheet metal or membrane materials that resist water. Flashings serve multiple roles:
- Seal joints – Horizontal joints between roof planes, adjoining walls, dormer interfaces, etc.
- Waterproof penetrations – Pipes, vents, chimneys, skylights, etc. passing through the roof.
- Direct water flow – Valleys, wall/roof intersections to divert and control runoff.
- Terminate edges – Anchor points along edges, eaves, rakes to seal the roof perimeter.
Common Flashing Types
Understand flashing materials to select the right product for each application:
- Step flashing – Small overlapping sheets along roof-wall joints. One piece per course or step in masonry.
- Valley flashing – Long continuous sheets under roof valleys. Often open or closed valley styles.
- Drip edge – Angled flashing along eaves and rake edges to shed water away.
- Pipe boots – Preformed rubber or neoprene sleeves that seal around pipe penetrations.
- Reglets – Receivers built into walls or joints to receive counterflashing. Made of metal, vinyl, or elastomeric sheeting.
Where Flashing is Needed
Some key areas that require flashing:
- Horizontal wall/roof intersections
- Vertical wall intersections such as chimneys and dormers
- Deck, patio cover, and porch roofs tied into house
- Valleys, hips, ridges, rakes, eaves
- Skylights, vent pipes, chimney crickets (check out our article on leaking skylights here)
- Roof-to-siding termination points
- Deck ledger board, kick-out diverter flashing
- Transitions between roof shapes or slopes (check out our article on different type of roof structures here)
Consult codes and manufacturer instructions for full flashing requirements. When in doubt, add flashing.
Installation Best Practices
Follow these standards to ensure quality flashing application:
- Extend up wall at least 4″ and integrate with housewrap.
- Embed at least 2″ into mortar bed, not just tarpaper.
- Overlap pieces by at least 3-4″. Interlock shingle-style.
- Secure with non-corrosive fasteners. Seal laps with plastic cement.
- Apply matching counterflashing over to complete waterproof seal.
- Run full length of valley from ridge to eaves. Overlap pieces min 6″.
- Valley lining should be smooth, wide, and without kinks or creases.
- Anchor 18-24″ on center with nails or blind rivets into roof sheathing.
- Keep edges raised up from roof surface to allow proper drainage.
- Cut tight-fitting flange around base without gaps or tears. Make X-pattern cuts to seal around irregular shapes.
- Set flange in thick bed of roofing mastic and secure with clamping ring.
- Ensure assembly allows flexing as pipe moves.
- Seal top edge to pipe with flexible sealant.
- Install drip edge flashing first at lower edges of roof. Overlap pieces 2-4″.
- Extend out 1/4″-3/4″ past roof edge. Angle downward to divert runoff.
- Secure every 4-6″ into the roof decking, not just the fascia board face.
Common Flashing Install Mistakes
- Not overlapping horizontal seams enough
- Poor adhesion to roof or wall surface
- Flashing terminates too high or too low
- Gaps around penetrations or corners
- Rips, crimps, creases impede drainage
- Reusing old flashing instead of new material
Use the optimal flashing material for each application:
- Aluminum – Lightweight, corrosion resistant, needs special fasteners. For soffits, siding.
- Copper – Durable, naturally pathogen resistant. Used for architectural detail.
- Galvanized – Zinc coated steel. Economical option for metal roofs.
- Lead – Soft, workable traditional flashing. Being phased out for toxicity risks.
- EPDM – Flexible synthetic rubber sheets. For irregular shapes and roof penetrations.
- Modified Bitumen – Asphalt/polymer sheets with adhesive backing. Low cost.
- PVC – Durable plastic sheeting. For valley flashing and detail work.
- Peel-and-Stick – Self-adhesive rubberized asphalt membrane. Quick installation but shorter lifespan.
Use compatible materials – don’t mix copper and steel or different metals to avoid galvanic corrosion.
When to Call a Roofer
Some flashing projects are best left to roofing professionals:
- Steep pitch or multi-story installations
- Intricate architectural details and joints
- Tying new into existing roofing
- Limited access areas like confined vents
- Moisture issues or leaks below roofline
- Unfamiliar roofing materials or techniques
For DIY work, limit scope to straight forward flashing with standard materials on simple roof geometry.
Signs of Flashing Failure
Watch for these red flags that indicate flashing problems:
- Visible light or sky through roof joints
- Rusting, warping, or fastener popping
- Ponding water at penetrations or walls
- Leaks originating high on walls or ceilings
- Mold or rotting wood along roof line
- Ice dams or uneven icicle formations
- Insects, moisture in soffits or attic
Promptly correct any compromised or substandard flashing to avoid expensive moisture damage. We have an article about the costs of roof replacement if it really goes pear-shaped. Best to fix the flashing early on!
Proper flashing craftsmanship is vital to roof weather-tightness and longevity. Take time to install it meticulously with overlapping seams, full adhesion, and fail-safe terminations. Quality materials matched to the application also ensure maximum performance. Make flashing a priority for roofs that endure the elements year after year.
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