Snow load, ice dam and freeze-thaw damage repair for Manhattan, NY roofs: drift surveys, split seams, cracked drain leaders and flashing pulled by sliding ice.
A drift measured at thirty inches against a bulkhead, on a roof where the open field holds eight, shows where winter will do its damage: at the walls, the parapets and the steps in the roof, where wind-blown snow collects and meltwater later pools. Central Park received a record 27.3 inch snowfall on January 23, 2016, and it averages 29.8 inches in an ordinary year. The roof carries that load, and then it carries the thaw. Snow and ice damage on a Manhattan roof shows up at the details, weeks after the storm, as stains below and split seams above. Photograph the drifts before they melt and record the date of the thaw.
We survey and repair snow load and ice damage on commercial roofs.
How does snow load a flat roof, and where does it concentrate?
Fresh snow is light, and wet or refrozen snow can be many times heavier. Wind moves snow across a flat roof and deposits it where the surface steps up: against parapets, bulkheads, penthouses and equipment. On a setback roof in Midtown, snow from an upper roof can slide or blow onto a lower one and add to the drift.
The deck is designed for a stated snow load, and the drift can exceed it locally. Signs of overload are a sagging ceiling, a deck that has deflected, a door that sticks and cracks at walls. If any are present, the area is vacated and the structural engineer is called before anyone walks on the roof. We do not shovel a membrane with steel tools. Plastic tools and a protective layer of snow prevent cuts. Measure the drift depth and keep the date and reading in the log.
Melt rate matters as much as depth. A storm followed by a quick warm spell sends a large volume of water to the drains in a short time, and a storm followed by a hard freeze locks the water in place. The urban heat island keeps some rooftops warmer than the surrounding air, so snow may melt on one roof and stay on the next. We note orientation, shading and nearby buildings, since a shaded roof beside a tower melts slowly and refreezes overnight, which is when drains freeze shut.
What does ice do to drains, seams and flashings?
Snowmelt sends a steady flow toward the drains, and if a drain is frozen or buried in ice the water stacks up against the parapet and exploits any gap in the flashing. Ice also forms inside drain bowls and leaders, and as it expands it can crack a cast or plastic fitting, which shows itself weeks later as a stain on a ceiling several floors down.
Ice also grips the membrane. A pond that freezes lifts seams and flashings as it expands, and when it melts the lifted section may not settle back. Chipping ice with tools cuts the sheet, so the safe method is melting channels to the drain or waiting for the thaw. Our seam failure repair reseals seams opened by frost, and the survey checks leaders with a camera when a stain pattern suggests a cracked fitting. Photograph any ice formation before it is cleared.
Where a building has gutters or canopies they carry a risk of their own. Along the edge ice builds up, hangs from the eave and can drop onto the sidewalk and injure a passerby, while the edge metal bearing that weight may bend or pull loose. Sagging gutters, bowed metal and damaged hangers are checked, and ice is removed from the edge with warm water or with hand tools used from a safe position, and the metal is never pried. Pedestrians are protected by a sidewalk shed or barricade until the edge is clear.
Why do the coldest days matter to the membrane?
Central Park has recorded minus 15 F, on February 9, 1934, and ordinary winters bring hard freezes. Cold stiffens a membrane. Thermoplastic and rubber sheets lose their flexibility, and a sheet already stretched at a wall or curb can split. Sealants harden and crack, and contraction tugs at every attachment.
After a hard freeze the evidence appears as fresh splits at corners and walls, open laps and fasteners backed up out of the deck. Prewar five and six story buildings across Washington Heights, Inwood, the East Village and the Lower East Side carry roofs with many walls and bulkheads, and each is a point of stress. We inspect those details first, because they take the strain. Our flashing failure repair work rebuilds walls where the cold has pulled the flashing. Check the roof after the first deep freeze each year.
What does a snow and ice repair involve?
Repairs follow the survey. Blocked drains are cleared, cracked fittings are replaced and leaders are tested. Split seams are re-welded or re-adhered. Flashing pulled loose by sliding ice is reseated, resealed and, if needed, rebuilt. Lifted membrane is refastened, and wet insulation found by an infrared scan is removed and replaced.
Many materials need dry, moderate conditions, so temporary measures protect the building until weather allows permanent work. The approach in storm damage roof repair is to dry in first and repair second. Where damage is widespread, the survey compares the repair to a commercial roof replacement and prices both. Record every temporary measure with a date so it is not forgotten.
The insulation absorbs damage quietly. Meltwater entering through a cracked seam soaks the board, and the next freeze expands that water and breaks the board apart. From above, wet insulation on a thawed roof looks like dry insulation, which is why the infrared scan after the thaw is the way to find it. Cores confirm the reading, and the wet section is cut away and replaced; repairing only the visible split would leave saturated board beneath to fail the following winter.
How can an owner prepare a roof for the next winter?
Preparation is mostly drains and details. In autumn, clear leaves from strainers and scuppers, check flashing sealant, secure loose edge metal and note the locations where drifts have formed. Know the design snow load. Heat tracing or insulation on exposed drain leaders helps where it was installed, and it is checked before the cold.
One preventive roof maintenance visit ahead of the first freeze and another after the thaw record what changed. On Roosevelt Island, which is about 2 miles long and no more than 800 feet wide, a roof has little shelter from wind that drives snow into drifts, so the autumn visit matters. Add the next storm's drift reading to the log.
Commercial Roofing Questions
Where is the line between a heavy snow and an overload?
The limit is the building's design snow load, which the structural drawings or an engineer can state. Wet snow and drifts can exceed it in places, so stress signs like a sagging ceiling count for more than depth alone.
Is it safe to shovel the roof?
Steel tools are out, and so is going up without fall protection. A protective layer of snow stays down, plastic tools do the work, and when the load is a worry the engineer and we should be called first.
Why would a leak appear weeks after a storm?
Leaks can start long after the snow is gone from a cracked drain fitting, a flashing dragged loose by ice or a seam frost has opened, and the stain appears when the next rain finds the damaged spot.
Can repairs be made in freezing weather?
Some temporary repairs can. Permanent work using adhesives and welds needs suitable temperatures, so the roof is dried in and the final repair waits for the first good window.
Which records matter after a snowstorm?
Drift photographs, the thaw date, depth readings and the state of the drains, all of which back the repair scope and any claim.
