Metal roof flashing is the set of formed metal components that close a panel roof everywhere the panels stop: at the eave, at the gable edge, in the valleys, where the roof meets a wall, at the ridge, and around every pipe, curb and unit on the roof. The panels shed water. The flashings handle it at every transition, and those transitions are where almost every metal roof leak begins.
That is worth knowing before you read a bid, because the flashing package is where bids differ most and where the difference is least visible. Two prices for the same metal roof can be a straightforward comparison on the panels and a completely different roof on the details. The panels are a repeating product. The flashings are made for your building.
What a metal roof needs, and what each piece does
| Location | Component | What it does | How it fails |
|---|---|---|---|
| Eave | Drip or eave metal | Starts the roof, throws water clear of the fascia | Fixed too tight to move, or lapped the wrong way |
| Gable edge | Rake trim | Closes the panel end against wind driven rain | Lifts at the fixings, loses its closure |
| Valley | Valley metal | Carries the concentrated flow from two slopes | Too light a gauge, or panels cut short of it |
| Roof to wall, uphill | Headwall flashing | Takes water running down onto a vertical | Not carried high enough behind the wall finish |
| Roof to wall, alongside | Sidewall flashing | Closes a slope running beside a wall | Relies on sealant instead of a formed upstand |
| Ridge and hip | Cap and closures | Closes the top of the panels, allows venting | Closures omitted, so wind and pests get in |
| Penetrations | Boot or sleeve | Seals a pipe to the panel while allowing movement | Mastic used instead of a clamped boot |
| Equipment | Curb and cricket | Lifts the unit clear and diverts water around it | No cricket, so the uphill side holds water |
Roof flashing covers the same components on membrane roofs, where the principles are identical and the materials are not.
Formed, lapped and free to move
Three rules govern every detail in the table, and a detail that breaks any of them will leak eventually.
Formed. The shape sheds the water. A flashing is bent so that water running onto it has nowhere to go but out and down. Sealant inside the lap is a second line of defense, not the first one.
Lapped with the flow. Upper pieces lap over lower ones, always, so water crossing a joint stays on the outside. A lap made the other way is a funnel, and it looks perfectly tidy from the ground.
Free to move. Metal expands and contracts, and the flashings have to move with the panels they touch. Clamping a long trim tight at both ends puts the thermal load into the fixings, which is how screws back out, holes elongate and trim ripples along its length. Standing seam metal roof installation explains the same principle as it applies to the panels and their clips.
Dissimilar metals, and the slow failure nobody looks for
Put two different metals in contact with moisture present and the less noble one corrodes. On a roof that shows up in three ways: direct contact between a flashing and a panel of another metal, fasteners of the wrong material driven into either, and runoff from a component uphill landing on one below.
The last of these catches people out because nothing is touching. Water running off a copper component onto galvanized steel further down the roof carries the problem with it. The remedies are to keep the roof in one metal family, to isolate any contact that cannot be avoided, and to look uphill from every component and ask what drains onto it. Commercial metal roofing covers how these roofs age, which is mostly a story about fixings and details.
Long runs, and the joints inside the trim itself
Trim arrives in lengths, so a long eave or ridge is made from several pieces joined end to end. How those joints are made matters as much as the main detail, and they are easy to get wrong because they look finished either way.
A joint in trim should lap in the direction of the flow with butyl tape between the pieces, and it should not be fixed through both pieces with a screw that locks them together. If it is locked, the two lengths cannot move independently, and the thermal load goes into the fixing or buckles the metal between joints. The usual signs on an older roof are rippled trim, elongated screw holes and a line of rust where a hole has worked itself open.
On a long building the roof itself may need an expansion joint, which is a designed break in the assembly rather than a flashing. It is formed as a raised detail with a flexible closure, and it is the one place on the roof where metal is deliberately not continuous. Ask whether the design includes one and where it falls, because an owner who does not know it is there will eventually have it sealed over by somebody chasing a leak.
Where the flashing package gets cut
Watch for these in a bid, because each one saves the contractor money and costs you a detail:
- Flashings in a lighter gauge than the panels, stated nowhere
- A different coating system on the trim, which fades to a different color
- Valley metal sized for the slope, not for the flow it actually collects
- Closures omitted at the ridge and the eave
- Pipe penetrations sealed with mastic rather than a clamped boot
- Curbs without crickets on the uphill side
- Headwall flashing stopped short behind a wall finish that will have to come off later
- Site cut edges left untreated, which rust from the cut inward
The honest way to compare two bids is to count the details on your own roof first, then check that both prices have the same number of them. Metal roof cost sets out the rest of what moves the number.
