How the two differ in kind
Before either material is chosen, there is a rule that decides whether the installation works, and it is the same rule for both: flashing is SHINGLED. Water running down the drainage plane behind the cladding has to pass over every horizontal joint and never into one, which means each piece is lapped over by the piece above it, all the way up the wall. Reverse a single lap and the flashing actively directs water into the wall instead of out of it — and the defect becomes invisible the moment the cladding is on. It is the most common cause of water damage around openings and it is a sequencing error rather than a material one.
Given the correct sequence, the two materials do different things. SELF-ADHERED MEMBRANE is a flexible sheet with an adhesive back. Its strengths are conformability and self-sealing: it wraps a rough opening's corners and returns, it bonds continuously to the sheathing and to the weather-resistive barrier, and it seals around the fasteners driven through it, which is what makes it the right material for a sill pan and for integrating a window flange into the drainage plane.
FORMED METAL holds a shape. It spans an opening without sagging, it carries a DRIP — a formed edge that throws water clear of the face below instead of letting it run back along the underside — and it is durable where it is exposed. That drip is the thing membrane cannot do: a head flashing has to project and shed, and a flexible sheet adhered to the wall simply carries water down the wall.
Which is why the usual answer at an opening is both. A membrane sill pan with end dams and a back dam, membrane up the jambs lapping over the pan, the window set and flanged, membrane over the head flange, and then a metal head flashing over that with a drip and end dams — each material doing the part the other cannot.
The factors that actually differ
Show
| Self-adhered membrane | Formed metal | |
|---|---|---|
| The rule that governs both | Shingled laps. Every piece lapped over by the one above; the barrier laps over the head flashing, never behind it. | Identical. The material changes nothing about the sequence. |
| Conformability | Excellent — wraps corners, returns into a rough opening, follows irregular substrates. | None. It holds the shape it was formed into, which is the point of it. |
| Sealing around fasteners | Yes. Self-sealing membranes close around nails and screws driven through them. | No. Every fastener through metal is a hole, which is why fixings go where water does not run. |
| Carrying a drip | Cannot. Water adhered to the wall runs down the wall. | This is its distinctive capability — a formed edge that throws water clear of the face below. |
| Spanning an opening | Sags without support, so it needs a substrate behind it. | Spans on its own, which is why a head flashing over a window is metal. |
| Exposure | Intended to be covered. Ultraviolet exposure degrades most membranes, and each has a stated exposure limit. | Designed to be exposed, subject to the metal's own durability and finish. |
| Compatibility | Asphaltic membranes and some sealants, and some rigid foams, react badly together — check compatibility rather than assuming. | Galvanic pairing matters: dissimilar metals in contact, or runoff from one onto another, corrodes. |
| Cold-weather installation | Adhesion falls with temperature; low-temperature grades and primer exist, and installing below the rated temperature is a bond that will release. | Unaffected by temperature for installation, though it expands and contracts in service. |
| End dams | Formed by turning the membrane up at each end of a sill pan — the detail that stops water running off the side. | Formed into the metal at each end of a head or sill flashing, for the same reason. |
| Where each is indispensable | The sill pan, the jambs, and integrating a window flange into the drainage plane. | The head, and anywhere water must be thrown clear rather than run down the wall. |
Which one, and when
Choose self-adhered membrane when…
- Forming a sill pan with end dams and a back dam — the detail that catches whatever gets past the window.
- Wrapping jambs and corners, where the material has to turn and stay sealed.
- Integrating a nailed flange into the weather-resistive barrier, where sealing around fasteners is the requirement.
- An irregular or uneven substrate that a rigid piece could not follow.
Choose formed metal when…
- The head of an opening, where the flashing must span and must carry a drip.
- Anywhere the flashing is exposed and has to survive weather and sunlight.
- A transition that has to shed water clear of the face below — a band course, a roof-to-wall step, a parapet coping.
- A long straight run where a formed profile is faster and more consistent than working a sheet by hand.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- What does 'shingled' actually mean here?
- That every horizontal joint is arranged so water flowing down the face passes OVER it, exactly like roof shingles — the piece above always laps over the piece below. Applied to an opening, the order is specific: the sill pan goes in first, the jamb pieces lap over the ends of the pan, the head piece laps over the tops of the jambs, and the weather-resistive barrier above laps over the head flashing. The error that causes most of the damage is tucking the barrier BEHIND the head flashing, which is quick and looks neat and turns the flashing into a funnel pointing into the wall. The same logic applies at every transition on the building. If you can trace a drop of water from the top of the wall to the ground and it never has to travel upward or inward, the sequence is right.
