Flashing is the material that closes the joint wherever a roof meets something that is not roof. A parapet, a wall, an equipment curb, a pipe, a skylight, a drain, the edge of the building. The membrane or the panels take care of the open field. Flashing takes care of every interruption in it.
That division explains most of what happens to roofs. The open field is one material doing one job across an uninterrupted surface, and it rarely fails. Every flashing is two or three materials meeting, usually moving at different rates, often at a corner. When a roof leaks, the detail is where to look first, and an inspection that walked the field and glanced at the edges has not really inspected anything.
The two part detail, and why it is two parts
Where a roof meets a vertical surface, the standard answer is a pair.
Base flashing belongs to the roof. It is the membrane, or a sheet compatible with it, turned up the wall or the curb and secured there.
Counterflashing belongs to the wall. It is normally metal, set into a groove cut in the masonry, called a reglet, or surface mounted and sealed along its top edge. It comes down over the base flashing.
They overlap. They are not joined.
That last point is the one that gets undone in the field. A building and its roof deck move differently, through temperature, through loading, through settlement. A rigid joint between them has to put that movement somewhere, and it puts it into whichever material is weakest until something tears. Two overlapping parts absorb it instead: water sheds from the counterflashing onto the base flashing and away, while each is free to move.
So when you find sealant smeared along the line where a counterflashing meets a base flashing, somebody treated a leak and built a new problem. Ask why the water was getting behind in the first place.
Height, and the reroof trap
NRCA recommends a base flashing height of around eight inches above the finished roof surface. The number is not arbitrary: it is the room snow, ponded water and wind driven rain need before they reach the top of the detail.
Note the words finished roof surface. Not the deck.
This is what catches buildings during a recover. A new layer goes over the old one, the surface rises by the thickness of the recover board and any new insulation, and every existing base flashing is now shorter by exactly that much. An eight inch flashing becomes a five inch one without anybody touching it.
Two consequences follow. Rebuilding flashings higher is a real line in a reroof budget and is routinely left out of the cheaper bid. And the available flashing height is one of the hard limits on how much insulation a recover can add, which is why a recover sometimes cannot meet current energy code without rebuilding every wall detail. That belongs in the conversation before the design is settled.
The details worth naming
Edge metal. Where the roof ends, the edge is secured and the membrane is terminated into it. This is not trim. Roof edges carry the highest wind uplift on the building, there is a recognized standard for their securement, and a failed edge takes the first run of roofing with it.
Curbs. Rooftop units sit on curbs, and the curb is flashed like a small wall on four sides with four corners. Corners are where two directions of movement meet, so they are the most worked part of the detail.
Pipes and small penetrations. A boot or a sleeve, clamped and sealed at the top. Simple when installed with the roof, awkward when the pipe arrived afterward.
Drains. The membrane is clamped into the drain body. A loose clamping ring leaks continuously and invisibly, because the water goes straight into the deck rather than appearing on a ceiling.
Expansion joints. Where the structure is designed to move, the roof has to move with it. These are built details, not sealed gaps.
Pitch pockets. An open container around a penetration that cannot be flashed properly, filled with sealant. The sealant shrinks. It is the one detail on a roof that is maintenance by design, and every one on your roof is a standing commitment.
What changes on a metal roof
The principles hold, and two things about the execution differ enough to matter.
The first is movement. A metal panel moves further with temperature than a membrane does, and it moves along its length. A flashing fixed rigidly to both the panel and the wall fights that movement every day. Metal roof details use slip joints, cleats and cover plates so the two parts can travel against each other, and a detail that looks over engineered on paper is usually doing exactly that job.
The second is compatibility. Dissimilar metals in contact, or one draining onto another, corrode at the junction. A copper flashing above a steel panel is a slow problem with a certain outcome. This extends to the fasteners and the clips, not just the visible sheet, so a repair made with whatever screws were in the van can start a failure that takes years to show.
The practical consequence for an owner is that metal roof flashings are a specialist repair rather than a general one, and that patching one with sealant is even less durable than it is on a membrane.
What a specification should actually name
A proposal that says flashings included has told you nothing. A usable one names:
- The base flashing material, and that it is compatible with the membrane or panel
- The base flashing height above the finished surface, in inches
- How the counterflashing is secured: a reglet cut into the masonry, or surface mounted, and what seals its top edge
- The edge metal, and the standard its securement is tested against
- How corners are formed, since that is where the detail is most worked
- Which penetrations get a boot, which get a curb, and which, if any, get a pitch pocket
- Who reworks existing flashings if the roof surface rises
That last line is the one that separates two bids on a recover. One of them has priced rebuilding the wall details to the new height and the other has not, and until you ask, both look like the same job.
Why they fail
Four causes account for most of it.
- Movement. The joint works every day, through every temperature swing, for as long as the roof exists. Details that allow for it last; details that resist it eventually tear.
- Sealant used as the waterproofing. Sealant has a service life measured in years, not decades. It belongs as a secondary defense at the top of a counterflashing, not as the thing holding water out.
- Equipment installed after the roof. New units, conduit, cable trays and brackets arrive, fitted by trades who are not roofers, and each one is a new penetration detailed by whoever was holding the drill.
- Height lost at a reroof. Covered above, and common enough to be worth repeating.
Notice how many of those are about what happened to the roof after it was built, rather than how it was built.
Parapets and skylights, the two that catch people out
Parapets are the most demanding wall detail on a commercial roof because they are exposed on both sides. The roof side carries a base flashing. The top takes a coping, which is metal capping that has to shed water away rather than let it sit, and which is itself a wind loaded component. The outside face is building envelope. Water that gets into the top of a parapet travels down inside the wall and appears somewhere unrelated, which is why a parapet leak is so often misdiagnosed as a roof leak in the wrong place.
