Flashing, WRB & Drainage Planes
~13 min read · Explain kickout flashing, weep holes and the drainage plane — and what happens when each is missing.
Siding is not waterproof — walls survive because of what's underneath: the water-resistive barrier, the flashings at every hole, and the drainage plane that gives intruding water a way down and out. Kickout flashing and weep holes are the exam's favorite details because their absence causes the most damage.
The drainage plane concept
Modern walls are screen systems: cladding sheds most water; the WRB (housewrap, felt) behind it catches the rest and drains it down; flashings direct water OUT at every interruption. The rule of assembly is shingle-lapping — every layer laps OVER the one below so gravity carries water outward; reverse laps (upper layer tucked BEHIND lower) pump water inward. The inspector rarely sees the WRB directly — its performance is read at the edges, penetrations, and stains.
- Cladding sheds; WRB drains; flashing exits
- Shingle-lap everything; reverse laps = built-in leaks
- WRB judged by evidence at edges and stains
The flashing roster
Head flashing over windows, doors, and trim (with end dams) — missing above openings is a top rot source. Pan flashing under windows and doors catches leaks. Kickout (diverter) flashing where a roof edge dies into a wall — it kicks roof water INTO the gutter instead of letting it run down inside the cladding; its absence is the classic hidden-rot factory, betrayed by staining on the wall or soffit below the intersection. Ledger flashing at decks; penetration flashing/sealant at pipes, vents, fixtures; weep holes at veneer bottoms and above veneer openings.
- Head + pan flashing at openings
- Kickout at every roof-to-wall termination
- Weeps stay open; penetrations sealed and flashed
Reading failures
Evidence hierarchy: stains (tea-colored streaks below intersections and openings), bubbled or peeling paint in vertical bands, soft trim and sheathing (probe), interior drywall stains at matching locations, and sealant relied on where flashing belongs — caulk is maintenance, flashing is architecture; 'caulked instead of flashed' is a finding, not a fix.
Worked example
A gable roof ends against a two-story wall. Below the intersection: a fan of brown staining down the siding, paint flaking, and the trim board at the bottom is punky to a probe. The seller says 'we caulk that joint every spring.' Diagnose and report.
The geometry is the tell: roof water concentrates along the wall edge; without a kickout flashing at the eave end, that stream slips behind the siding and runs down inside the wall — the stain fan, banded paint failure, and rotten trim map its path, and annual caulking never addressed the mechanism (water enters at the roof edge above the caulk). Report: missing kickout flashing at the roof-wall intersection (condition); chronic water entry behind cladding with visible rot — concealed sheathing/framing damage likely (implication); install kickout and repair the wall, opening it enough to assess hidden damage (action). The one-piece $15 flashing versus an open-the-wall repair is the lesson the exam wants told.
Common exam pitfalls
Accepting caulk as the water strategy.
Sealant is a maintenance layer — where flashing is absent, report the missing flashing, not the caulk's condition.
Missing the kickout location by habit.
Trace every roof edge that terminates on a wall — the eave-end of each such intersection either has a kickout or has a problem.
Calling reversed laps 'installed siding.'
Layer order is the system — upper-behind-lower laps channel water inward and are defects even when new.
Shed, drain, exit — lapped like shingles, kicked out at the walls, weeping at the bottom.
Recap
- Walls are screen systems: cladding + WRB + flashings
- Shingle-lapping is the assembly law
- Kickout flashing at roof-wall terminations — absence = hidden rot
- Head/pan flashing at openings; weeps at veneer
- Stains, banded paint failure, soft trim: the evidence chain
- Caulk substituting for flashing is a finding
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