Parapet wall base flashing: the detail where low-slope roofs leak

Parapet wall flashing, and why the transition is the weak point

A parapet is the wall that extends above the roof at the edge of a low-slope building. The roof membrane runs across the field, reaches that wall, and has to stop, turn up the vertical face, and get sealed off at the top where a coping caps the whole assembly. That turn-up and its termination is the parapet base flashing, and it is one of the most leak-prone details on the building.

The reason is geometry. The field of the roof is a flat plane laid to shed water, and it almost never leaks on its own. The trouble starts where the plane has to change direction and go vertical, where it gets fastened, where the wall above moves differently than the deck below, and where another trade built the wall you have to flash against. Every one of those problems lands in the same eight to twelve inches of detail.

Done right, the assembly is layered so water always sheds over a lower piece and never into a joint: base flashing carried up to the right height, a cant or cove at the corner so the membrane is not forced into a hard 90, a mechanical termination at the top, and a counterflashing or through-wall flashing that laps over the termination and keeps water out of it. The companion guide on roof flashing types maps every flashing on the roof, and the leak-diagnosis guide walks how to trace water once it is already inside. This guide goes deep on the parapet wall.

Why do parapet walls leak?

Parapet walls leak because they collect water on three sides and move independently of the roof they sit on. The roof side gets the runoff. The outside face takes wind-driven rain. The top bakes in the sun and freezes at night. All of that water is looking for a joint, and the parapet has more joints per foot than anywhere else on the roof.

Movement is the quiet killer. The parapet is a thin wall sticking up into the weather, so it heats, cools, and flexes on a different schedule than the insulated deck below it. If the base flashing is fastened rigidly to both the deck and a wall that moves, the cycling works the flashing until it splits at the corner or tears at the fastener line. That is why base flashing is generally not anchored tight to a parapet that can move on its own, and why the cant and the termination detail matter so much.

Then there is the wall itself. On a masonry parapet, water that gets behind the cap or through the joints travels down inside the wall and shows up on the ceiling well away from where it entered, which sends crews chasing the wrong spot. The parapet is where a roof that looks perfect in the field still puts water on the floor. Treat it as the number one detail, not an afterthought at the edge.

The layered assembly and the shingle principle

The parapet detail is a stack of layers, and the whole thing works on one rule: each upper layer sheds water over the top of the layer below it, never into a joint pointed uphill. Same logic as a shingle. Get the order right and the wall stays dry. Reverse any single lap and you have built a funnel that collects every drop.

From the bottom up the stack reads like this. The field membrane runs out to the base of the wall. A cant or cove fills the inside corner so the membrane turns up without a hard right angle. The base flashing carries up the vertical face to height. A termination bar or other mechanical fastening holds the top of the base flashing. Then a counterflashing, or a through-wall flashing built into a masonry parapet, laps down over the top of that base flashing and sheds water across the termination instead of into it. The coping caps the wall above everything.

Every leak on a parapet is some version of breaking that order. The base flashing stops too low. There is no counterflashing, so the termination is exposed and relying on a bead of sealant. The counterflashing is tucked behind the base instead of over it. The coping joints dump water into the wall above a flashing that was never meant to catch it. Walk the stack from the top down when you inspect, and water-test it as a layered system, not a single piece.

The base flashing: the membrane turned up the wall

Base flashing is the lower layer of the two-part wall detail. It is the roofing material, usually the membrane itself or a compatible flashing sheet, carried off the field and up the face of the parapet, then secured at the top. On a single-ply roof it is TPO, PVC, or EPDM flashing membrane. On a modified-bitumen roof it is a flashing-grade cap sheet. On some assemblies it is metal. The counterflashing is the upper layer that caps it.

How the base flashing attaches to the wall depends on the system. Single-ply flashing membrane is generally fully adhered to the parapet substrate with the manufacturer's bonding adhesive, or set against a fastened cover board, so it cannot trap an air pocket that flutters in the wind and works the seam loose. Modified bitumen is torched, mopped, or set in cold adhesive per the system. However it goes on, the bond carries the flashing up the wall and the mechanical termination holds the very top.

