Flashing is the collective name for every detail that closes a place where the roof surface stops. Each type takes its name from the thing it closes: a wall, an edge, a drain, a pipe, a curb, a change in direction. There are perhaps a dozen on a typical commercial building, and the useful way to learn them is by location rather than by shape, because location decides how each one gets built and how each one fails.
This matters on paper as much as on the roof. A scope that says flashings as required has priced nothing and promised nothing, and two bids written that way cannot be compared. Our page on roof flashing covers how the details work and why they leak. This one is the list.
The principle every type shares
Almost every flashing is two pieces, not one. A base flashing is part of the roof and turns up whatever it meets. A counterflashing is part of the wall or the cap and comes down over the top of the base, overlapping it without being fixed to it.
That separation is what lets the roof and the wall move independently and still shed water, and it is why the overlap is never sealed shut along its bottom edge. One piece details, sealed to both sides, are the most common cause of repeat leaks on an otherwise sound roof.
The types, by where they sit
| Type | Where it sits | What it closes | How it usually fails |
|---|---|---|---|
| Base flashing | Any upturn: wall, curb, parapet | The roof’s own edge at a vertical face | Short upstand, no mechanical termination |
| Counterflashing | Over a base flashing | The top edge of the upstand | Surface fixed to the face, sealant gone |
| Through wall flashing | Within the wall construction | Water inside the wall itself | Omitted, or blocked at the weeps |
| Coping | On top of a parapet | The whole top of the wall | Open joints, loose cleats, no drip |
| Drip edge and gravel stop | Roof perimeter, no parapet | The edge of the membrane or deck | Backed out fixings, water tracking behind |
| Step flashing | Pitched roof against a wall | Each course against the wall | Pieces too short or lapped backward |
| Valley flashing | Where two slopes meet | The line carrying concentrated water | Debris dammed, fixings in the water path |
| Apron flashing | Below a wall or chimney face | The downslope face of a penetration | Too narrow, no end dams |
| Curb flashing | Around rooftop equipment | The base of units, hatches, skylights | Curb too low, cap missing |
| Pipe boot and collar | Around pipes and conduit | A round penetration | Clamp loose, sealant the only barrier |
| Pitch pocket | Around awkward penetrations | Anything that cannot be flashed | Filler shrinks, nobody refills it |
| Drain and scupper flashing | At every outlet | The hole water is meant to leave by | Clamping ring loose or short of bolts |
| Expansion joint cover | Where the structure moves | A deliberate gap in the deck | Treated as a seam and sealed flat |
Two rows deserve a note. A pitch pocket is an admission that something could not be flashed properly, and it needs refilling on a schedule for as long as the roof lasts. An expansion joint is not a seam and must never be welded or sealed across; doing so transfers structural movement into the membrane, which then splits in a straight line.
Wall and parapet details
These are the ones that cost owners the most, because the roof and the wall are different structures that move at different rates and the detail has to absorb that.
The things that decide whether a wall detail lasts are the height of the upstand above the finished roof surface, whether the top edge is held mechanically rather than by sealant alone, and whether there is anything dealing with water inside the wall. Masonry is not waterproof; it absorbs water at the top and sheds it out lower down, which is what through wall flashing and weep holes exist for. A roof detail built perfectly against a wall with no internal drainage still delivers water to the inside face.
Parapets add the coping on top, and a coping with open joints feeds water straight into the wall core where nothing can be seen until it appears inside, often a long way from the source. What sits under the flashing and why it matters is worth reading alongside this, because the upstand height is set by the insulation thickness, and a reroof that adds depth without raising the detail is the single most common way a sound flashing becomes a short one.
Perimeter details
Where there is no parapet, the roof ends at the edge, and the edge is where wind pressure is highest. Drip edge, gravel stop, fascia and edge metal all do the same job: terminate the membrane, hold it down against uplift and throw water clear of the face below.
What makes them work is the fixing, not the metal. Continuous cleats, closely spaced fasteners, laps backed by a cover plate rather than sealant, and a profile that actually projects far enough to drip clear. On flat roofs in particular, a perimeter that lifts in a storm is where a wind loss usually starts, and the membrane peels inward from there.
Penetration details
Everything that comes through the roof is flashed either on a curb or with a collar, and the preference is always the curb. A curb raises the penetration above the water, gives the membrane a vertical face to terminate on, and lets the equipment be serviced or replaced without a roofer being involved.
Pipes and conduit get boots clamped above the water line. Groups of small penetrations are better handled by one curb with a sealed cover than by a dozen individual details, and far better than by a pitch pocket. On panel roofs the geometry changes, and metal roof flashing covers how these details are made where the surface itself is ribbed and moving.
Drainage details
Drains, scuppers and overflows are flashings even though nobody calls them that, and they are the ones inspected least. The membrane has to enter the drain bowl and be clamped, the area around it has to be the low point, and there has to be an overflow route for when the primary outlet blocks.