What to specify before anyone prices it
- Flashing metal, gauge and finish, as their own lines rather than implied
- Valley metal gauge, stated separately
- Fastener type for each metal on the roof
- Where butyl tape or sealant sits in each lap, and which sealant
- How curbs and crickets are formed, and who supplies them
- How site cut edges are treated
- The equipment layout, agreed before the panel layout is set
Where to go next
If you are specifying a new metal roof, read this alongside standing seam metal roofing, because the panel choice and the detail package have to be decided together. If the roof is already on and leaking, the failure is almost certainly in the table above rather than in the panels, and knowing which row it is turns an expensive guess into a defined repair. How a commercial roof is specified covers where flashing sits in the wider assembly.
Common questions
What flashings does a metal roof need?
At minimum: eave or drip metal, rake trim at the gable edges, valley metal where two slopes meet, headwall and sidewall flashing where the roof meets a vertical, ridge and hip caps with closures, and a formed detail at every pipe, curb and penetration.
- Eave metal starts the roof and sets the drip line
- Rake trim closes the gable edge against wind driven rain
- Valley metal carries the heaviest concentrated flow
- Headwall and sidewall details join roof to building
- Ridge and hip caps need closures to stop wind and pests
Each of those is a separate formed component with its own gauge, profile and lap direction, and each is priced separately in a proper bid. A quote that lists flashings as a single allowance has not worked out what the building needs, and the shortfall appears as substitutions once the panels are going on. Count the penetrations on your own roof before reading a bid, since the number of details rather than the area of the roof is what sets the detail cost.
Can dissimilar metals be used together on a roof?
Only with care, because two different metals in contact with moisture present form a galvanic cell and the less noble one corrodes. Copper against steel is the classic failure, and so is water running off copper onto galvanized or aluminum components further down the roof.
- Contact between dissimilar metals needs separating
- Runoff carries the problem downhill without any contact
- Copper and galvanized steel is the worst common pairing
- Aluminum against uncoated steel fasteners corrodes too
- Fasteners must suit the metal they are driven into
The practical rules are to keep the whole roof in one metal family where you can, to use an isolating layer where contact is unavoidable, and to look uphill from any component for something that drains onto it. Fasteners are the detail most often got wrong, because a box of the wrong screws costs nothing and ruins a roof slowly from a hundred points at once. Ask which fastener is specified for each metal on the roof and ask to see the box.
How are rooftop units flashed into a metal roof?
On a raised curb, not directly onto the panels. The curb is built up above the roof plane, panels and flashings run to it and lap against it, and the unit sits on top of the curb so that no part of the equipment penetrates the waterproofing itself.
- A curb lifts the penetration clear of running water
- Flashings lap against the curb in the direction of flow
- A cricket diverts water around the uphill side
- Equipment can be serviced without standing on details
- Pipes and conduit get their own sleeved penetrations
The uphill side is the part that gets missed. Water running down the slope hits the back of the curb and has to be pushed around it, which is what a cricket does, and without one the back of the curb becomes a dam that holds water against a joint. Get the equipment layout agreed before the panel layout is set, because a unit that lands across a seam makes a difficult detail out of a straightforward one and nobody can move it later.
Should metal roof flashing be sealed with caulk?
Sealant belongs inside a joint that already sheds water by its shape, never instead of one. A properly formed and lapped flashing works without sealant, and the sealant is a second line of defense in the lap. A joint relying on a bead on the outside will fail when the bead does.
- Formed geometry does the work, sealant backs it up
- Sealant inside a lap is protected from sunlight
- An exposed external bead is the first thing to fail
- Butyl tape in laps outlasts a surface applied bead
- Sealant has a service life shorter than the roof
A useful test when you are looking at a bid or at an existing roof is to ask what happens to each joint when the sealant has gone, because it will. If the answer is that water still runs out and away, the detail is right. If the answer is that water gets in, the detail was never right and no amount of maintenance will fix it. Visible beads of mastic on a metal roof are a sign that somebody solved a shape problem with a tube.
Does metal roof flashing have to match the panel gauge and finish?
It should match the finish system and it should not be significantly lighter than the panels. Flashings handle more water and more movement than the panels do, so a thinner gauge in a different coating fades at a different rate, deforms more easily and reaches the end of its life first.
- Flashing gauge is often downgraded quietly in a bid
- A different coating system weathers to a different color
- Light gauge trim dents and oil cans at every fixing
- Valley metal in particular deserves a heavier gauge
- Matching finishes keeps the roof repairable later
Ask for the flashing gauge and finish to be stated as their own lines in the specification rather than implied by the panel line above them. The reason is partly appearance and mostly repairability: a roof whose details are in the same metal and finish as the panels can be repaired in twenty years with material that still matches, while a roof trimmed in whatever was cheapest cannot. Valley metal is worth paying up for in any case, because it carries the concentrated flow from two slopes.