The base flashing is field-made, by hand, at the hard spot, often late in the day. That is exactly why it fails more than the field. Lap the flashing onto the field membrane with the field-to-flashing weld or seam the manufacturer specifies, probe every seam, and treat the corners as separate work, not a continuation of the run. The manufacturer's parapet and base-flashing details govern the attachment method, the seam, and the materials, and following them is what keeps the warranty intact.

How high should base flashing be?

Base flashing should generally extend at least 8 in above the finished roof surface, and many details and high-snow regions call for more, commonly 12 in or higher. That 8 in minimum is widely cited by NRCA and carried in most membrane manufacturers' details, but it is the floor, not a target, and the manufacturer detail and the AHJ control the number on your job.

The height exists to keep standing water, ponding, and drifted snow below the top of the flashing and below the termination. Flash too low and the first hard rain that ponds against the wall, or the first snow that piles into the corner, sits above the sealed top and runs straight behind the flashing. This is one of the most common parapet failures there is, and it is usually baked in at design or created later when someone adds insulation or an overlay and raises the roof surface without raising the flashing.

Measure the 8 in from the finished surface you will actually have, not the deck. If a recover adds two inches of height to the roof, the flashing that used to clear by 8 in now clears by 6 in, and the detail that passed last time fails this time. On a reroof, raising a too-low flashing to regain the height is part of the job, not an extra. Hedge the exact dimension to the membrane manufacturer, NRCA, and the AHJ, but never let it drop below their stated minimum to save a strip of membrane.

The cant strip: no hard 90 at the base of the wall

A cant strip is the angled fillet, usually wood fiber, perlite, or treated wood cut to a 45, set into the inside corner where the roof meets the parapet. It turns the sharp 90-degree corner into two gentler angles so the base flashing can make the transition without being bridged or stressed across a hard edge.

Membrane and especially bitumen do not like a hard inside corner. Bridge a stiff flashing sheet across a 90 and it never fully contacts the corner, so you get a void that flexes, fatigues, and eventually splits right at the angle. A torched or mopped cap sheet forced into a square corner cracks there first. The cant gives the flashing a surface to lie against the whole way around the turn, which spreads the stress and kills the void.

Whether you need a cant, and what size, depends on the membrane. Built-up and modified-bitumen systems almost always call for one. Many single-ply systems allow a smaller cove or a prefabricated inside-corner piece instead of a full cant, and some adhered details turn the corner without one if the manufacturer's detail permits it. Follow the system. The principle holds across all of them: do not force the flashing into a square inside corner and expect it to last.

The termination bar at the top of the base flashing

The top edge of the base flashing has to be held mechanically so it cannot sag, peel, or get peeled by wind. The common method on single-ply is a termination bar: an extruded aluminum bar fastened through the membrane into the wall, clamping the top of the flashing tight against the substrate. Many bars have pre-punched holes around 6 in on center and a top lip shaped to take a bead of sealant against the wall.

Fastener type and spacing come from the manufacturer and the substrate, not from habit. Into masonry you use the right anchor for the block or brick. Into concrete you use a concrete fastener that will actually hold. Into a stud-and-sheathing parapet you hit the framing or use the specified anchor for the sheathing. Spacing is commonly in the 6 to 12 in range depending on the system and the wind exposure, but the detail and the wind design control it, so confirm the number rather than guessing.

The bar is a mechanical termination, not a waterproofing seal by itself. The sealant at the top lip, the lap sealant where the bar joints meet, and the bar alone do not make the top of the wall watertight for long. They are the reason the counterflashing exists. Set the bar, seal it per the detail, and then cover it with a counterflashing or through-wall flashing that sheds water over the whole termination.

What is counterflashing?