- Why does a sill pan need end dams?
- Because without them, water that has collected in the pan runs off the sides into the wall, which defeats the entire purpose of having a pan. A sill pan is the last line of defence at an opening: it catches whatever gets past the window itself — and windows do leak, at their corners and their joints — and directs it back out onto the drainage plane. For that to work the pan has to be a tray rather than a shelf, which means turning the membrane up at each end to form a dam, and turning it up at the back as well so water cannot run inward onto the framing. Sloping the pan outward helps it drain rather than pond. Formed metal and moulded plastic pans exist with the dams built in, which removes the most-skipped detail from the site's discretion.
- Does a head flashing really need a drip?
- Yes, and its absence is a slow-acting defect rather than an immediate leak. Water running down a surface will cling to it and follow it around a corner — surface tension carries it back along the underside of a horizontal projection and onto the face below. A drip edge interrupts that: the formed edge presents a sharp break the water cannot follow, so it falls clear instead of tracking back to the wall. Without one, water from the head flashing runs onto the window head and down the frame, which over years stains, degrades sealant and finds any gap in the perimeter. It is a small piece of geometry with a large effect, and it is the clearest reason a head flashing is formed metal rather than an adhered sheet.
- Can membrane be left exposed?
- Only within the exposure period its manufacturer states, which is typically measured in months rather than seasons and varies substantially between products. Ultraviolet light degrades most self-adhered membranes: the surface embrittles, the adhesive changes, and the sheet stops behaving as it was tested to. Because building programmes slip, this is a real risk — a facade left open over a winter with the flashing and the barrier exposed can need replacing before the cladding goes on. Two practical consequences. Check the stated exposure limit of the specific product before assuming a few months are fine. And where a longer exposure is unavoidable, specify a membrane rated for it rather than covering the shortfall with hope, since replacing degraded flashing after the windows are in is a much larger job.
- What compatibility problems should I watch for?
- Chemical ones with membranes and galvanic ones with metals, and both are avoidable by checking rather than assuming. Asphalt-based membranes can react with certain sealants, with some plasticised materials, and with some rigid insulations, producing softening, bleed-through or loss of adhesion — so the sealant used at a membrane flashing should be one the membrane manufacturer lists. For metal, dissimilar metals in contact corrode galvanically, and runoff from one metal onto another can corrode the second even without contact: copper runoff onto galvanised steel or aluminium is the classic case, and fasteners of the wrong metal through a flashing are the most common small version of it. The rule is to keep the flashing, its fixings and anything draining onto it within a compatible family.
- How cold is too cold to install membrane?
- Below whatever the product states, and the limit matters because the failure is delayed rather than immediate. Adhesion depends on the adhesive flowing into the substrate, and that flow falls as temperature drops — so membrane applied below its rated temperature can appear to stick, pass a visual inspection, and release later as the assembly moves and the bond was never fully developed. The responses are a low-temperature grade formulated for it, a primer that improves adhesion on cold or dusty substrates, or waiting. Substrate condition matters as much as air temperature: frost, condensation or dampness on the sheathing prevents bonding regardless of the grade, and a substrate can be below the air temperature on a cold clear morning. Rolling the membrane firmly after application is what actually makes the bond, in any weather.
- What is the correct order at a window?
- Sill first, then jambs, then head, with the barrier over the top — and each step laps over what is below it. The sill pan goes in before the window, formed with end dams and a back dam and sloped to drain outward, lapped over the barrier below the opening. The window is set and, if flanged, fastened — with the sill flange typically left unsealed so that anything reaching the pan can escape. Membrane then goes up the jambs, lapping over the jamb flanges and over the ends of the sill pan, and finally over the head flange. The metal head flashing goes on over that with a drip and end dams, and the weather-resistive barrier above is lapped down OVER the head flashing. Particular window systems vary the details, so the manufacturer's instructions govern, but the shingling never does.
- Is sealant an acceptable substitute for either?
- No, and treating it as one is a recognisable pattern of failure. Sealant is a joint filler, not a flashing: it relies on adhesion to two faces that keep moving relative to one another, it has a service life much shorter than the building's, and when it fails it does so invisibly from behind. A flashing works by geometry — by directing water somewhere — and continues to work whether or not anything is stuck to anything. Sealant has a legitimate supporting role, closing a joint that flashing has already made watertight and keeping wind-driven rain out of a gap, and the specification should say where. Where it becomes a problem is as a remedy: an opening that leaks and is repaired by running a bead around the perimeter has had its drainage path closed, and the water that gets in now cannot get out.