Roof Flashing: Valleys to Penetrations
~12 min read · Trace valley, step, chimney and vent flashing details — where most roof leaks actually start.
Roofs rarely leak in the field — they leak at the interruptions: valleys, walls, chimneys, and pipes. Flashing inspection is a tour of the roof's joints, and 'sealed with tar' is the finding that says a flashing already failed.
Valleys and edges
Valleys concentrate two planes' water: open metal valleys (exposed W-metal) fail by corrosion and nail placement; closed/woven shingle valleys fail by wear-through — debris dams and granule wear in the valley line are early signs. Drip edge at eaves and rakes keeps water off the fascia and deck edge; its absence shows as stained, delaminating deck edges. Ice-barrier questions live at the eaves in cold climates.
- Valleys carry double water — check wear, rust, debris
- Drip edge protects fascia and deck edge
- Eave staining marks the missing/failed edge details
Wall and chimney flashings
Where roof meets vertical: step flashing — individual L-pieces woven shingle-by-shingle up the wall — is the correct detail; continuous 'straight' flashing smeared to the wall is the shortcut that fails. Above it, counterflashing laps down over the step flashing (at chimneys, let into mortar joints and sealed). The chimney's uphill side needs a cricket/saddle on wide chimneys to split the flow. Kickout flashing finishes the bottom of every roof-wall run. Rust, lifted counterflashing, and tar-over repairs are the finding set.
- Step flashing woven per course — not one long strip
- Counterflashing laps OVER, let into masonry joints
- Crickets behind wide chimneys; kickouts at run bottoms
Penetrations
Plumbing vent boots: the neoprene collar cracks years before the shingles die — the most common roof leak in America; look for split collars and sealant cobwebs. Metal vents and flanges: uphill edge under shingles, downhill over. Satellite mounts, solar standoffs, and abandoned fasteners: every hole is a flashing question. The universal tell: roofing cement (tar) troweled anywhere marks a failed detail being managed rather than fixed — report the underlying flashing, not the goo's condition.
Worked example
Behind a wide masonry chimney, debris is piled to the brick; the flashing is one continuous strip tarred to both wall and shingles, rusted at the bottom; inside, the ceiling below shows a stain that grows after snowy weeks. Diagnose the assembly.
Three failures stack. No cricket: the wide chimney dams the uphill flow — debris piles, water ponds against brick, and snow sits longest exactly there (the stain's snowy-week rhythm is the cricket finding announcing itself). Wrong flashing type: a continuous tarred strip instead of woven step flashing with let-in counterflashing — the tar is the failure's bandage, now cracking. Rust at the bottom marks where ponded water has been working. Report: install cricket, rebuild chimney flashing as step + counterflashing let into joints, assess sheathing rot below the debris field, and note the interior stain's correlation. The tar didn't fail to work — it was never the right answer.
Common exam pitfalls
Reading heavy tar as maintenance diligence.
Roof cement marks failed flashing under management — report the detail that should exist, not the sealant's coverage.
Accepting continuous strip flashing at shingled walls.
Step flashing woven per course is the standard — a straight strip sheds into the wall over time.
Overlooking the humble vent boot.
Cracked neoprene collars are the most frequent roof leak — check every boot's collar and seal.
Roofs leak at their joints: step it up the walls, counter it over, cricket the chimneys, and never trust the tar.
Recap
- Valleys and drip edges handle concentrated and edge water
- Step flashing + counterflashing at walls and chimneys
- Crickets split flow behind wide chimneys
- Vent-boot collars: the most common leak point
- Tar-over repairs flag failed details
- Correlate interior stains to the joint above
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