Flat roof flashing is the set of membrane strips, formed metal and preformed boots that seal every place where the roof surface ends or is interrupted: walls and parapets, roof edges, curbs under equipment and skylights, pipes, vents, drains and scuppers. Many flat roof leaks trace back to one of these joints rather than to the open membrane, so flashing deserves the closest look on any new roof or repair. For a flashing inspection, call NEMA Roofing at 866-631-3366.
Where does a flat roof need flashing?
California’s building codes spell out the places. The 2025 California Building Code (1503.2) and the residential code (R903.2) require flashing that keeps water out of the wall and roof at joints in copings, through moisture-permeable materials, at intersections with parapet walls and at other penetrations, and they list the locations: wall and roof intersections, every change in roof slope or direction, and around roof openings (the building code adds gutters). Metal flashing must be corrosion resistant and at least 0.019 inch thick, the equivalent of No. 26 galvanized sheet.
| Location | Usual flashing | Where it goes wrong |
|---|---|---|
| Walls and parapets | Membrane base flashing turned up the wall, covered by metal counterflashing | Base flashing too short, counterflashing loose, cracked stucco above letting water in behind |
| Parapet tops | Metal coping or a membrane cap | Open coping joints, a top that slopes toward the roof side and drips down the wall |
| Roof edges without a wall | Metal edge or gravel stop, stripped in with membrane | Edge metal loosened by wind, split strip-in at the joints |
| Equipment, skylights, hatches | A raised curb wrapped in membrane, metal cap above | Curb too low, water ponding on the upslope side |
| Pipes, vents, conduits | Preformed boot or a pitch pocket filled with sealer | Boot cracked by sun, pitch pocket filler shrunk away from the pipe |
| Drains and scuppers | Membrane clamped into the drain or wrapped through the scupper | Loose clamping ring, poorly sealed scupper corners |
How are walls and parapets flashed?
Where the flat roof meets a wall, the membrane does not simply stop. It turns up the wall as base flashing, fully bonded, and its top edge is then covered by counterflashing: formed metal or elastomeric sheeting fixed on or into the wall to shield the upper edge of the base flashing and its fasteners (the definition used in the U.S. Department of Defense roofing criteria, UFC 3-110-03). The counterflashing may be set into a cut in the wall (a reglet) or surface-mounted with a sealed top edge.
The wall above matters as much as the flashing. Water that runs down cracked stucco or through a poorly sealed window above can get behind the counterflashing and show up inside as a “roof leak”. On parapets, the building code requires the top surface to provide positive drainage (1503.3), so coping should shed water, not collect it.
How are pipes, vents and rooftop equipment flashed?
Small round penetrations get a preformed boot made for the membrane, or a pitch pocket (a formed collar filled with sealer) where a boot will not fit. The DoD criteria for TPO roofs call for one of the two on every penetration, vertical or horizontal. Pitch pockets need their filler topped up as it shrinks; boots need checking for sun cracks.
Anything larger (air conditioning units, skylights, roof hatches) sits on a curb that the membrane wraps. The same criteria set curbs at least 8 inches above the roof surface and at least 6 inches above the high point of any cricket, and require a cricket on the upslope side of penetrations wider than 24 inches so water flows around them instead of ponding against them. For houses, the residential code (R903.2.2) requires a cricket or saddle on the ridge side of any chimney or penetration more than 30 inches wide.
Two habits prevent most penetration leaks: keep new penetrations away from membrane seams, and have the roofer flash anything added later. Cable, solar and HVAC installers who cut through a membrane and seal it with caulk are a frequent source of new leaks on an otherwise sound roof.
What about roof edges, copings and scuppers?
On a roof without parapets, the edge metal both finishes the look and holds the membrane’s perimeter down; on a low-slope commercial roof it must be a wind-tested system (see our guide to flat roof wind uplift). Where a parapet has a scupper, the membrane wraps through the opening; the DoD criteria note that scuppers in TPO systems need hand-welded seams and extra inspection attention, because the corners are hard to seal.
Why does flashing fail before the rest of the roof?
- Movement. Walls, curbs and the roof deck expand and move differently in heat and cold, and the flashing sits at the joint between them.
- Sun and heat. Vertical flashing and boots take direct sun on their sides and edges, and sealants harden and pull away.
- Ponding. Water held against a curb or wall base tests every seam in the flashing for hours or days.
- Later work. Penetrations added after the roof was built, and patches made with the wrong materials.
A bead of caulk over a failed flashing buys time at best; why tube sealant rarely fixes a flat roof leak is covered in our guide to flat roof waterproofing.
Repair or replace the flashing?
A single cracked boot, a loose piece of counterflashing or an open coping joint is a normal repair: the part is replaced or re-stripped with membrane compatible with the roof. When the membrane itself is replaced, all flashing is replaced with it; old metal and old base flashing are not reused under a new roof, because they would carry the old roof’s age into the new one. If a roof deck, new equipment or solar is planned, do it before or with a reroof so the new flashing goes in once. Planning a deck on a flat roof raises its own flashing questions at posts and door thresholds, covered in our flat roof deck guide.
What you can check yourself
- From a safe vantage point or a photo, look at the base of walls and curbs for open seams, wrinkles or membrane pulling away.
- Look for rust, gaps or missing fasteners in counterflashing and coping.
- Check pipe boots for cracks and pitch pockets for filler that has sunk below the rim.
- After rain, note where water stands; ponding against a curb or wall means the flashing there is under constant test.
- Inside, match any stain to what sits above it on the roof. A stain under an AC unit or near an outside wall points to flashing first.
Flashing done as part of the roof, not an afterthought
We treat flat roofing as a specialty: on every flat roof we install, each seam, edge and penetration is sealed as part of the system. NEMA Roofing’s flat roofing service covers flashing repair, reflashing and full replacement for homes and commercial buildings. From five locations (Santa Barbara, Oxnard, Woodland Hills, Tarzana and Downey) we reach the whole of Santa Barbara, Ventura, Los Angeles, Riverside and San Bernardino counties, including every city on our San Bernardino County page. To have your flashing checked, call 866-631-3366.
