What flashing does
Flashing is a metal, membrane, or formed component that moves water across an interruption in the building's drainage path. You need it at roof-to-wall intersections, openings, penetrations, roof edges, valleys, and other transitions where water could move behind the finish.
Gravity is the driving force behind drainage. A durable detail gives water a continuous path down and out rather than trying to make the exterior face perfectly waterproof. Building Science Corporation's flashing guide explains that flashing integrates with drainage planes and can direct water from behind cladding back outside, protect an interface, shed water over a projection, or perform all three jobs.
The practical distinction is simple: roofing, siding, windows, and sealant are products; flashing is the transition logic between them. Start with the water path, identify the drainage plane on each side of the transition, and connect those planes without a reverse lap or dead end.
The cardinal rule: every layer drains down and out
Flashings are needed wherever a drainage plane ends or is interrupted. That includes roof edges, the bottom of a wall, windows, doors, control joints, penetrations, and intersections between assemblies. The common-flashing details show the same sequence repeatedly: lower layers go in first, upper layers overlap them, and the completed detail discharges onto the exterior or the next drainage layer.
Use this field check before covering a detail:
- Identify where water above the detail lands.
- Trace its path over each lap.
- Confirm the path ends outside, not behind cladding or inside a rough opening.
- Check corners and terminations, where a two-dimensional sketch often fails in three dimensions.
- Use sealant or compatible tape where the tested assembly calls for it, but do not use a surface bead to replace a missing drainage path.
A fast way to find a reverse lap is to imagine pouring water above the detail with the finish removed. If any layer catches and directs that water inward, rebuild it before the next trade hides it. For the wall-side drainage plane, our house-wrap and weather-barrier guide explains how the field membrane fits into this sequence.
Roof flashing: match each transition to its detail
Step flashing is for the intersection of a sloped shingle roof and a vertical wall. Step flashings are interwoven with the shingles and act like shingles with an upturned leg. The wall leg connects behind or seals to the wall drainage plane, while the roof leg lies over the roof drainage plane. See the step-flashing description and figures.
A kick-out belongs at the lower end of that roof-wall run. The kick-out flashing directs water away from the adjoining wall and keeps the step flashing from concentrating runoff on the wall surface. It must discharge where the gutter or exterior drainage path can receive the water, not behind the cladding.
Other roof transitions require their own details: valleys move water where two roof planes meet; eave and rake edges terminate the roof assembly; and pipe, vent, skylight, and chimney penetrations must reconnect the roof drainage plane around an opening. Product-specific kits can help, but only when their flanges and laps connect to the surrounding roof system.
Asphalt shingles should not be installed directly on a wood roof deck; the deck should first be covered by roofing underlayment. ARMA also describes self-adhering underlayment as secondary protection at ridges, hips, valleys, and around vents, skylights, plumbing stacks, and chimneys. Read ARMA's underlayment bulletin, then compare roof underlayment types before specifying felt, synthetic, or self-adhering material. Underlayment supports the water-management system, but it does not replace correctly formed flashing at transitions.
Wall, window, door and penetration flashing
Head flashing above windows and doors should slope toward the exterior. It should extend laterally beyond both sides of the opening, and cladding above it should not force it to bend or slope back toward the wall. Those installation points come from the Building Science Corporation detail guide.
At a window, think in sequence rather than isolated strips: prepare the sill drainage path, set the unit as its instructions require, integrate jambs, then lap the wall drainage plane over the head detail. The exact membrane and corner treatment depends on the window and wall assembly. The non-negotiable outcome is a sill path that can drain incidental water and a head detail that does not direct water behind the unit.
Flanged wall penetrations follow the same logic. With building paper, the lower sheet goes beneath the flange and the upper sheet laps over it. With housewrap or a sheathing drainage plane, compatible tapes or membranes integrate the flange. The illustrated sequence applies tape from the lower edge to the sides and then the top so the layers remain shingle-lapped.
Do not copy a window detail onto every wall opening without checking materials. Fluid-applied barriers, mechanically fastened wraps, taped sheathing, stucco, masonry veneer, and panel systems can require different transitions even though all must drain down and out.
Choose materials as a compatible assembly
Material selection is not just a durability ranking. The flashing, tapes, sealants, drainage plane, fasteners, cladding, substrate, and manufacturer instructions have to work together.
Use sheet metal when the detail needs a rigid shape, positive projection, or durable exposed edge. Use flexible flashing membranes where they can form around corners and connect flanges to a drainage plane. Use premanufactured boots, caps, or pans when they fit the geometry and can be integrated into the surrounding layers. Building Science Corporation notes that premanufactured flashings can provide effective solutions at drainage-plane penetrations, while site-built metal or membrane details still must follow the down-and-out rule. Review the penetration guidance.
Before installation, check:
- chemical and galvanic compatibility with adjacent materials and fasteners;
- minimum laps, primer needs, temperature limits, and substrate preparation;
- whether the product can remain exposed before cladding or roofing goes on;
- corner and end-dam requirements;
- how the upper drainage plane connects to the flashing;
- where the lower edge discharges.
Do not mix products only because each one is individually labeled waterproof. A bond between incompatible materials can fail even when both products perform well in their tested systems.
Common flashing failures and the field test for each
Most flashing failures are continuity failures. The material may be intact while the water path is wrong.
| Failure | What it does | Field check |
|---|---|---|
| Reverse lap | Directs water behind the lower layer | Trace the path from the top of the assembly to discharge |
| Missing kick-out | Dumps roof runoff onto or behind the wall finish | Inspect the bottom of every sloped roof-to-wall run |
| Back-sloped head flashing | Sends water toward the opening | Check for positive slope to the exterior |
| Short head flashing | Lets water wrap around an opening corner | Confirm projection beyond both sides |
| Poor penetration integration | Leaves an opening in the drainage plane | Check lower, side, and top laps in sequence |
| Termination with no discharge | Traps water behind cladding or trim | Identify the visible exit before covering the work |
| Sealant-only repair | Hides a missing lap until movement opens the joint | Ask what drains the joint if the bead separates |
Building Science Corporation warns that flashing must not direct water onto another element in a way that concentrates moisture loading. Its kick-out example is the clearest roof-wall case: the final piece redirects concentrated runoff away from the adjoining wall.
Photograph critical layers before roofing, siding, trim, or drywall conceals them. Include a wide shot for location and close shots showing lap direction, corners, fasteners, and discharge. That record is more useful than a photo of the finished surface.
Flashing FAQ
Is caulk a substitute for flashing?
No. Sealant can be part of a tested flashing assembly, but a surface bead does not replace the layered drainage path. The detail still needs to move water down and out if the sealant ages or the joint moves.
What is kick-out flashing?
It is the diverter at the bottom of a sloped roof-to-wall intersection. It sends water away from the adjoining wall and prevents step flashing from concentrating runoff on that wall.
How does roof underlayment relate to flashing?
Underlayment is a secondary layer over the roof deck. It must integrate with edge, valley, wall, and penetration details, but it does not replace formed flashing at those transitions.
What should be checked before flashing is covered?
Confirm lap direction, positive drainage, corners, end conditions, compatibility, fastener placement, and the final discharge point. Photograph the full location and the critical close-up layers before concealment.