Flashing, House Wrap & Weather Barriers
Key Takeaways
- The drainage plane is the continuous path (usually the WRB face plus flashing) that moves incidental water down and out of the wall
- Install housewrap/WRB shingle-fashion with proper horizontal and vertical laps and integrate it with openings—not under pans the wrong way
- Head flashing/drip caps, window and door pan flashing, step flashing, and kick-out flashing each protect specific leak paths
- Kick-out flashing at roof-to-wall intersections diverts water into the gutter/away from the wall—omitting it is a classic leak cause
- Caulk is a sealant aid, not a substitute for correctly lapped flashing and WRB
Flashing, House Wrap & Weather Barriers
Quick Answer: Behind cladding, a water-resistive barrier (WRB/housewrap) and flashing form the drainage plane. Install wrap shingle-fashion, integrate it with window/door pans and head flashing, interweave step flashing at roof-walls, and always include kick-out flashing where a roof dumps water into a wall. Exterior finish fails when water is directed into the sheathing instead of out.
Module 27204 Exterior Finishing builds on envelope basics: you are expected to install (or coordinate) weather barriers so siding, trim, and roof edges work as a system. This section is about sequence and direction of water, not brand names.
Drainage Plane Concept
Rain hits cladding first. Wind drives some water past joints, nail holes, and cracks. That water must meet a continuous, outward-draining surface—not a sponge of reverse laps and open RO corners.
| Term | Meaning for carpenters |
|---|---|
| Drainage plane | The face of the WRB plus integrated flashing that water follows downward and out |
| Shingle fashion | Upper layer overlaps lower layer so gravity keeps water on the outer face |
| Drainage gap / rainscreen | Intentional space (furring, drainage mat, or product design) that lets water drain and air dry the back of cladding |
| Reverse lap | Lower material overlapping upper material—water is driven behind the barrier |
Exam frame: “Where does the water go if this joint leaks?” If the answer is “into the stud bay,” the detail is wrong.
Housewrap and WRB Installation
Common WRB types (housewrap, building paper/felt, liquid-applied membranes, integral sheathing coatings) differ in product rules, but share install logic:
- Cover the sheathing continuously where the design requires a WRB.
- Horizontal laps: upper course over lower course; use the manufacturer’s minimum lap (often a few inches—read the sheet).
- Vertical laps: similarly shingled or taped per product; windward exposure may demand tape even when untaped was once common.
- Fasten with staples or cap nails as specified; overdriven fasteners that tear the wrap create leak points.
- Repair tears before cladding hides them.
- Integrate openings so the window/door system drains to the WRB exterior face, not behind it into the rough opening framing without a path out.
Integrating WRB with Openings (Concept Sequence)
Exact tape brands and flap folds vary by manufacturer, but a teaching sequence many systems follow is:
- WRB installed on the field of the wall (sometimes with an opening cut and flaps folded).
- Sill/pan flashing first—sloped pan or flexible membrane that turns up the jambs and drains out over the WRB face or to the exterior.
- Window/door set on the pan per instructions (often with a bed of sealant at specific flange locations—not a full four-side hermetic glue-in that traps water).
- Jamb flashing overlapping the pan legs and the unit flanges as required.
- Head flashing (flexible or rigid drip cap) over the unit and under the WRB or lapped so water from above runs over the head detail to the exterior.
- WRB or tape shingled over the head flashing so water on the wrap hits the head flash and exits.
Trap: Tucking the bottom of the WRB under the window sill flange in a way that creates a trough into the RO. The pan must empty outward.
