Roof edge flashing is the metal that finishes the roof where it runs out of building. It has four jobs at once: terminate the membrane, hold the roof down against wind trying to lift it, throw water clear of the wall below, and do all of that while expanding and contracting every day of its life. It is the most highly loaded detail on a commercial roof and usually the cheapest line on the bid.
That mismatch is why the perimeter is where so many roofs fail first. Wind does not press evenly on a roof: it accelerates over the edge and around the corners, and the uplift pressure there is a multiple of what the middle of the roof ever sees. A membrane that is perfect in the field and loosely terminated at the edge is a membrane waiting for one storm. Our page on roof flashing covers the principle behind every detail, and types of roof flashing lists them all; this one is the perimeter in depth.
The parts of a perimeter
| Part | Where | Job |
|---|---|---|
| Drip edge | Open edge, no parapet | Terminate the membrane, shed water clear of the face |
| Gravel stop | Open edge on a surfaced roof | The same, plus retain loose aggregate |
| Fascia | Over the edge, as a finish | Cover the edge detail and the deck end |
| Coping | Top of a parapet | Cover the whole wall top, both faces |
| Continuous cleat | Behind the outer edge of the metal | Hold the metal down along the whole run |
| Termination bar | Top of a membrane upstand | Hold the membrane mechanically to the backing |
| Counterflashing | Over a base flashing at a wall | Cover the top of the upstand |
| Scupper and overflow | Through the parapet | Let water leave, and let it leave when the drain blocks |
Everything in that list except the fascia is doing structural work. The fascia is the only part that is genuinely cosmetic, and it is the one most people can name.
Uplift, and why corners are different
Wind flowing over a building separates at the edge and creates suction. That suction is strongest at the corners, strong along the perimeter, and comparatively mild across the field. Tested roof assemblies and the edge systems that go with them recognize this by dividing the roof into zones and asking for more fixings, closer spacing or heavier sections at the corners and the perimeter than in the middle.
Two things follow for an owner reading a bid.
- The edge metal is part of a tested system, and in commercial work the relevant edge standard is ANSI/SPRI ES-1. Ask which tested assembly the detail follows.
- Fixing spacing is not uniform, and a quote that gives one spacing for the whole roof has either simplified the detail or not read it.
How edge details fail, in order of frequency
- Face fixings elongate. Metal screwed through its face into the wall carries the whole uplift load through a handful of holes, which work oval and then loose.
- Laps open. Lengths of metal butted together and sealed, with no cover plate, separate as they move and let water behind.
- The membrane termination lets go. Adhesive alone at the edge, no termination bar, no mechanical hold.
- The drip is too shallow. Water tracks back along the underside and runs down the wall instead of off it.
- Coping joints split. Sealant across a moving joint lasts a few seasons and then feeds the wall core.
- The detail is too low. Usually after a reroof, covered below.
Fixing, cleats and laps
Three details separate an edge that lasts from one that does not, and none of them are visible once the roof is finished.
A continuous cleat. A strip fixed for the full length of the run that the metal’s outer edge hooks over. It converts a point load at each fastener into a line load along the whole edge, and it lets the metal move without working anything loose.
Laps backed by a cover plate or a formed splice, with the sealant inside the joint rather than smeared across it. Metal moves. A joint designed to move survives; a joint sealed rigid opens.
Fixings into something solid. A nailer at the perimeter, properly anchored, sized for the uplift in that zone. Fastening edge metal into insulation or into a decayed wood nailer is a detail that passes inspection on the day and fails in the first strong wind. How a flat roof is installed covers where the nailer goes in the sequence.
Height, and what a reroof does to it
A new roof is nearly always thicker than the one it replaces, because insulation requirements have risen and because a recover adds the whole new build up on top of the old one. Every detail at the perimeter and every upstand at a wall gets shorter by that amount.
This is the single most common way a sound detail becomes a failing one, and it is entirely avoidable: the edge is rebuilt at the new height rather than reused at the old one. Where a bid keeps the existing edge metal on a roof that is getting thicker, ask what the remaining upstand height will be. Our page on roof recover against tear off covers the wider consequences, and roof insulation explains where the extra depth comes from.
Scuppers and overflows
A parapet turns the roof into a tray, so the way water leaves becomes a detail rather than an accident. Scuppers through the wall and overflow scuppers above the normal water line are both flashings, and both get inspected less than anything else on the roof.
The overflow matters more than owners expect. A primary drain or scupper will block eventually, and a parapet roof with no overflow route holds the water until something gives, which may be the structure rather than the membrane. If there is no overflow provision, that is worth raising before the next roof rather than during the next storm.
