Improve Your Flat Roof Water Drainage
Picture a July cloudburst in Brooklyn-twenty minutes of downpour that drops 1.2 inches of rain. On a typical 1,200-square-foot flat roof, that’s roughly 900 gallons of water landing on your building in less time than it takes to walk to the bodega. If your flat roof water drainage systems aren’t sized correctly, maintained properly, or sloped the right way, that water has nowhere to go except into puddles that sit for days-or worse, under your membrane and into your ceiling.
I’m Dana Kwan, and I’ve spent over a decade fixing exactly this problem on brownstones, rowhouses, and small apartment buildings across Brooklyn. Most drainage issues I see aren’t catastrophic failures-they’re slow, preventable problems that got ignored until ponding turned into leaks. The good news? Once you understand how water is actually behaving on your roof, the fixes become obvious.
What’s Really Going On With Water on Your Flat Roof?
If water is hanging around on your flat roof longer than a day or two after it rains, you don’t just have a cosmetic issue-you have a drainage problem that will shorten the life of your roof and raise your leak risk. The good news is that flat roof water drainage systems can almost always be improved once you know what you’re dealing with.
On this page, you can quickly figure out:
- Whether your flat roof drainage is actually a problem or still within normal limits
- Simple checks you can do yourself before you call anyone
- The main types of flat roof water drainage systems used in Brooklyn
- When you need a roofer or engineer to redesign drains, scuppers, or slopes
Step 1: Decide If Your Flat Roof Drainage Needs Attention Now
Before you panic or spend money, take a systematic look at what’s actually happening after the next rainfall. Flat roofs are allowed a little temporary ponding-the question is how much, where, and for how long.
Ask yourself these 5 questions after the next rainfall:
- Is water still standing in puddles more than 48 hours after the rain stops?
- Are the same low spots or puddles getting bigger over time?
- Do you see ceiling stains, bubbling paint, or musty smells inside?
- Are roof drains, scuppers, or gutters overflowing during storms?
- Has your roof already needed more than one leak repair in the last two years?
How to read your answers:
- Mostly “no”: Your drainage is probably acceptable, but basic cleaning and checks are still smart.
- Some “yes”: You likely need targeted improvements-clearing, small slope corrections, or drain adjustments.
- Many “yes”: You’re in the danger zone. A professional flat roof drainage assessment in Brooklyn is strongly recommended.
Step 2: Do the Safe, Simple Checks You Can Handle Yourself
Safety first on Brooklyn flat roofs: Only go onto your roof if it’s safe, accessible, and you’re comfortable doing so. Many Brooklyn roofs have soft spots, hidden edges, and limited guardrails. When in doubt, stay off the roof and use photos or a professional inspection instead.
Quick checks that often improve drainage right away:
- Clear leaves, trash, and gravel buildup from around drains and scuppers.
- Make sure downspouts are not clogged where they meet the ground.
- Look for moss, plant growth, or debris dams creating small “walls” that trap water.
- Check that temporary objects (pallets, boards, equipment) aren’t blocking water paths.
- Note with your phone camera where water actually sits after a storm.
Pro tip: Taking photos of ponding areas and outlets after heavy rain gives you and any roofer a clear map of how your drainage is performing in real conditions. I’ve diagnosed dozens of drainage problems from photos before I ever climbed a ladder.
How Flat Roof Water Drainage Systems Actually Work
To fix drainage, you need to think about three interconnected parts-slope, collection, and discharge. Miss one, and water finds ways to stay on your roof.
Three parts you always have to think about together:
- Slope: The subtle tilt of the roof surface that tells water where to go.
- Collection: The drains, scuppers, or gutters that receive that water.
- Discharge: Where the water ends up-downspouts, leaders, storm drains, or safe splash areas.