Skylights combine every difficulty at once: a curb with four corners, a frame that moves differently from the roof, glazing seals with their own service life, and condensation that forms on the underside in cold weather and drips exactly like a leak. Before chasing a skylight leak, establish whether the water is coming in or forming on the inside, because the two have completely different fixes and the second one is not a roofing problem at all.
Both are worth looking at specifically rather than as part of a general walk. A parapet needs its coping joints checked; a skylight needs its curb corners and its seals checked. Neither shows a problem from the middle of the roof.
The one thing to check this week
Walk to the largest rooftop unit on your building and look at the four corners of its curb.
Corners are where two directions of movement meet, which makes them the most worked part of the most worked detail on the roof. The unit also attracts service traffic, so people stand there, set tools down there, and run new conduit from there. If a roof is going to leak, the odds favor that spot over anywhere in the open field.
You are looking for lifted or open laps, sealant that has pulled away or cracked, a flashing that stops short of the recommended height, fasteners backing out, and any conduit or pipe that has clearly arrived since the roof did. None of it requires a specialist to spot.
If what you find is a surprise, that tells you something about the rest of the details as well.
What to do about it
Put the details on a schedule. Two inspections a year and one after any serious storm, and make the route go to every curb, pipe, drain, corner and wall junction rather than around the perimeter. How a surveyor tests a suspect detail covers what that actually involves.
Keep a list of the pitch pockets by location and treat it as a maintenance item, not a defect list.
Control what gets fitted to the roof. A written rule that no trade penetrates the roof without the roofing contractor doing the detail is one of the cheapest protections available to a building.
And when a leak appears, resist the urge to seal the stain. Water travels along a deck before it comes through. Start at the nearest detail uphill and work out, which is the method behind tracing a leak back to the detail that caused it.
Next: commercial roofing systems for how the field is built, and flat roofs explained for where these details are most tested.
Common questions
What is roof flashing?
Flashing is the material that closes the joint wherever a roof meets something that is not roof: a wall, a parapet, a curb, a pipe, a skylight or the edge of the building itself. The membrane or the panels handle the open field; flashing handles every interruption in it.
- It is a system of parts, not a single material
- Base flashing turns up from the roof onto the vertical surface
- Counterflashing comes down over the base flashing from above
- Edge metal closes the perimeter where the roof ends
- Pipe boots, curbs and corners each have their own detail
Think of a roof as a flat area plus a list of interruptions. The flat area rarely fails. Every item on the list is a place where two materials meet, usually two materials that move at different rates, and that is what a flashing detail has to absorb. The quality of a roof is mostly the quality of that list.
What is the difference between base flashing and counterflashing?
Base flashing belongs to the roof and turns up the vertical surface. Counterflashing belongs to the wall and comes down over the top of it. They overlap without being joined, so water sheds from one to the other while each is free to move independently.
- Base flashing is usually membrane or a compatible sheet
- Counterflashing is usually metal, set into or onto the wall
- The overlap sheds water; a sealed joint would not be better
- Sealing the two together is a common and damaging mistake
- A reglet is the groove cut into masonry to receive counterflashing
The reason they are not bonded is movement. The building and the roof deck move differently, and a rigid joint between them transfers that movement into the weakest material until it tears. A two part detail absorbs it. When you see sealant smeared between a base flashing and a counterflashing, someone has treated a symptom and created a new problem.
How high should roof flashing go up a wall?
NRCA recommends a base flashing height of about eight inches measured above the finished roof surface. The figure exists so that snow, standing water and wind driven rain all have somewhere to go before any of them reaches the top of the detail and gets behind it.
- Measure from the finished surface, not the deck
- Adding insulation at a reroof raises the surface and shortens the flashing
- A short flashing is one of the most common reroof defects
- Where the height cannot be met, the wall detail has to change
- Confirm the requirement with your membrane manufacturer as well
This catches owners during a recover, when a new layer lifts the roof surface and the existing flashings are suddenly too short. The fix is to rebuild them higher, which costs money nobody budgeted. It is also one of the limits on how much insulation a recover can add, so it belongs in the conversation before the design is settled rather than after.
Why do most roof leaks happen at flashings?
Because every flashing is a joint between materials that behave differently, and a joint is harder to keep watertight than a continuous surface. The field of a roof is one material doing one job. A flashing is two or three materials, movement, and usually a corner.
- Thermal movement works at the joint every day of the year
- Corners concentrate stress where two directions of movement meet
- Rooftop equipment sits on curbs, and service traffic lives around them
- Sealant has a service life and the details relying on it need revisiting
- Wind driven rain is pushed upward at a vertical surface
This is why an inspection that walks the open field and glances at the perimeter has not inspected anything. The time belongs at the details: every curb, every pipe, every wall junction, every corner, every drain. It is also why the first question after a leak is which detail is above it, not where the stain is.
What is a pitch pocket and should my roof have one?
A pitch pocket is a small open container built around an awkward penetration and filled with sealant. It is a last resort, used where nothing can be flashed conventionally, and it needs topping up for as long as the roof exists.
- The sealant inside shrinks and cracks with age and weather
- It is the one roof detail that is maintenance by design
- A properly flashed penetration needs no routine attention
- Several pitch pockets usually means the equipment came after the roof
- Put every one of them on the maintenance schedule by location
Count them on your roof. Each is a scheduled commitment, and a roof with a dozen of them has a dozen small reservoirs quietly drying out. When something is next replaced, ask whether the penetration can be reworked into a proper curb or boot instead of rebuilding the pocket, because that converts a recurring job into a finished one.