A Brooklyn flat roof drains through one or two interior drains, not gutters. One mat of October leaves at the strainer keeps rain on the roof, and one inch of standing water weighs 5.2 pounds per square foot. MLM Construction Group clears and flood tests drains before the freeze. (516) 713-9199.
On a pitched roof the water runs off the edge into a gutter you can see from the sidewalk. On a Brooklyn flat roof there is no edge to run off. The roof is built with a slight pitch toward one or two interior drains, and those drains are pipes that run straight down through the inside of the building to the sewer line. On older row houses and brownstones you may have scuppers instead, which are openings cut through the base of the parapet wall that let water out over the side.
Either way, the entire roof depends on one or two openings the size of a dinner plate. Block them and the water has nowhere to go. It does not overflow somewhere harmless. It sits.
The sequence is the same every year and it takes about six weeks.
Leaves come off the London planes and the maples along the street and land on the roof. They do not sit where they land. The next rain carries them toward the low point of the roof, which is exactly where the drain is. The strainer dome over the drain is designed to stop debris from entering the pipe, and it does its job: the leaves stop there and build into a mat. Add the grit that washes off an aging roof surface and the mat packs tight.
Now rain arrives and cannot leave. A roof that was draining in September holds two or three inches of water in a ring around the drain by the first week of November. On a roof with any sag in the deck, and most hundred year old Brooklyn roofs have some, the water collects in that low spot instead and stays there.
Then the temperature drops below freezing overnight. That is when a nuisance turns into a repair.
Water weighs about 62.4 pounds per cubic foot. That works out to roughly 5.2 pounds per square foot for every inch of depth.
A pond 20 feet by 20 feet and two inches deep is 400 square feet carrying about 10.4 pounds per square foot. That is 4,160 pounds, concentrated on one part of the roof, for weeks at a time. The insulation under the membrane compresses under that load and does not spring back. The compressed area becomes a permanent bowl, the bowl holds more water next time, and the problem deepens on its own every season.
That is also why a roof that ponds tends to keep ponding in the same place even after the drain is cleared. The deck and the insulation have taken the shape of the puddle.
Water expands about 9 percent when it turns to ice. New York City runs through more than 30 freeze and thaw cycles in a normal winter, so this is not one event, it is thirty.
Standing water finds every seam on the roof. On a flat roof the weak points are the laps where one sheet of membrane overlaps the next, the base flashing where the field of the roof turns up the parapet wall, and the clamping ring around the drain itself. Water that sits on a lap all week works into the edge of it. When it freezes it opens that edge a fraction of a millimeter. When it thaws, water moves further in. Repeat it thirty times and the lap is no longer a lap.
The drain bowl is the worst spot. Ice forms in the bowl first because the metal is colder than the roof surface. Once the bowl is plugged with ice, even a warm afternoon that melts the pond cannot drain it, and the water simply refreezes that night with more volume behind it.
A pitched roof leak usually shows up near the wall, under a valley, or around the chimney. A ponding flat roof leak shows up in the middle of the top floor ceiling, away from any wall, because the water entered at the low point in the middle of the roof.
Two things make it confusing. First, the stain appears in January, months after the leaves landed, so nobody connects it to the fall. Second, it often appears during a thaw rather than during a storm. People tell us it leaks when it is not raining, which sounds impossible and is actually the clearest symptom of ice on the roof.
The other version we get called for is a stain on a lower floor with a dry ceiling above it. That is the interior leader. The drain pipe runs down through the building, and if it is blocked at a bend, water backs up and finds the joint. We have traced a second floor ceiling stain to a drain on the roof four floors up. Related: how we find the actual entry point of a roof leak.
Do this on a dry day two or three days after rain. The timing matters more than anything else you do.
1. Is there still water up there? A flat roof should be dry within 48 hours of rain. Water after 48 hours means it is not draining, full stop. Note where the water is and how deep it is at the deepest point.
2. Lift the strainer. The dome over the drain either twists off or lifts off. Pull the leaves and grit out by hand. People stop here and assume they fixed it, which is only true about half the time.
3. Run a hose into the open drain. This is the step that tells you the truth. Water that swirls and goes is fine. Water that rises in the bowl, or gurgles and burps air back at you, means the blockage is down in the leader where you cannot reach it.