Counterflashing is the upper metal layer that laps down over the top of the base flashing and sheds water across the termination instead of letting it run behind. It is the cap of the two-part wall detail: base flashing carries the water up and turns it back at the bottom, counterflashing covers the top and turns water out at the top. The two pieces overlap but are not bonded together, so the wall and the roof can move independently without tearing the seal.

The job of the counterflashing is to keep water out of the termination. Water running down the parapet face hits the counterflashing and is thrown clear of the term bar and the top edge of the base flashing. Without it, every drop that runs down the wall arrives at the exposed top of the base flashing and the only thing standing between that water and the building is a bead of sealant in the sun. Counterflashing is what lets the detail shed by its shape rather than depend on a sealant joint.

Counterflashing should lap well down over the base flashing, commonly on the order of 3 to 4 in of overlap, with its own joints lapped and sealed so the runs do not leak at the splices. It comes surface-mounted to the wall, set into a saw-cut reglet, or formed as part of a through-wall flashing in masonry. Surface-mounted is the most sealant-dependent and the first to fail when that sealant goes. Reglet and through-wall are more durable because they tuck into the wall and shed without leaning on a bead. The manufacturer and SMACNA details govern the laps, the metal, and the profile.

Reglet counterflashing vs through-wall flashing

On a masonry parapet there are two main ways to terminate and shed over the base flashing, and they are not equal. A reglet counterflashing is metal whose top edge tucks into a groove cut into the masonry, then is sealed. A through-wall flashing is a continuous waterproof layer built into the wall during construction, spanning the full thickness and turning down on the roof side as the counterflashing. Through-wall is the stronger detail because it stops water that is inside the wall, not just water on the face.

A reglet is usually a saw-cut into a mortar joint or the masonry, commonly around 1 to 1-1/2 in deep, with the counterflashing's bent top edge inserted and held with sealant or a lead wedge. A reglet sheds face water well and is far better than a surface-mount, but the cut still relies on sealant at the top and it does nothing for water already traveling inside the wall. Reglets sit within roughly the same band as the base flashing height, commonly 8 to 12 in above the roof, so the lap over the base flashing works.

Through-wall flashing is the better answer on a masonry parapet and the harder one to add later, because it has to be built into the wall. When the wall is being built or rebuilt, this is the detail to insist on. When you are flashing against an existing wall you usually cannot get one, so you use a reglet and accept that the wall above it needs its own water management. SMACNA and masonry practice cover both; the choice is driven by whether you control the wall.

Through-wall flashing and weeps on a masonry parapet

A masonry parapet is not waterproof. Brick, block, mortar joints, and the coping above all let water in, and that water travels down inside the wall. If nothing stops it, it runs right past the base flashing and the counterflashing on the inside and shows up as a roof leak that no amount of work on the membrane will fix. Through-wall flashing is what stops it.

Through-wall flashing is a continuous membrane or metal layer built into the wall, spanning the full thickness, that catches water moving down inside the masonry and directs it back out. It is paired with weep holes or weep vents in the head joints just above the flashing, commonly spaced around 24 in on center, so the collected water has a way out instead of pooling on the flashing and finding the next path in. On the roof side, the through-wall flashing turns down to serve as the counterflashing over the base flashing, which is why it is the cleanest version of the whole detail.

If you are chasing a recurring parapet leak on a masonry wall and the membrane checks out, the wall is the suspect. Water entering the coping joints or the face and running down inside the masonry behaves exactly like a flashing leak from the ceiling, but the entry point is feet away and above. On a wall being built or rebuilt, get the through-wall flashing and the weeps in. On an existing wall that lacks them, the realistic fixes are managing the water at the coping and the face and, where the masonry allows, retrofitting a through-wall detail, which is real work and worth scoping honestly.

The coping interface at the top of the wall

The coping is the cap that covers the top of the parapet and is the first line of defense for everything below it. It sheds water off both faces of the wall and protects the top of the masonry, the top of the base flashing, and the counterflashing termination. The coping is its own detail with its own wind-uplift and expansion requirements, and the companion guide on edge-metal coping wind design covers the cap itself in depth.