What a specification should state
- The tested edge assembly the detail follows
- Metal, gauge and finish, and the profile with its projection and drip
- Continuous cleat, and its fixing spacing by zone
- Lap treatment: cover plate or formed splice, and the sealant inside it
- The nailer, its size, its material and how it is anchored
- Membrane termination method at the edge and at every upstand
- Upstand height above the finished surface after the new build up
- Scupper and overflow provision, with sizes
Where to go next
If the roof is a panel roof rather than a membrane, the perimeter is made differently and metal roof flashing covers it. If you are walking an existing building and want one thing to check, lift at the outer edge of the metal by hand along the windward side: anything that moves is already carrying load it cannot hold, and it is the cheapest problem on the roof to fix today and the most expensive to fix after a storm.
Common questions
What is the difference between a drip edge and a gravel stop?
A drip edge terminates the roof and throws water off the face below, so its defining feature is the downward kick on its outer edge. A gravel stop does the same and adds a raised lip on the roof side to hold loose aggregate back from the edge.
- Both terminate the membrane at an open edge
- A drip edge is shaped to shed water clear of the wall
- A gravel stop adds an upstand to retain aggregate
- A gravel stop without a drip detail stains the wall
- Both need a continuous cleat and a sealed membrane termination
In practice the names get used interchangeably and what matters is the profile on the drawing rather than the word in the specification. Look for two things: that the outer edge projects far enough past the face and kicks down, so water leaves the building rather than running back along the underside, and that the roof side of the section is high enough to terminate the membrane above any water that collects at the edge. A flat strip of metal does neither and appears on more buildings than it should.
What is a coping, and does every parapet need one?
A coping is the cap that covers the whole top of a parapet wall. Every parapet needs one, because masonry absorbs water at its top surface and releases it lower down inside the wall, where nothing on the roof can intercept it.
- Covers the full width of the wall, both faces
- Needs a fall, usually back towards the roof
- Needs a drip on each exposed edge
- Joints are the failure point, not the metal
- Fixed on cleats, not face fixed through the top
Joints deserve the attention. A coping run is made of lengths, and each joint moves with temperature, so a joint sealed flat eventually opens and feeds water straight into the wall core. The detail that lasts is a loose cover plate or a formed splice with the sealant protected inside it, allowing the lengths to move without opening a path. Face fixing through the top of a coping is the other common fault: every fastener is then a hole in the one surface that has nowhere to drain.
Does roof edge metal need a continuous cleat?
On a commercial roof, almost always. A continuous cleat is a strip fixed to the wall or the deck for the full length of the run, which the outer edge of the metal hooks into. It holds the edge down along its whole length rather than at points.
- Spreads uplift load along the run instead of at fixings
- Lets the metal expand without working its fasteners
- Keeps the outer edge from peeling in wind
- Allows face fixings to be omitted or reduced
- Must be fixed at the spacing the system was tested at
The alternative, screwing the metal through its face into the wall, puts every bit of the uplift load into a few fasteners and a few holes. Those holes elongate, the metal rattles loose, and the edge is then a lever for the next storm. Where you see face fixings with washers along a perimeter, the detail has usually been improvised rather than specified. Ask which tested assembly the edge follows and at what fixing spacing, because the test is the whole basis of the performance.
What is a termination bar, and where does it go?
A termination bar is a metal strip screwed through the top edge of a membrane upstand to hold it mechanically against the wall or the curb, with a sealant bead run along its top edge. It is the belt that stops the membrane relying on adhesive alone.
- Fixed through the membrane into solid backing
- Sealed along its top edge, not along the bottom
- Needs a counterflashing or coping above it
- Not a substitute for a proper upstand height
- Exposed bars need the sealant renewed on a schedule
A termination bar on its own, exposed to the weather with nothing over it, is a maintenance item for the life of the roof rather than a finished detail. The sealant along its top is the only thing between the water and the fixings, and sealant is the shortest lived element on any roof. Where you have them, they belong on the annual inspection list. Where a new roof is being specified, ask for metal over the top of every bar, which converts a recurring task into a detail nobody has to think about.
Why does water run back behind the edge metal?
Because the metal has no drip, or the drip does not project far enough past the face. Water leaving a flat surface clings to the underside and travels back along it unless the edge turns down sharply and stands clear of whatever is below.
- Water follows the underside of any flat edge
- A downward kick breaks the surface tension
- Projection has to clear the wall finish below
- Staining under the edge is the visible symptom
- Wind driven rain defeats a shallow drip
The result is not usually a leak into the building, which is why it gets tolerated. It is staining down the face, saturation of the top of the wall and, on a masonry parapet, water being drawn into the core where it eventually appears inside a long way from the edge. On an existing building this is fixable without touching the roof, by replacing the edge metal with a section that projects and kicks properly. It is one of the cheapest improvements available on an older building.