9 min read · Guide
There is a piece of bent metal that should sit above every window, every door, and every horizontal joint where one cladding material meets another. It costs a few dollars a linear foot. It is invisible once the siding is on. And when it is missing, reversed, or replaced with a bead of caulk, it produces the most common and most expensive water-damage pattern we open walls to find. This guide explains what head flashing is, the one rule that makes it work, what the code requires, why the correct detail is deliberately not sealed, and how a homeowner can tell whether theirs was built or skipped.
What it is and what it does
Head flashing — also called a drip cap, or a Z-flashing where it crosses a material transition — is a bent metal profile installed at the **top** of an opening or a horizontal joint. Its job is to catch water that gets behind the cladding above and redirect it back out, past the opening below, instead of letting it run down into the head of the window frame or into the wall cavity. The name for the shape describes the function: a vertical leg that tucks up behind the water-resistive barrier, a horizontal leg that crosses the top of the opening, and a downturned drip leg at the front edge that breaks surface tension so water falls clear rather than curling back under. The whole thing works by gravity and geometry. There are no moving parts and nothing to maintain — which is exactly why getting it right at installation is the only chance you get.
The one rule: shingle-lapping, and why reversing it is catastrophic
Everything in a wall assembly sheds water onto the layer beneath it, in the same way roof shingles do. That means every layer must overlap the one below it **on the outside**. For head flashing, the sequence is specific and unforgiving: the flashing's vertical leg goes **behind** the weather-resistive barrier above, the barrier laps **over** it, and the siding then laps over both. The flashing's horizontal leg sits **on top of** the window's head trim or nailing flange below. Done that way, any water arriving from above lands on the outside of the next layer down and keeps travelling outward and downward until it exits. Reverse a single lap — tuck the barrier *behind* the flashing instead of over it — and you have not merely failed to help. You have built a funnel that collects water off an entire elevation and delivers it into the wall at one point. That is why reversed laps produce such concentrated damage: the failure is not a leak, it is a collection system.
What the code requires
This is not a best-practice upgrade; it is a requirement. California Residential Code §R703 requires flashing at exterior wall openings and at the junctions where materials change, integrated with the water-resistive barrier so that water is directed to the exterior. The parallel provision for buildings under the Building Code is Ch. 14, §1402–1404, which requires a weather-resistant exterior wall envelope with flashing integrated to drain. This is also the thing an inspector checks at a pre-cover inspection on a permitted re-side — the barrier, the flashing and, on stucco, the weep screed — precisely because none of it is visible afterwards. A crew that wants to skip it will want to close the wall before that inspection happens, which is a reason to insist on the permit rather than treat it as paperwork.
Why the correct detail is NOT sealed
This surprises homeowners, and it is worth explaining because it looks like sloppiness. On a correctly built wall, the underside of the head flashing's drip leg is left **unsealed**, and on many assemblies the ends are left open too. That is deliberate. The drainage plane behind the cladding is designed to let incidental water out, and the bottom of a flashing is one of the places it exits. Caulk that gap and you close the exit while leaving every entry point — laps, joints, penetrations — open. Water still gets in; it just cannot get out, and it sits against sheathing and framing until something rots. Building Science Corporation's work on drainage planes is the standard reference for why a wall must be allowed to drain rather than sealed shut. If a contractor's answer to water management is 'we caulk it well', that is the wrong answer to the wrong question.
The failure pattern, and how it shows up inside
Missing or reversed head flashing rarely announces itself as a dripping window. It shows up as staining or softness in the **drywall below the window sill**, or as peeling paint on the interior casing, or as a musty smell in a corner — because the water enters at the head, runs down the inside of the wall behind the finish, and appears somewhere lower. It shows up outside as a stripe of dark staining on the siding below an opening, or as a sill that reads spongy under a screwdriver point. And it shows up during a re-side as the thing that turns a two-week job into a three-week job, because rotten sheathing and framing around an opening cannot be sided over. Our why windows leak after siding guide covers the diagnostic side, and flashing failure behind siding covers the broader family this belongs to.
How to tell whether yours exists
You can do a reasonable first check from the ground. Look at the top of a window: is there a visible metal edge with a downturned lip between the top trim and the siding above? On many correctly built walls you will see a thin, clean shadow line. If the siding runs straight down to the top of the window trim with nothing between them, and there is a bead of caulk across that joint, that is a strong signal — caulk in that position is usually a substitute for flashing rather than a supplement to it. Two caveats before you conclude anything. Some assemblies conceal the flashing behind a trim profile, so absence of a visible edge is not proof of absence. And on a stucco or older wall the detail may be different in appearance while still being correct. The reliable confirmation is opening a small area, which is why we look at this specifically during scoping rather than guessing from the driveway.