In Brooklyn, I see three main flat roof water drainage systems, each with its own set of strengths and typical failure modes:
| System Type | How It Works | Best For | Common Problems |
|---|---|---|---|
| Internal roof drains | Drains located in the middle of the roof or low areas that connect into internal piping | Larger buildings or roofs surrounded by parapet walls | Clogs you can’t see, frozen lines in cold snaps, leaks at drain connections |
| Scuppers through parapet walls | Openings through parapet walls that let water flow horizontally off the roof into downspouts or onto lower roofs | Rowhouses and small apartment buildings | Leaves and trash block the openings; ice forms in winter; water can stain or damage walls below |
| Perimeter gutters and downspouts | Traditional gutters hung at the roof edge catching water as it flows off the surface | Mixed-slope roofs and smaller flat roofs with exposed edges | Overflow onto facades or neighboring properties when undersized or clogged |
Brooklyn Conditions That Make Flat Roof Drainage Tricky
I work almost exclusively in Brooklyn, and the buildings here have specific quirks that turn drainage into a custom puzzle every time.
Why drainage fixes here are not one-size-fits-all:
- Older brick parapet walls that have settled and changed water flow.
- Multiple old roof layers adding weight and hiding original slopes.
- Tree-lined streets dropping leaves into scuppers and gutters every fall.
- Tight lot lines that limit where you can safely dump roof water.
- Freeze-thaw cycles that open cracks around outlets every winter.
A typical Brooklyn scenario: A small apartment building in Crown Heights had internal drains that worked fine when the building was new in 1989. Over time, extra roof layers and ponding changed the water path. One clogged drain during a summer downpour sent water under the membrane and into three top-floor units. The fix wasn’t just clearing the drain-it was rethinking slope and adding an emergency overflow path. That building now has two scuppers as backup so water always has somewhere to go.
Low-Impact Ways to Improve Your Flat Roof Drainage
Not every drainage problem requires tearing up the roof. If your membrane is still in good shape and your structure is sound, these targeted adjustments often make a big difference:
Re-level or lower existing drains: Sometimes the drain flange sits higher than the surrounding roof surface because of extra layers or bad patching. Carefully lowering or resetting drains can restore proper flow. I did this on a Park Slope rowhouse last spring-dropped the drain body about half an inch and eliminated a 6-foot-wide pond that had been there for two years.
Add or resize scuppers: Where parapet walls trap water, additional or larger scuppers can give water more ways to exit, especially during intense Brooklyn storms. The building code allows scuppers as small as 4 inches square, but I usually recommend at least 6 inches to handle debris without clogging.
Fine-tune gutters and downspouts: Larger downspout diameters, better outlets, or additional drops can keep water moving instead of spilling over the gutter edge. Switching from 2-inch to 3-inch downspouts on a Fort Greene commercial building cut overflow events from five storms per year to zero.
Install crickets and saddles: Small built-up slopes near walls, behind rooftop units, or around chimneys push water toward drains instead of letting it sit in pockets. These are usually made from tapered insulation covered with membrane-simple but incredibly effective.
When You Need to Redesign Slope and Drainage, Not Just Clear Drains
Sometimes cleaning drains and adding scuppers isn’t enough. When the roof itself has lost its shape or was never sloped correctly to begin with, you’re looking at a more serious intervention.
Signs your flat roof needs new slope or major drainage upgrades:
| Sign | What It Suggests |
|---|---|
| Water ponds in wide, shallow areas for days | Insulation or deck has sagged or original slope was never correct |
| Multiple leak locations over time | Water is traveling under the membrane, following poor slope toward weak points |
| Visible dips or soft spots underfoot | Damaged or saturated substrate that likely needs removal and rebuilding |
| Drains work, but water never really reaches them | Roof needs tapered insulation or reframing to redirect water paths |
What a re-slope project can look like: On many Brooklyn flat roofs, improving drainage properly means removing wet or failing areas, rebuilding slope with tapered insulation or new framing, and sometimes adding or relocating drains and overflows. It’s more involved than a patch job but can add many years of life to the roof. I worked on a Clinton Hill mixed-use building last year where we installed a full tapered insulation system-essentially creating new slopes from ¼ inch per foot up to the high points-and tied everything into two new internal drains and three scuppers. Ponding time dropped from 48 hours to under 2 hours, even after our heaviest summer storms.
Upgrading or Changing Flat Roof Water Drainage Systems
Sometimes the existing drainage type just isn’t right for the building anymore. Here are the upgrade paths I see most often in Brooklyn:
Old internal drains constantly clog and leak: Replace or reline drains, improve strainers, and add overflow scuppers so water has a backup path if a drain blocks.