What a specification should name
Before a bid means anything, it should state, detail by detail:
- The upstand height above the finished roof surface
- How the top edge of each flashing is terminated, and into what
- The metal, its gauge and its finish
- Whether existing metal is reused or replaced
- Fixing type and spacing at the perimeter
- Which penetrations get curbs and which get collars
- How many pitch pockets remain, and why each one is unavoidable
- How the expansion joints are covered
Commercial roof installation covers where in the sequence these go in, which is earlier than most owners expect.
Where to go next
If you are comparing bids on an existing building, read the flashing lines first and the membrane line second: the membrane is a commodity and the details are where the real difference in price and life sits. Roof recover against tear off covers what happens to every one of these details when a new roof goes over an old one.
Common questions
What metal should roof flashing be made of?
Whatever is compatible with the roof around it, the wall it meets and the water running over it. Coated steel and aluminum cover most commercial work, stainless steel goes where corrosion is likely, copper where longevity matters more than cost, and lead where a shape has to be dressed by hand.
- Coated steel suits most wall and perimeter details
- Aluminum is light and will not stain, but moves more
- Stainless steel resists salt, chemicals and runoff
- Copper lasts longest and stains what sits below it
- Lead and lead substitutes dress into awkward shapes
The mistake that matters is mixing metals that react with each other, which corrodes the less noble one at the junction and does it fastest where water carries the reaction along. Copper over coated steel is the classic example. Where two metals have to meet, they need separating, and a specification should say with what. Gauge matters as much as material: a thin flashing on a long run oil cans, works loose at its fixings and opens its joints long before the metal itself gives up.
What is a reglet, and does my flashing need one?
A reglet is a groove or a formed channel in a wall that the top edge of a counterflashing tucks into and is sealed within. It gives the detail a mechanical termination rather than a sealed joint on the face of the wall, which is why it lasts.
- Cut into masonry, or formed as a surface mounted section
- Holds the counterflashing edge mechanically, not with sealant
- Sealant inside it is protected from sun and water
- A through wall version runs right across the wall
- Surface mounted on the face is the weakest version
The reason to care is sequence. A reglet cut into masonry is straightforward on a wall you are already working on and awkward on a finished one, so the detail tends to get downgraded to a surface mounted section stuck to the face. That version relies on sealant exposed to full sun, which is the shortest lived element of any roof. If a bid shows surface mounted counterflashing on a wall where a cut reglet is possible, ask why, and ask what the renewal interval for the sealant is.
How is a roof drain flashed into the membrane?
The membrane is carried down into the drain bowl, clamped between the bowl and a clamping ring, and the insulation around it is tapered down so water arrives at the drain rather than beside it. The clamping ring is the whole detail, and it is the part most often left loose.
- Membrane runs into the bowl, not up to it
- A clamping ring compresses it against the bowl
- Insulation is sumped so the drain is the low point
- A strainer keeps debris out of the outlet
- Overflow provision matters as much as the drain
Two checks tell you most of what you need to know about a drain. Lift the strainer and see whether the clamping ring bolts are tight and complete, because a ring short of bolts leaks without any visible damage anywhere. Then look at whether the drain actually sits at the lowest point of its area, since a drain on a high spot is a decoration. Both are quick, both are commonly wrong, and neither shows up in a photograph taken from the middle of the roof.
What is a curb, and why does every rooftop unit need one?
A curb is a raised frame built up from the deck that a rooftop unit, hatch, skylight or duct sits on. It lifts the equipment clear of the roof surface so the membrane can be turned up the side of the frame and terminated above the water line instead of around the equipment itself.
- Raises the penetration above standing and driven water
- Gives the membrane a vertical face to terminate on
- Lets the unit be serviced without touching the roof
- Lets the unit be replaced without a new roof detail
- Takes a cap or counterflashing to shed water outward
The alternative, sealing the membrane straight onto a unit sitting flat on the roof, fails every time: the equipment vibrates, the sealant has nothing mechanical holding it, and the joint is exactly where water collects. Where equipment is already flat on the roof, retrofitting a curb at reroofing is almost always worth the cost, because it converts a detail that leaks every few years into one that is maintained by looking at it. Ask whether the curb height clears the insulation thickness as well as the water.
Can flashing be reused when the roof is replaced?
Sometimes the metal can, but usually it should not be. Flashings are the oldest and hardest worked part of the assembly, the new roof will sit at a different height, and reusing the metal puts a detail with an unknown remaining life under a roof with a long warranty.
- A new roof is almost always thicker than the old one
- Reused metal rarely suits the new termination height
- Fixing holes do not line up a second time
- Coatings and finishes are near the end of their life
- A membrane warranty may exclude reused details
The one case for keeping existing metal is heavy architectural work, copings and copper in particular, where the material has decades left and the cost of replacing it is out of proportion. Even then it comes off, gets inspected and goes back with new fixings and new upstands behind it. If a bid keeps the existing flashings to save money, price the alternative as well, because a flashing renewed with the roof is renewed once and a flashing left in place is the thing that brings somebody back.