The flashing defects we find most often on GTA commercial roofs are base flashing that has pulled away from the parapet, counterflashing with open laps or a failed reglet, split corners at curbs and walls, shrunken pitch pockets, and drain flashing that has separated from the membrane. Each has a specific fix.
What is actually meant by flashing on a flat roof?
Flashing is every detail carrying the waterproofing from the horizontal field of the roof onto something vertical, or through something. That covers a lot of ground.
- Base flashing. The membrane itself turned up the parapet, curb or wall, over a cant strip, and terminated at the top with a bar or a nailer.
- Counterflashing or cap flashing. The metal above it that sheds water over the top of the base flashing, either surface-mounted, set into a reglet cut in the masonry, or formed as a full cap over a parapet.
- Curb flashing. The same idea around rooftop units, skylights, hatches and duct openings.
- Penetration flashing. Pipe boots, vent stacks, and pitch pockets around structural steel and conduit that cannot be sleeved.
- Drain flashing. The membrane and lead or target sheet clamped into the drain body.
- Expansion joint covers. Where the structure is designed to move and the roof has to move with it.
None of this is factory-made waterproofing. All of it is site-built by hand, against irregular existing conditions. That is why it fails first, and why we treat roof flashing and siding as its own discipline rather than an afterthought to membrane work.
Why does base flashing pull away from the parapet?
Three mechanisms, usually acting together.
Thermal movement. A dark membrane swings through a wide temperature range across a Toronto year. The parapet, often masonry or a framed wall with metal cladding, moves at a different rate. Base flashing is the joint between two materials that want to move differently, fixed at the top by a termination bar. Over enough cycles the fasteners elongate their holes and a gap opens behind the bar.
Missing or failed cant strip. Where the horizontal roof meets the vertical wall, the membrane should turn through a chamfered cant rather than a sharp ninety degree corner. A sharp corner concentrates all the stress on one line, and the membrane splits there. On older roofs the cant has often been omitted, crushed or rotted out.
Sealant relied on as waterproofing. The bead at the top of a termination bar is a secondary defence with a short service life. When it hardens and cracks, water runs in behind the flashing if the counterflashing above is not doing its job.
The fix. Strip the failed base flashing back to sound membrane, correct or install the cant, run new base flashing to the correct height with the plies lapped into the field, refasten the termination bar into sound substrate, and make sure the counterflashing above overlaps it. Where the wall is deteriorated, the masonry is dealt with first or the detail fails again.
What goes wrong with counterflashing and cap metal?
Counterflashing fails in ways that are visible from the roof deck if you know what you are looking at.
- Open laps. Cap metal comes in lengths, and the joints between lengths are the weak point. Laps that were sealed with caulk alone open as the metal expands and contracts. Properly detailed joints use a cover plate or a standing seam that accommodates movement rather than resisting it.
- Failed reglet. Where counterflashing is let into a saw cut in the masonry, the sealant in the reglet eventually shrinks and cracks. Water then runs behind the counterflashing and lands on top of the base flashing termination.
- Reverse slope on parapet caps. A cap that slopes back onto the roof rather than outward, or that has no drip edge, sheds water down the inside face of the parapet and behind the base flashing.
- Backed-out or missing fasteners, common on windward elevations, and where cap metal lifts in a wind event.
- Coating failure. Rust perforation on older galvanised metal, or dissimilar metals in contact.
The fix. Refabricate the affected metal in a suitable gauge and profile, with laps detailed for movement, a proper drip and an outward slope on caps. Reglets are raked out, cleaned and resealed, or the counterflashing is converted to a surface-mounted detail with a compression seal where the masonry will not hold a cut. Piecemeal caulking of an old cap is a holding action, not a repair.
Why do corners split before anything else?
Because a corner is where movement from two directions meets in one piece of material. Along a straight parapet the membrane is only being stretched in one axis. At an inside or outside corner it is being worked in two, and the material has nowhere to relieve the stress.
On modified bitumen roofs, corners are reinforced with a separate cut piece before the main base flashing goes on. On TPO and EPDM, corners use moulded corner pieces or field-fabricated patches welded in place. The failure we see most is a corner that was simply folded, with the fold cracking open after enough freeze-thaw cycling.
The same applies at curb corners under rooftop units, at skylight curbs and at mechanical screens. Corners are the highest-value thing to look at on your own roof, because a split corner is a direct path into the assembly and shows as a hairline crack rather than a hole. Found early it is a straightforward flat roof repair; found after two winters it has saturated a metre of insulation.
What about pitch pockets, pipe boots and drains?
Pitch pockets
A pitch pocket is a metal collar around an irregular penetration, structural support legs, guy wires, bundles of conduit, filled with sealant. Sealant shrinks. Every pitch pocket on an older roof is a maintenance item, and the classic finding is a dished surface with a visible gap around the penetration. The correct fix is to clean out the old filler and refill to a crowned profile with a suitable two-part material, or, better, to eliminate the pocket entirely by fitting a proper flashed sleeve or a manufactured penetration seal.
Pipe boots and vent stacks
Boots crack at the top clamp where the material has been exposed to sun and movement. On single-ply roofs a failed boot is cut out and a new prefabricated boot welded in. On modified bitumen roofs the stack is re-flashed with a metal sleeve and stripping plies.
Drains
Drain flashing fails when the clamping ring loosens, when the lead or target sheet under the membrane has corroded or torn, or when the drain sump has compressed so the drain sits proud of the surrounding insulation. The fix involves lifting the clamping ring, replacing the target flashing, resetting the ring evenly, and correcting the sump so water actually reaches the drain rather than ringing it. Anything that changes how water moves across the roof is properly considered alongside the wider drainage design, which we cover in our post on flat roof drainage systems.
How do you tell a flashing leak from a membrane leak?
Pattern and timing usually give it away before anyone opens anything.
- Water appearing near an exterior wall, especially running down the inside face of the wall rather than dripping from the ceiling, points at the parapet detail rather than the field.
- Leaks only in wind-driven rain from one direction point at a wall or counterflashing detail on that elevation.
- Leaks that start during a thaw rather than during rain point at ice damming at the parapet or around the drains, with meltwater backing up behind ice and over the base flashing.
- Leaks tied to heavy rain only point at drainage and ponding rather than flashing.
- Leaks under or beside a rooftop unit point at the curb, and frequently at flashing that a mechanical contractor cut to run new conduit and never reinstated.
That last one is worth its own note. On multi-tenant buildings, a large share of curb flashing damage is caused by other trades on the roof. If your building has regular mechanical service visits, roof access control and a post-work check are cheap insurance, and both belong in a flat roof maintenance routine.
Is it worth re-flashing a roof that is near the end of its life?
Sometimes, and the test is whether the field of the roof still has service life in it.
If the membrane across the field is sound and the failures are confined to perimeter and penetration details, a comprehensive re-flashing programme is good value. It addresses where the roof is actually failing and it can add usable years without a capital project. This is common on modified bitumen roofs where the cap sheet is still granulated and intact but the metal work and terminations have aged out.
If the field is crazing, laps are opening in unrelated areas and fasteners are backing out through the membrane, new flashing is being tied into a substrate that is finished. The money goes further as part of a planned flat roof replacement, where every detail is rebuilt to one specification.
A walk that records the field and the details separately, with photographs and moisture readings, tells you which case you are in. If you manage buildings across the GTA and want that written up for ownership, our property manager services produce documents that go straight into a report.