Snow on a Flat Roof in Brooklyn: When to Clear It and Why It Leaks

Most Brooklyn flat roofs do not need shoveling after a normal storm. NYC designs roofs around a ground snow load of 25 pounds per square foot, and a foot of fresh snow weighs about 5. Wet snow, deep drifts and rain on top are different. MLM Construction Group checks roofs after storms at (516) 713-9199.

What Snow on a Roof Actually Weighs

People picture a foot of snow as a heavy load. It depends almost entirely on how wet it is. The numbers below are rough, but they are the ones that matter.

• Fresh, dry snow: about 3 to 5 pounds per square foot for every foot of depth.
• Wet, packed snow: 15 to 20 pounds per square foot per foot of depth, sometimes more.
• Ice: close to 57 pounds per cubic foot. An inch of ice on a roof weighs about as much as a foot of fresh snow.
• Rain on snow: the snow soaks it up like a sponge. A deep snowpack that gets rained on can double its weight overnight.

So a foot of light powder on a row house roof is not an emergency. Two feet of wet snow drifted against a parapet, with rain in the forecast, can be.

When Snow Becomes a Problem

On Brooklyn roofs the trouble is rarely the average depth. It is where the snow ends up and what the roof underneath has been through.

• Drifts two feet or more against a parapet, a stair bulkhead, or the wall of a taller building next door. Wind stacks snow there, and that is where the weight concentrates.
• Rain or a warm spell forecast on top of deep snow.
• Frozen drains and scuppers, so meltwater has nowhere to go and freezes back into ice.
• An older roof with a leak history. Joists that have been wet for years under a patched roof do not carry what they were built to carry.
• Warning signs inside: a sagging top floor ceiling, new cracks in the plaster, doors that suddenly stick, creaking you have not heard before. That is not a roofing call. Get people off that floor and call a structural engineer.

Why Roofs Leak When the Snow Melts

The most common winter call we get is not a collapse. It is a ceiling that starts dripping three or four days after the storm, on a roof that never leaks in the rain. There are three reasons.

1. The water stays on the roof for days

A rainstorm is over in hours and the roof drains. Snow melts slowly, so the roof sits wet for a week. Any seam, flashing lap or patch that is only slightly open gets far more time to let water through.

2. Snow piles against the flashing

The drift against the parapet or the bulkhead keeps the base flashing and counterflashing soaked the whole time. That is exactly where most flashing leaks start, and snow finds them before rain does.

3. Frozen drains turn the roof into a pool

When the drain or scupper freezes, meltwater ponds across the roof. At night it refreezes, expands in every small gap, and opens it a little wider. By the end of a cold week a hairline gap is a leak.

If You Clear the Roof, Clear It Right

• Leave 2 to 3 inches of snow on the roof. Scraping down to the membrane is how shovels cut roofs.
• Use a plastic shovel. No metal edges, no ice picks, no chipping at ice.
• Start with the drains and scuppers so meltwater can leave, then take down the drifts against parapets and bulkheads.
• Do not use rock salt or sidewalk ice melt on the roof. It corrodes metal flashing and gutters.
• Do not throw snow onto the sidewalk, the neighbor's roof or a lower extension roof. You are just moving the weight.
• Stay well back from the edge, which is hard to see under snow, and never go up alone. If it is icy, do not go up at all. Hire an insured crew.

What to Check After a Big Storm

• Top floor ceilings, especially under the parapet walls and around the skylight or roof hatch.
• The inside face of the parapet, where a wet stain means water is getting in at the top of the wall.
• Whether the drains and leaders are actually running when the snow melts, or backing up.
• Gutters and leaders on the back of the house, which freeze solid and split.

Get the Roof Ready Before the First Snow

The cheapest winter repair is the one done in October or November. Clear and test the drains, fix the seams and flashing that are already open, and look at the parapet caps before they spend a winter under snow. If a drain freezes every single year, a heat cable in the drain line stops it for good. On shingle roofs the matching problem is ice dams, and the prep is different. Our fall roof maintenance checklist covers both.

Frequently Asked Questions

Should I shovel snow off my flat roof in Brooklyn?

After a normal storm, usually not. A foot of fresh, dry snow weighs roughly 5 pounds per square foot, well under what a sound roof is built for. Clear it when snow is wet and deep, when drifts pile two feet or more against a parapet or bulkhead, when rain is forecast on top of it, or when you see sagging or new cracks in a top floor ceiling.

How much snow can a Brooklyn row house roof hold?

The NYC building code uses a ground snow load of 25 pounds per square foot to design roofs. A healthy roof handles an ordinary storm easily. The risk is an older roof with joists weakened by years of leaks, or heavy wet snow and drifts that concentrate weight in one spot. If the ceiling sags or doors on the top floor start to stick, get people off that floor and call an engineer.

Why does my roof leak after snow but not in the rain?

Rain runs off in hours. Snow sits for days. Meltwater keeps the flashing at the parapet wet the whole time, drains and scuppers freeze so water ponds, and every night it refreezes and pries open seams a little more. A small weakness that never shows in a rainstorm leaks during a week of melting.

Can I use salt or ice melt on my flat roof?

Avoid it. Most ice melt products are made for sidewalks, not roofing. The salt corrodes metal flashing, drains and gutters, and some products are hard on membranes. Clear drains by hand and let the sun and a clean drain do the rest. If a drain freezes every winter, a heat cable in the drain is the lasting fix.