A cold-storage building with refrigeration lines coming up through the roof has more holes than a building manager usually counts. Each pipe, curb and vent is a place where the membrane stops and a flashing takes over, and flashing is where roofs fail first. A manager should check the penetrations before the open field, because a roof can be sound everywhere but at one pipe boot. We rebuild skylight curbs, stair hatches, pipe boots, conduit supports and vent flashings so that every hole in the roof is sealed on purpose.
What Counts as a Penetration
Anything that passes through the roof surface or rises from it is a penetration. That includes plumbing vents, exhaust fans, refrigeration lines, electrical conduits, gas lines, skylights, roof hatches, stair bulkhead doors, antenna masts and equipment curbs. A pre-war apartment building has a stair hatch, a bulkhead and a few vents. A warehouse has dozens of vents and several curbs. A cold-storage building may have a forest of pipe supports.
The work is the same in each case. The membrane is turned up the penetration, a flashing is formed around it, a counter-flashing or clamp seals the top, and sealant closes the gaps. Any of those layers can fail, and the failure is usually a few inches wide, out of view from the ground.
How Flashings Fail
Four failures cover most leaks at penetrations, and each is a flashing failure of a different kind. A pipe boot cracks in the sun and opens a ring around the pipe. Sealant shrinks and pulls back from the curb. The flashing seam opens along an edge, usually the lowest. A fastener backs out and lifts the sheet. Heat is part of the cause: the Central Park station averages 95.7 F in July and has dropped to -15 F, so rubber, plastic and sealants live through a wide thermal range.
Pitch pans, the open metal boxes packed with filler around old pipes, fail by shrinking. The filler pulls away from the pipe and leaves a gap. Older roofs can still have them. We replace the pan with a prefabricated boot or a flashed sleeve whenever the pipe allows it.
Skylights and Roof Hatches
A skylight leaks three ways: at the curb flashing, at the glazing seal and through condensation inside the frame. Condensation looks like a leak and is not, so we check before we repair. Curb leaks are common because the curb is a short wall on a roof, and the base flashing at its foot takes standing water. A skylight set at the low side of a slope is a worse risk than one set high.
Roof hatches and stair bulkheads have their own habits. The curb on a hatch is low, its cover seals with a gasket, and the gasket hardens. A bulkhead door sill can sit below the surrounding roof surface and take water at the threshold. We rebuild the flashing and raise the sill detail where we can. On any roof we find during a roof inspection, the hatch gets a close look because the superintendent walks through it every day.
Equipment Curbs and Supports
Rooftop units sit on curbs, and the curb is a flashing problem. A cracked sealant line at the corner, a gap where the unit meets the curb or an uninsulated curb that sweats can all produce a stain below. Where refrigeration lines run across the roof on supports, each support foot punches or sits on the membrane, and a loose one chafes it. Larger units need equipment curbs and supports built to carry their weight as well as shed water.
At Hunts Point the Cooperative Market holds 700,000 square feet of refrigerated space, and any roof over cold space that carries piping or equipment has to keep those penetrations watertight. Condensation and temperature swing at the curb accelerate wear. Flashing there is detailed with compatible materials and a clean, dry substrate.
Tracing the Leak to the Penetration
When a stain sits near a pipe or a curb, we start there. We look at the flashing, press on the sealant and check each fastener. A hose test, running water on one side of the penetration at a time, finds the entry. An infrared moisture survey maps the wet insulation around the curb, which shows how far the water has traveled.
Most penetration leaks are cheap to fix once found. The expensive ones are those that went unnoticed long enough to soak the insulation below. A leak repair at a pipe boot costs little, while wet insulation around it costs more.
Repair Methods
For a pipe, we use a prefabricated boot or a sleeve with a clamp ring and a bead of compatible sealant. For a curb, we cut back old flashing, clean the substrate, form a new base flashing and tie it into the membrane with a welded or adhered lap. For a skylight, we reflash the curb and replace gaskets or the unit when the glazing has failed. For a bulkhead, we rebuild the wall base and counter-flash above it.
Where liquid-applied flashing resin suits an awkward shape, such as a cluster of pipes, we use a reinforced resin that cures into a continuous seal. The method follows the shape and the membrane type, not the other way around. The Department of Buildings issues the 2022 NYC Construction Codes. Code questions go there.
Cost and Access
Price follows the number and type of penetrations, the membrane, the access and the amount of wet material. A single pipe boot is a short visit. A roof with fifty penetrations is a project. We count and list each one with a photograph, so the manager sees what is being fixed and can approve by group. Montefiore has Bronx campuses in Morris Park, Norwood and Wakefield, and on any hospital roof, access and work hours are arranged around patient areas.
Commercial Roofing Questions
How can I tell whether a skylight leak is condensation?
Condensation drips from the inside of the frame on cold mornings and stops when the room warms. A true leak follows rain. We check by wetting the exterior in sections while someone watches inside, and we look at the frame for water tracks.
Can one sealant be used on every penetration?
No. Sealants have to match the materials: some bond to metal and not to rubber, and some break down on contact with certain membranes. We choose by pairing, and we do not rely on sealant alone where a mechanical flashing is the better detail.
Why do the same pipes keep leaking after a patch?
A patch on a moving joint tears again. If the pipe expands and contracts, the flashing has to flex with it. A boot with a clamp or a sleeve with room for movement solves the problem a patch only hides.
Do old pitch pans need to come out?
Usually yes. They fail by shrinking and are hard to maintain. A prefabricated boot or a flashed sleeve replaces the pan and removes the recurring leak. Where the pipe is too close to a wall, we use a custom-formed flashing.
What access do you need to the hatch?
We need the hatch clear and open on the day of the walk, and a contact who can open the stairwell. For skylights, we also need access from below to look at the glazing and the curb ceiling.
Send the Building Type and the Last Tenant Complaint
Tell us the building type, the roof age and the last tenant complaint, and ask for a flat-roof walk before anyone writes a scope. A photo of the stain and the nearest pipe or curb saves us time.
Request a Roof Assessment