What matters here is the relationship between the coping and the wall flashing below it. The counterflashing or through-wall flashing has to integrate with the coping so water shed by the cap lands outboard of the termination, not into it. Where a reglet or surface counterflashing terminates high on the wall, the coping above it needs to keep its joints and its underside from feeding water down onto that termination. The two details have to be coordinated, because a coping that leaks at its joints dumps water straight onto the flashing you worked to keep dry.

The most common failure at this interface is treating the coping and the wall flashing as separate scopes that never get reconciled. The roofer flashes the wall, someone else sets the coping, and the lap between them is nobody's responsibility. Water finds that gap. Coordinate the coping joints, the cleats, and the underside drip with the counterflashing below so the whole top of the wall sheds as one system.

The back side of the parapet and the wall cladding

The roof side gets all the attention, but water gets into a parapet from the outside face too, and that water ends up in the same place. The exterior cladding, the mortar joints, the sealant at windows or penetrations in the wall, and the coping joints all admit water that travels down inside the wall and can surface on the roof side or on the ceiling below.

This is why a parapet is a wall problem as much as a roof problem. On a masonry parapet, water entering the back face has to be caught and discharged by the through-wall flashing and weeps the same way water from the top is. On a clad or EIFS parapet, the back-side water management is part of the wall assembly, and if it dumps water into the parapet without a way out, the roofer gets blamed for a leak that started on the building's exterior.

When a parapet leak does not track to the membrane or the base flashing, look at the whole wall, both faces and the cap, before you re-flash the roof side again. The roof crew owns the roof-to-wall detail. The wall, its cladding, and its through-wall management are where a lot of stubborn parapet leaks actually live, and the fix is on the wall, not the membrane.

Inside and outside corners: the hardest leak spot

Corners are where parapet flashing fails first. A straight run of base flashing is simple. A corner forces the membrane to turn in two directions at once, which means stretching, bunching, or cutting and patching the sheet exactly where it is already under the most stress. Inside corners pull the membrane into a pocket. Outside corners stretch it over an edge. Both are the spots a leak finds before the field of the flashing ever gives up.

The fix the manufacturers settled on is the prefabricated corner: a molded inside or outside corner piece made to fit the geometry, welded or bonded into the field flashing so the hardest part of the detail is a factory-made shape rather than a hand-cut one at the end of a shift. Use them. A clean prefab corner welded in beats a field-cut corner sealed with mastic every time, and on most warranted systems the prefab corner is the required detail, not an option.

Reinforce the corners and probe every weld around them. On a masonry parapet, the through-wall flashing and counterflashing also have to turn the corner, and metal corners are soldered, welded, or made from prefabricated corner pieces with lapped and sealed joints. When you inspect, go to the corners first. That is where twenty years of chasing leaks tells you to start, and it is almost never wrong.

Scuppers and penetrations through the parapet

Anything that goes through the parapet is a hole in the wall flashing, and it has to be flashed as carefully as the wall itself. The most common is the scupper: an opening through the parapet that lets water drain off the roof through the wall, either to a downspout or to open discharge. A scupper combines a roof penetration, a wall penetration, and the base flashing all in one spot, which is why it leaks.

A scupper has to tie the roof membrane, the base flashing, and the scupper sleeve or box into a continuous waterproof transition, with the membrane sealed to the scupper flange and the flange lapped correctly so water sheds out through the opening and not into the wall around it. The scupper also has to be set low enough to actually drain the roof and relieve ponding, since a scupper above the water line does nothing. Drainage sizing and overflow are their own subject, and the roof drainage details cover scupper and overflow sizing.

Other penetrations through a parapet, conduit, pipe, overflow lines, get the same treatment as any roof penetration: a flashed, sealed transition that sheds over the membrane, not a hole caulked shut and hoped over. Every penetration through the wall is one more entry point in the most leak-prone part of the roof, so keep them out of the parapet where you can, and detail the ones you cannot avoid.