Flashing Types You Must Name and Place
| Flashing | Location | Job |
|---|---|---|
| Head flashing / drip cap | Top of windows, doors, and often horizontal trim bands | Catches water from above and drips it out beyond the unit/trim face |
| Pan flashing | Sill of windows and exterior doors | Catches water that enters the unit/RO and drains it out |
| Step flashing | Roof-to-sidewall (and similar upslope intersections) | Interwoven with each roof course; water steps down beside the wall |
| Kick-out flashing | Bottom of roof-wall where roof edge meets vertical wall | Diverts concentrated roof water into gutter or away from the wall cladding |
| Base / transition flashing | Cladding base, ledger intersections, material changes | Starts the drainage plane above grade or above a lower roof |
| Counterflashing | Over step or base flashing into masonry/reglets | Covers the top edge of base flashing so water cannot get behind it |
Head Flashing and Drip Caps
Even “flanged” nail-fin windows often still need a proper head detail in the water-management system. Rigid drip caps extend past the unit and kick water off the face. Flexible head flashing tapes must be shingled: water from the WRB above must land on top of the head tape, not run behind it.
When siding or trim is installed, do not bury the drip edge of a head flash so water is forced inward. Leave the drip free to shed.
Window and Door Pan Flashing
Pans are not optional decoration:
- They create a back dam or upturned legs so water cannot run into the interior finish.
- They slope or drain to the exterior (or to a designed weepage path).
- Door thresholds are high-risk: wind-driven rain and tracked water hit the sill constantly. A continuous pan under exterior doors is a frequent spec requirement.
Step Flashing at Roof-Walls
At a sidewall, do not run one long L-shaped coil after all shingles are on and hope caulk saves it. Correct practice teaches:
- Place a step-flash piece on each shingle course.
- Cover that piece with the next course up.
- Wall cladding/WRB covers the vertical leg appropriately, with counterflashing or cladding details that still allow drainage.
Step flashing without a kick-out at the eave/wall corner still fails at the bottom of the run.
Kick-Out Flashing (High-Frequency Leak)
Where a roof terminates into a wall (dormer sides, lower roof into multi-story wall, porch roof into house wall), water running down the step-flash path must be kicked out into the gutter or away from the cladding. Missing kick-outs force water behind siding, staining interiors and rotting sheathing. Install kick-out before or as part of the roof-wall flashing sequence—not as caulk after the leak appears.
Water Management Sequence at a Wall Opening (Scenario)
Scenario: A vinyl-sided wall with a new window. Crew installs housewrap, cuts an X in the RO, folds flaps in, sets the window on a flat 2× sill with no pan, tapes the head only, then hangs siding tight to the nail fin on all four sides with no drip cap.
Failures: No pan → sill leaks enter framing. Head not integrated with WRB → water from above runs behind the unit. Siding tight on all sides can block drainage paths and hide reverse laps. Correction path: pan first, unit set per manufacturer water details, jamb/head flashing shingled, drip path clear, siding integrated without trapping the head flash.
Caulk vs Flashing
Sealants close small gaps and protect paint lines. They do not replace:
- Shingle-fashion WRB laps
- Metal or membrane head flashings
- Pans at sills
- Kick-outs at roof-walls
UV, movement, and poor surface prep kill caulk joints. Design for drainage first; seal second.
Coordination with Siding and Trim
- Install WRB and primary flashing before cladding hides the plane.
- Leave access for kick-outs and step flashing when roofs and walls meet on different schedules—sequence the trades.
- At horizontal trim bands (belly bands, water tables), use Z-flashing or equivalent so water on the upper cladding face cannot run behind the lower cladding.
- Fastener penetrations through WRB should be minimized and made with compatible tapes/sealants when required.
Quality Checks Before Cladding Closes the Wall
- WRB continuous, lapped correctly, tears repaired.
- Every window/door has pan + jamb + head logic that drains out.
- Roof-wall intersections have step flashing and kick-out where the roof dumps into a wall.
- No reverse laps at horizontal transitions.
- Photos of flashing (common on commercial jobs) taken before siding covers work.
Master the drainage plane and you will install exterior finish that lasts. The final section covers soffit, fascia, and exterior trim—the pieces that close eaves and corners while supporting attic intake ventilation.
What is the drainage plane in a wall assembly?
How should housewrap courses be lapped on a wall?
Where is kick-out flashing required in the roof-wall water path?
In a proper window water-management sequence, which component is typically established at the sill to catch and drain water out of the rough opening?