Where else it belongs besides windows
Openings get the attention, but the same detail is required at every **horizontal transition**. Where siding sits above a band board or a water table. Where a new cladding meets an existing stucco field — our stucco-to-siding transition guide covers that specific junction. Above garage doors. Above deck ledgers. Where a lower roof meets a wall the detail becomes a kick-out flashing, which is the same logic turned through ninety degrees and is, if anything, the more commonly omitted of the two. The generalisation worth carrying: **any place where water running down a wall meets a horizontal interruption needs metal, lapped correctly, and not sealed at the bottom.** If a bid does not mention flashing at all, that is not because the flashing is assumed. It is because nobody has priced it.
Correct head flashing vs the three ways it goes wrong
| Condition | What you'd see | What happens over time |
|---|---|---|
| Built correctly | Metal edge with a downturned drip leg; laps shingle-style; bottom left open | Incidental water lands outside the next layer and drains out |
| Missing entirely | Siding meets window trim directly, usually with a caulk bead | Water enters at the head and travels down inside the wall |
| Reversed lap | Looks correct from outside; barrier tucked behind the flashing | Collects water off the elevation and funnels it into one point — the worst case |
| Sealed underneath | Caulk along the drip leg's bottom edge | Exit closed, entries still open; water accumulates against sheathing |
Key takeaways
- Head flashing (drip cap, Z-flashing) sits above every opening and horizontal material transition, catching water from above and directing it back out.
- The rule is shingle-lapping: the vertical leg goes BEHIND the weather-resistive barrier, the barrier laps OVER it, the siding laps over both.
- A single reversed lap does not merely fail — it builds a funnel that collects water off a whole elevation and delivers it into the wall at one point.
- CRC §R703 requires flashing at openings and material junctions, integrated with the WRB to drain. It is checked at the pre-cover inspection on a permitted job.
- The bottom of the drip leg is deliberately NOT sealed. Caulking it closes the exit while leaving every entry open.
- It shows up inside as staining below the sill rather than at the head, because the water travels down behind the finish before it appears.
- The same detail is required above band boards, garage doors, deck ledgers and stucco transitions — and at roof-to-wall corners it becomes a kick-out flashing.
FAQ
Quick Answers
A bent metal profile installed at the top of a window, door, or horizontal material transition. Its vertical leg tucks behind the weather-resistive barrier, its horizontal leg crosses the top of the opening, and its downturned drip leg makes water fall clear of the wall instead of curling back underneath. Also called a drip cap, or a Z-flashing where it crosses a change of cladding.
Yes. California Residential Code §R703 requires flashing at exterior wall openings and at material junctions, integrated with the water-resistive barrier so water is directed to the exterior; the Building Code's parallel provisions are in Chapter 14. On a permitted re-side an inspector checks it at the pre-cover inspection, before the wall is closed up.
No. Caulk is a sealant, not a drainage detail, and it fails on a schedule — sun and seasonal movement open it. A flashing works by geometry and gravity and does not depend on adhesion. A bead of caulk across the top of a window where flashing should be is one of the more reliable signs that the detail was skipped rather than built.
No, and this is deliberate rather than an oversight. The drainage plane behind the cladding is designed to let incidental water escape, and the underside of a flashing is one of its exits. Sealing it closes the way out while leaving every way in — laps, joints, penetrations — open. Water then sits against sheathing and framing.
From the ground, look for a thin metal edge with a downturned lip between the top of the window trim and the siding above. Siding running straight down to the trim with a bead of caulk across the joint is a strong warning sign. Two caveats: some assemblies conceal the flashing behind a trim profile, and stucco details look different while still being correct. Confirming properly means opening a small area.
Water enters at the head of the opening and runs down inside the wall, so it usually appears as staining or softness in the drywall below the sill, peeling interior casing paint, or a musty corner — not as a drip at the top. Outside it can show as dark staining on the siding below the opening. Left long enough it rots sheathing and framing around the opening, which is work that has to be done before new siding goes on.
Sources
Authoritative references
- 2025 California Residential Code §R703 — wall covering, flashing and water-resistive barrier (via UpCodes)
- 2025 California Residential Code, Ch. 7 — Wall Covering (§R703)
- 2025 California Building Code, Ch. 14 — Exterior Walls (weather protection, §1402–1404)
- Building Science Corporation — water management and drainage-plane research
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.