Thermal and structural movement at the parapet

The parapet moves on a different schedule than the roof, and the flashing has to absorb that movement instead of fighting it. The wall heats and cools faster than the insulated deck, the structure deflects, and a long parapet expands and contracts measurably between a hot afternoon and a cold night. Metal moves three to four times more than masonry over the same temperature swing, which is why metal copings and counterflashings need slip joints while the wall barely moves.

If the base flashing is anchored rigidly to both the deck and a parapet that can move on its own, the cycling tears it. This is why base flashing is generally not fastened tight to a parapet that is structurally separate from the deck, and why the cant and the flexible turn at the base matter: they let the detail flex. Where the building has an expansion joint, the roof and the flashing have to carry that joint through with a detail built to move, not a rigid flashing run straight across it.

Rigid flashing across a moving joint cracks, and it cracks on a schedule, usually at the corner or the joint within a season or two. When you see a parapet flashing split in a straight line at a consistent spot, suspect movement that the detail never accounted for. The fix is a detail that flexes, not a heavier bead of sealant over a crack that will just reopen.

Membrane base flashing vs metal, and compatibility

Base flashing is either the membrane system carried up the wall or a metal flashing, and the choice follows the roof. A TPO roof gets TPO flashing membrane and TPO-coated metal where metal is used. A PVC roof gets PVC flashing and PVC-coated metal. EPDM gets EPDM flashing with the system's seam tape or adhesive. A modified-bitumen roof gets a flashing-grade cap sheet. Mixing chemistries is how you build in a failure: uncured EPDM detailing on a TPO wall does not weld, and the wrong metal coating will not bond to the field.

The counterflashing and any exposed metal are usually a coated or compatible metal, galvanized or stainless or aluminum depending on the environment and the system, and on coastal or high-corrosion sites the metal selection matters more. The point is to keep the whole detail inside one manufacturer's system so the materials are tested to bond and the warranty stands behind the assembly, not just the field membrane.

When you substitute, you own the result. A field-expedient flashing in a material the system was not designed around may pass a glance and fail the first real weather. Stay in the manufacturer's parapet detail and use their flashing membrane, their adhesive, their prefab corners, and their specified metal. That is the difference between a warranted assembly and a detail you are personally guaranteeing.

Masonry, concrete, and stud-framed parapets

What the parapet is made of changes how you attach to it and how it leaks. The three you see most are masonry, concrete, and stud-and-sheathing, and each one wants a different anchor and a different water strategy.

Masonry parapets, brick or block, are not waterproof and they move differently than the deck, so they want a fully adhered base flashing, the right masonry anchor at the termination, and through-wall flashing with weeps to manage water inside the wall. Concrete parapets are more stable but still need the correct concrete fastener and a sound, primed surface for adhesion, and a cast-in reglet is a clean way to take the counterflashing. Stud-and-sheathing parapets are common on newer construction and behave like a wall: the flashing bonds to a fastened cover board or sheathing, you anchor the termination into framing or the specified sheathing anchor, and the wall's water-resistive barrier and back-side cladding have to be integrated so water does not get behind the flashing from the exterior.

Confirm the substrate before you order anchors, because the termination bar fastener that holds in concrete spins out of soft block, and the anchor that bites in masonry does nothing in sheathing. The attachment method, the fastener, and the spacing all follow the substrate and the manufacturer's detail.

Where do parapet walls leak most?

Parapet walls leak at a short, predictable list of spots, and they rank by how often they bite. Base flashing carried too low, so ponding or drifted snow sits above the seal. No counterflashing, so the termination is exposed and the detail is riding on a bead of sealant. Failed sealant at the term bar where there is nothing capping it. An open or failed reglet where the cut has dried out. Badly detailed corners where the membrane was cut and patched instead of using a prefab. And on masonry, the absence of through-wall flashing and weeps, so water inside the wall bypasses everything you did on the roof side.