Small roof behind high parapet walls: Add new scuppers and exterior downspouts sized for intense storms, with proper splash protection or connection to storm systems where allowed by DOB.
Mixed flat and low-slope roof with chronic ponding: Insert tapered insulation system to restore positive slope to existing drains and replace or upsize key outlets.
Garage or small addition with edge-only drainage: Install a perimeter gutter system, possibly with a slight increase in slope using new sheathing or insulation.
Details That Make Drainage Systems Last on Flat Roofs
The difference between a drainage upgrade that works for 20 years and one that fails in 3 often comes down to small installation details:
- Choosing drain bodies and scuppers that match your roof membrane type (EPDM, TPO, modified bitumen, etc.).
- Using proper strainers and baskets that are easy to clean, not improvised screens that trap debris.
- Making sure insulation and coverboards taper cleanly into drains without creating a raised “donut” around them.
- Installing expansion joints and flexible flashings where roofs meet parapet walls or different structures.
- Keeping drainage hardware visible and accessible, not buried under coatings or new layers.
What You Can Maintain Yourself vs. What a Brooklyn Roofer Should Handle
Property owners can and should stay involved with routine drainage care, but some tasks require professional knowledge and access.
Homeowner or building staff tasks:
- Regular visual checks after storms from safe vantage points.
- Scheduling seasonal cleaning of drains, gutters, and scuppers.
- Documenting ponding areas with photos over time.
- Reporting changes like new leaks, sagging, or staining quickly.
Tasks for a flat roof drainage professional:
- Removing and rebuilding low or saturated sections of the roof.
- Re-sloping surfaces with tapered insulation or framing.
- Adding, relocating, or resizing drains and scuppers.
- Tying new drainage work into existing plumbing or storm systems.
- Ensuring all changes comply with NYC building code and DOB expectations.
Create a Simple Drainage Maintenance Plan for Your Flat Roof
The best drainage systems in Brooklyn aren’t necessarily the most expensive-they’re the ones that get checked and cleaned on a predictable schedule.
A realistic schedule for Brooklyn property owners:
Early spring: Check and clear winter debris and ice damage around drains and scuppers. Inspect for new ponding areas after freeze-thaw cycles.
Late summer / early fall: Clean drains and gutters before leaf drop season. Confirm downspouts discharge safely away from foundations.
After major storms: Verify drains and scuppers are flowing, not overwhelmed. Photograph any new or unusual ponding that appears.
Flat Roof Drainage Questions We Hear Most in Brooklyn
Is some standing water normal on a flat roof?
A little shallow water that drains or evaporates within 24-48 hours after a storm can be acceptable, depending on the roofing system. Deep or long-lasting ponds, especially near seams or drains, are a red flag.
Will a coating fix my drainage problem?
Coatings can protect a roof surface, but they don’t fix poor slope or undersized drainage. In some cases, extra coating can even make low spots worse by adding material where water already collects.
Can I just add more drains to solve ponding?
Sometimes additional drains help, but they must be connected correctly to piping or leaders and placed at the true low points. Without adjusting slope or structure, extra drains alone may not solve the underlying issue.
Do drainage upgrades always mean replacing the whole roof?
Not always. Many drainage improvements can be targeted to problem areas. That said, if the membrane is very old or the roof is saturated, combining drainage work with a reroof can be more cost-effective long term.
How often should I have a pro check my flat roof drainage?
Once a year is a good baseline for most Brooklyn buildings, with extra checks after major storms or if you notice new ponding, staining, or clogs.
Need Help Improving Your Flat Roof Water Drainage in Brooklyn, NY?
If you’re dealing with ponding, overflowing drains, or interior leaks linked to poor drainage, FlatTop Brooklyn can walk you through your options.
Get a flat roof drainage assessment tailored to your building:
- On-roof inspection focused on slope, outlets, and ponding areas
- Photo documentation and a clear explanation in plain language
- Options ranging from simple cleaning and tweaks to full drainage redesign
We regularly work with Brooklyn owners, supers, and property managers to improve flat roof water drainage systems without unnecessary disruption to tenants or businesses. Whether you need a single scupper added or a complete tapered insulation system, we’ll show you what’s realistic for your roof and your budget.