4. Look at the clamping ring. That is the metal ring that holds the membrane down into the drain. If the bolts are loose, if the ring is sitting proud of the roof surface, or if you can lift the membrane edge with a finger, the drain is a leak point regardless of whether it flows.
5. Check the scuppers and the parapet base. If your building drains over the side, put your hand through the scupper opening. They block with leaves and with bird nests, and they are hard to see from the roof surface because they sit at the bottom of the wall. While you are there, look at the base flashing where the roof turns up the parapet, because that is the second most common entry point after the drain.
Clearing the strainer is homeowner work and we would rather you did it yourself every October. The part worth calling for is the part below the roof surface.
We snake the leader from the roof to clear the bend that a hose only reveals. We flood test afterward, which means putting real volume down the drain and timing it, because a drain that passes a trickle can still fail in a downpour. We reset or replace the clamping ring and reflash the drain if the membrane around it has pulled, since a clear drain with a bad seal is still a leak. We check the pitch of the field around the drain and, where the insulation has compressed into a bowl, we build the slope back up with tapered insulation instead of leaving a dish that refills every storm. And we photograph all of it, so you can see the inside of your own drain rather than take our word for it.
On buildings with parapets we look at the parapet at the same time, because the same water that ponds at the drain sits against the base of that wall all winter. If your building is subject to Local Law 126 parapet inspection, the condition of the scuppers and the parapet base is part of what gets reported.
A blocked drain in October is a maintenance visit. The same blocked drain in March is a membrane repair, sometimes a section of insulation, and a ceiling inside.
There is also a practical reason to move now. Flat roof membrane work has a temperature floor. Adhesives and self adhered sheets stop behaving reliably in the cold, torch and hot work on an occupied building in January is slower and more disruptive, and snow on the roof means we cannot even see the problem. Everything is easier while the roof is dry and the deck is still warm. Related: what cold weather actually does to roofing work and when snow on a flat roof is a real load problem.
Your flat roof drains through one or two openings. Leaves are sitting on them right now. Go up on a dry day, check for water more than 48 hours after rain, lift the strainer, and run a hose into it. If the water backs up, the problem is in the pipe and not on the roof.
Because in October the water drains and in January it does not. Leaves collect at the drain strainer in the fall and form a mat. Rain then sits on the roof instead of leaving it. When the temperature drops below freezing that standing water turns to ice, the ice works into the seams and laps of the membrane, and the thaw pushes water sideways under the sheet. The leak shows up inside weeks after the leaves landed.
Water weighs about 62.4 pounds per cubic foot, so one inch of standing water is roughly 5.2 pounds per square foot. A pond 20 feet by 20 feet and two inches deep is about 4,160 pounds sitting on one spot of the roof. That is why ponding flattens insulation and creates a bowl that holds even more water the next time it rains.
Most do not. A Brooklyn row house or small apartment building usually drains through one or two interior roof drains piped down through the building, or through scuppers cut in the parapet wall. There is no gutter to look into from the ground, which is exactly why a blocked drain goes unnoticed until there is a stain on the top floor ceiling.
Go up after a rain. If there is still water on the roof more than 48 hours later, the drain is not doing its job. Lift the strainer dome and pull out the leaves and grit by hand. Then run a hose into the drain and watch it. Water that swirls and disappears is fine. Water that backs up or gurgles means the leader is blocked further down and needs to be snaked.
Yes, and often before the roof itself fails. An interior leader runs down through the building, so a blockage can push water out at a joint inside a wall or a ceiling on a lower floor. We have traced second floor ceiling stains to a roof drain four floors up. That is also why a ponding roof is worth clearing before the freeze rather than after.
It can be. Standing water adds load the roof was not designed to carry long term, and on buildings subject to Local Law 126 parapet inspections the condition of the parapet and the scuppers gets looked at directly. A drain that cannot clear is a maintenance item that turns into a repair item fast.
We go up, clear the strainer, snake the leader if it needs it, and flood test the drain so you know it moves real water and not just a trickle. You get photographs of the drain, the laps and the parapet base, and a straight answer on whether this is a 20 minute clean out or a repair. Brooklyn, Queens, Manhattan, the Bronx, Staten Island and Nassau County.
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