Coping joints belong on the list too. A coping that leaks at its splices feeds water straight down onto the flashing, and because the entry is above and the exit is below, it reads like a base-flashing leak when the cap is the culprit. The leak-diagnosis guide walks how to separate these once water is already inside, but the pattern is consistent: it is almost always a layering or a height problem, not the field of the flashing.

When you inspect a parapet, work the list. Check the height against the finished surface. Confirm there is a counterflashing and that it laps over, not behind. Probe the term bar and the reglet. Pull apart the corners with your eyes. On masonry, ask whether the wall has any through-wall management at all. The water is getting in at one of these, not in the middle of the run.

Sealant is the last line, not the system

Sealant has a place in the parapet detail, and that place is a backup, not the waterproofing. The assembly is supposed to shed water by its shape: the base flashing turns water back, the counterflashing throws it clear of the termination, and the laps all run downhill. Sealant fills the small gap at the top of a term bar or a reglet as a secondary defense. It is not what keeps the wall dry.

The reason is simple. Sealant on a parapet lives in the worst exposure on the building, full sun, full weather, constant movement, and it has the shortest service life of anything in the detail. It dries, cracks, and pulls loose, often within a few years, while the membrane and metal around it last decades. Build a detail that leaks the day the sealant fails and you have built a maintenance problem that comes due on a schedule and gets ignored until the ceiling is wet.

The test for any parapet detail: if every bead of sealant disappeared tomorrow, would the assembly still shed water? A counterflashing that laps well over the base flashing passes that test. A term bar with a caulk joint and no cap fails it. Design and build so the sealant is the redundancy, then maintain the sealant on a real cadence because it is the part that wears out first.

Inspection and maintenance of parapet flashing

The parapet is the first place to inspect and the most recurring leak source on a low-slope roof, so it earns a real look on every inspection, not a glance from the field. Walk the whole perimeter. Check the base flashing height against the current finished surface, look for the counterflashing and confirm it laps over the base, probe the term bar and the reglet sealant, and open up the corners with your eyes and a seam probe.

The maintenance item that comes due first is sealant. Re-seal the term bar lip, the reglet, the counterflashing laps, and the coping joints on a cadence before they fail, because that is the part with the shortest life and the one a leak waits on. While you are up there, look at the masonry for cracks and open joints, check the coping for loose sections and open splices, and confirm the weeps are clear and not painted or mortared shut.

A recurring parapet leak that keeps coming back after spot repairs is telling you the problem is the detail, not the patch. Too-low flashing, a missing counterflashing, or no through-wall management on a masonry wall will not be cured by another bead of caulk. At that point the honest call is to re-flash the detail right, not to keep selling the same patch every spring.

Re-flashing the parapet at reroof

A reroof is the chance to fix the parapet detail, and it is the moment crews most often leave it half-done. The new field membrane goes on, the old base flashing gets tied into, and the too-low flashing that was marginal before is now worse because the new roof or the added insulation raised the surface. The height that barely cleared is now under the ponding line.

Re-flash the parapet as part of the reroof, do not just lap new field membrane onto old wall flashing. Carry fresh base flashing up to the full height measured from the new finished surface, add or rebuild the cant, set a new termination, and reinstate or replace the counterflashing so it laps the new base flashing. If a recover raised the roof, the flashing has to come up to match, and that is scope, not an extra you discover later.

Tie-ins to existing adjacent flashing have to be made with compatible materials and a real lap, not a smear of mastic at the joint. Where you are recovering over an old system, confirm the new flashing membrane is compatible with whatever it bonds to. The reroof that ignores the parapet hands the owner a roof that leaks at the edge within a season, and the callback lands on the crew that just left.

