Flashing + Weatherproofing Details Reference
Why this matters
Water intrusion through penetrations + transitions is the #1 callable failure on roofing + siding + HVAC + plumbing work. Wrong flashing = leak in 3 months. Right flashing lasts 25+ years. Tradesmen who understand drainage planes + flashing details prevent the calls; those who don't get the angry customer + lawsuit for property damage. This is the field card.
Drainage plane concept
Drainage plane: continuous surface that sheds water down + out, NEVER traps water.
Lap principles:
- Upper material laps OVER lower
- Water flows down + out
- Reverse lap = water gets behind + leaks
"Shingle-style" or "shingle fashion": each course laps over the one below.
Flashing materials
- Aluminum: most common, cheap, bends easily, roof valley/step/drip edge.
- Steel galvanized: stronger, industrial/commercial, prime if cut.
- Copper: premium 75+ yr life, patinas, premium installs.
- Lead: old roofs pre-1970s, lifetime durability, lead handling concerns.
- PVC flashing: newer, lightweight, window/vent.
- Self-adhered membrane: rubberized asphalt or EPDM, sticks to substrate, critical at valleys/eaves/penetrations.
- Sealants: polyurethane + butyl + silicone where flashing alone doesn't suffice.
Common flashing applications
Step flashing (shingle roof meets vertical wall): 5x7 or 4x8 pieces, L-shape with vertical leg on wall + horizontal under shingle, each piece laps over previous shingle + under next, caulk top of leg to wall, don't seal bottom (drainage).
Counter flashing: covers top of step flashing on chimney/wall; bent metal let into mortar joint; covers step flashing's vertical leg.
Apron flashing (front of chimney): single L-shape over front face, caulked to chimney, extends onto roof under shingles.
Drip edge (eaves + rakes): Z or T profile; eave under starter strip + over fascia; rake under shingles + over fascia; ~/LF aluminum standard.
Valley flashing: open valley (metal visible, shingles cut back) or closed/woven (shingles interlocked, no metal visible); underlayment + Ice + Water Shield + metal + shingles.
Vent pipe flashing (plumbing vents through roof): pre-formed boot with rubber gasket + flange, flange under upper shingles + over lower, replace every 15 - 25 yr.
Skylight flashing: manufacturer's kit only - apron at bottom, step + counter at sides, head flashing at top; common leak if wrong.
Window flashing: sill flashing first (slopes water out), jamb flashing over sill, head flashing over jamb, all upper laps over lower, self-adhered membrane (Tyvek, Vycor). Door flashing similar.
Roof penetration flashing
Combustion vent (B-vent, Type B):
- Manufacturer's storm collar + flashing
- Flange under upper shingles
- Collar around pipe
- Caulk at vent-collar interface
HVAC condenser line through wall:
- Wall flashing kit (LB conduit-style)
- Sealed with polyurethane sealant
- UV-resistant
- Internal wrap to prevent rodents
Antenna / mounting bracket:
- Lag bolts into rafter
- Sealant under bracket base
- Self-adhered membrane around base
Gutter flashing
Apron flashing into gutter:
- L-shape
- Bent over fascia + into gutter
- Prevents water from running behind gutter
Drip edge over fascia:
- Allows water to drip into gutter
- Critical for older homes without proper drip edge
Siding flashing
Window head flashing:
- Z-shape metal
- Above window header
- Diverts water away from window
- Under siding above
Door head flashing:
- Same principle
Belt course flashing:
- Transitions between siding types
- Z-shape diverts water away
Penetrations through siding (electrical box, hose bib, vent):
- Wrap with self-adhered membrane
- Caulk perimeter
Wall + window flashing sequence
Standard order (from bottom of wall up):
- WRB (weather-resistive barrier) - Tyvek, HouseWrap
- Sill flashing on window opening
- Window install
- Jamb flashing (over sill flashing + WRB)
- Head flashing (over jamb + WRB)
- Top of head flashed back to WRB
- Sheathing tape any seams
- Siding installed over WRB
Common flashing mistakes
Reverse lap: water gets behind flashing.
No flashing at all: relying on caulk = leak in years.
Caulk-only solution: caulk fails before flashing material; not a substitute.
Missing apron at chimney: water runs down vertical face into roof.
Skylight without manufacturer's flashing kit: leaks.
Drip edge upside down: water flows under fascia.
Pipe boot not replaced during reroof: old boot fails; leak.
Skipping ice + water shield: ice dams + leaks in cold climate.
Improperly cut step flashing: doesn't shed water properly.
Sealant in drainage path: blocks water; doesn't help.
Common service callbacks
Year 1 - 5 leaks:
- Flashing missing OR installed wrong
- Sealant fail (improper application)
- Penetration moved
Year 10 - 20 leaks:
- Pipe boots aging
- Aluminum corrosion at fasteners
- Sealant breakdown
- Movement causing failure
Year 25+ leaks:
- Original flashing reaching end of life
- Roof typically needs replacement
- Complete tear-off + re-flash
Ice + water shield specifics
Where required (cold climates):
- Eaves (3+ ft from edge inside heated wall line)
- Valleys
- Around chimneys, skylights, penetrations
- Low-slope areas (under 4/12 pitch)
Application:
- Self-adhered (peel + stick)
- Lap horizontal: 4" overlap
- Stick to dry, clean deck
- Don't expose to UV > 30 days
Code: ICC requires in northern climate zones; check local AHJ.
Industry standards + certifications
- NRCA (National Roofing Contractors Association)
- GAF + CertainTeed manufacturer training
- ASTM D8024 (asphalt shingle), D1970 (self-adhered membrane)
- Local AHJ permit requirements
The single most-impactful flashing habit is the SHINGLE-STYLE LAPPING - every upper piece overlaps every lower piece, every time, no exceptions. Mentally trace the water from the highest point down: it must run downward + outward, never sideways or upward into a joint. The tech who instinctively checks "would water flow down this seam?" catches errors before installation. Most flashing failures trace to a single reverse lap; the discipline of mental water-tracing prevents 80% of those.
References
- NRCA Roofing Manual
- ICC Building Code Chapter 14 (exterior walls)
- IRC + IBC roofing sections
- Manufacturer flashing manuals (GAF, CertainTeed, Owens Corning)
- Manuall internal: Sealant + Caulk Selection, Moisture Control Construction