Field checklist

What to document

A parapet detail you cannot reconstruct from the record is a detail you will rebuild blind at the next leak. Capture the assembly, the manufacturer system, and the inspection so the next person knows what is in the wall and whether it was ever right. A field tool like FieldOS keeps the photos, the detail callout, and the inspection notes tied to the building instead of scattered across phones and trucks.

Record the membrane system and flashing materials, the base flashing height as built off the finished surface, whether a cant or prefab corners were used, the termination method and fastener, the counterflashing type, whether the masonry has through-wall flashing and weeps, and the date and findings of each inspection. Photograph the corners and the terminations before the coping goes on, because once it is capped you cannot see the detail again without taking it apart.

Common mistakes

• Carrying the base flashing too low, so ponding or drifted snow sits above the sealed top.
• Skipping the cant strip and forcing the membrane into a hard 90-degree inside corner.
• Leaving the termination exposed with no counterflashing, so the detail relies on a bead of sealant.
• Tucking the counterflashing behind the base flashing instead of lapping it over the top.
• Letting a reglet dry out and open, or surface-mounting counterflashing and depending on the caulk.
• Hand-cutting and mastic-patching corners instead of using prefabricated corner pieces.
• Ignoring through-wall flashing and weeps on a masonry parapet, so wall water bypasses the roof detail.
• Running rigid flashing across a moving parapet or an expansion joint and waiting for it to crack.
• Recovering or reroofing without raising a now-too-low flashing to the new finished surface.

Standards and references

The membrane manufacturer's published parapet and base-flashing details are the first authority, because they govern the materials, the attachment, the heights, the prefab corners, and the warranty. Follow them as written. Where they call a height, a fastener, or a corner detail, that is the number that protects the assembly and the warranty, so do not improvise around it.

NRCA, the National Roofing Contractors Association, publishes the construction details and the roofing manual the trade works from, including the parapet base-flashing and metal cap details and the widely cited minimum flashing height of about 8 in above the roof, with more in high-precipitation regions. SMACNA, the Sheet Metal and Air Conditioning Contractors' National Association, covers the sheet-metal side, the counterflashing, reglets, and metal laps in its Architectural Sheet Metal Manual. For the masonry, through-wall flashing and weeps follow established masonry practice and the references that cover concrete and brick walls.

Heights, fastener spacing, reglet depths, and lap dimensions vary by system, by region, and by code cycle, so hedge them to the manufacturer detail, NRCA, SMACNA, and the AHJ rather than treating any one number as universal. What does not vary is the principle: carry the base flashing to height over a cant, terminate it mechanically, and shed water over it with a counterflashing or through-wall flashing instead of a sealant bead. On a masonry parapet, get the through-wall flashing and the corners right, because that is where the durable failures live.

Units and terms

The parapet detail carries a handful of terms that get used loosely on the jobsite, and getting them straight keeps a crew, an inspector, and a manufacturer talking about the same piece.

Heights and laps are in inches in NRCA and most US details and in millimeters in metric references, where 8 in reads as 200 mm and 12 in as 300 mm. The base flashing is the lower, roof-side layer; the counterflashing is the upper layer that caps it. Keep the two straight, because reversing them in a conversation or on a drawing is how a reversed lap ends up built.

Parapet

The wall that extends above the roof at the edge of a low-slope building

Base flashing

The lower, roof-side membrane or metal carried up the wall and terminated at the top

Counterflashing

The upper metal layer that laps over the base flashing and sheds water across the termination

Reglet

A groove cut or formed in masonry that receives the top edge of a counterflashing

Through-wall flashing

A continuous waterproof layer built across a masonry wall that catches interior water and discharges it at weeps

Cant strip

An angled fillet at the inside corner that lets the flashing turn up without a hard 90

Termination bar

An extruded bar fastened through the membrane to clamp and hold the top of the base flashing

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• What is roof flashing?
• Where do most roof leaks come from?
• How high should base flashing be on a commercial roof?
• Why do skylights leak?
• TPO vs EPDM vs PVC: which single-ply membrane is best?