Create Drainage Learning to Put Fall on Flat Roof

If Water Isn’t Leaving Your Flat Roof, You Don’t Really Have a Flat Roof – You Have a Shallow Pool

Every flat roof in Brooklyn is supposed to have a slight fall-just enough slope to push water toward drains, scuppers, or gutters. When that fall is missing or lost over time, you get ponding water, accelerated membrane wear, and leaks that seem to come from nowhere. Learning how fall is created on flat roofs won’t turn you into a roofer, but it will help you insist on the right solution instead of another surface patch.

Here’s something most Brooklyn owners don’t realize: your “flat” roof should actually pitch at least 1/4 inch per foot toward an outlet. That means a twelve-foot-wide roof should be three inches lower at the drain edge than at the high side. Skip that tiny slope, and every summer downpour or heavy nor’easter leaves standing water that sits for days, slowly destroying your membrane through UV exposure and chemical breakdown-even if the roof never springs an obvious leak.

You’re in the right place if you’re seeing:

• Shallow ponds that sit for days after rain instead of disappearing
• Roof coatings that look wrinkled or blistered in low spots
• Stains on the ceiling below the same areas where water sits
• Water often heading toward walls or door thresholds, not toward drains

What “Putting a Fall on a Flat Roof” Actually Means

Fall in simple terms: Putting a fall on a flat roof means building in a subtle, consistent slope-usually at least 1/4 inch per foot-so water has a direction to flow in. It’s not about making the roof visibly pitched; it’s about shaping the surface (or the layers above it) so water doesn’t have anywhere to sit.

When I sketch a drainage plan for a client in Carroll Gardens or Park Slope, I’m mapping out “water paths”-basically showing where every drop of rain will roll once it lands. If your roof doesn’t have these paths built in, the water makes its own, and it usually chooses the worst possible spots: along parapet bases, around door sills, or right over your kitchen ceiling.

A good fall design on a flat roof will:

• Move water away from vulnerable areas like walls, door sills, and skylights
• Deliver water efficiently to drains, scuppers, or gutters
• Avoid creating new low spots or “birdbaths” at transitions
• Work with the structure you have, not fight it

Before You Add Fall: Find Out Why Your Roof Lost Its Slope

On a hundred-year-old rowhouse in Cobble Hill last summer, we pulled back four layers of rolled roofing to find the original deck-and discovered it had been installed dead-level. No wonder the owner had been fighting leaks for twenty years. But on a 1970s walk-up in Sunset Park, the framing had the right fall when built; decades of patch coatings and extra membrane layers just slowly filled in the low spots until the whole thing flattened out.

Questions we ask on a Brooklyn flat roof with ponding:

• Was there ever proper slope, or was the roof essentially level from day one?
• Have multiple roof layers or coatings flattened or reversed the original fall?
• Are any joists or beams sagging enough to create structural low spots?
• Did new equipment, curbs, or previous repairs unintentionally dam water?
• Are drains and scuppers set too high compared to the roof surface?

Common Brooklyn causes of lost fall: On older rowhouses, we often see decades of added roofing and patching that slowly erase the original slope. In other cases, water damage has softened the deck, letting it sag between joists, especially near drains or heavy rooftop units. I’ve measured joists that dropped two inches in the middle span-no amount of surface coating will fix that kind of structural sag.

Three Main Ways Pros Create Fall on a Flat Roof

You can build fall into different layers of your roof assembly, and the method you choose depends on what’s failing, how much access you have, and what your budget allows. Here’s the hierarchy from bottom to top:

1. Structural fall (framing / joists / beams)

Build or adjust the structure itself to slope-common in new work or major rebuilds. This involves setting joists on a sloped bearing line, tapering joists, or adding sleepers on top. It’s the most invasive method, but when you’re replacing rotten framing anyway, it’s the time to get the bones right.

When to use it: New extensions, full roof reconstruction, or when structural sag is the underlying issue.

2. Deck-level fall (plywood / board sheathing)

Create slope by shimming or stepping the sheathing layers before waterproofing. We’ll add tapered sleepers or furring strips on top of joists, or use stepped sheathing thicknesses in a controlled pattern. This works well when you have access to the deck and the framing is still sound.

When to use it: Smaller roofs or local corrections when structure is mostly sound.

3. Insulation/surface fall (tapered insulation / screeds)

Shape the insulation or leveling layer above the deck. This is the go-to method for modern reroofs because it’s faster, lighter, and doesn’t require opening up the building structure. You’re essentially building a custom-shaped insulation “blanket” that gets thicker as you move away from the drain.

When to use it: Most reroofs that keep existing framing; widely used in modern flat roofing across all five boroughs.

Creating Fall with Tapered Insulation: The Modern Go-To

Why tapered insulation is so popular on retrofits: Instead of trying to re-frame an entire roof, we can lay insulation boards that are thicker on one side than the other. When they’re arranged in a planned pattern, they quietly build in fall across the roof while also improving thermal performance. I’ve used this method on probably 80% of the rowhouse and small multifamily reroofs I’ve handled in the past five years.

The boards come pre-cut from manufacturers in progressive thicknesses-say, 1 inch on one edge ramping to 3 inches on the other over a four-foot span. We map out the entire roof on paper first, marking high points (usually at walls or equipment) and low points (drains, scuppers), then order a custom “tapered kit” that creates exactly the slopes we need.

Key elements in a tapered insulation plan:

• Starting height: coordinated with door thresholds, parapet heights, and existing edges so you don’t accidentally raise the roof surface above your flashing or reduce code-required parapet heights
• Direction of fall: where you want the water to head-drains, scuppers, edges
• Slope rate: commonly 1/8″-1/4″ per foot, adjusted for your roof size and NYC Building Code requirements
• Crickets and saddles: small slopes that push water around obstacles or out of corners where ponds love to hide

Brooklyn example: On a rear roof extension in Boerum Hill last fall, we had a 14-by-20-foot flat section boxed in by parapets on three sides. The owner had tried coating it twice; water still pooled against the back wall. We designed a tapered scheme pitching gently toward two new scuppers at the front corners, using boards that ramped from 2 inches at the high (back) wall to 3.5 inches at the low (front) edge. After a heavy September rainstorm-2.8 inches in an hour-the roof was dry within 90 minutes.

When You Have to Fix the Structure to Get Proper Fall

Surface fixes can’t correct a failing structure. If joists are badly sagging, rotten, or undersized, trying to build slope entirely with insulation or sheathing is like putting a shim under a broken table leg-you may hide the wobble for a while, but the weakness remains. And every freeze-thaw cycle in a Brooklyn winter puts more stress on those compromised members.

Signs a structural correction may be needed:

• Large areas of visible sag viewed from the street or neighboring windows
• Joists that are cracked, deeply notched, or heavily bored for services
• Deck replacement repeatedly revealing severe rot or insect damage
• Load increases planned (deck, planters, equipment) on an already borderline roof

Structural fall is engineer territory. Reframing joists, changing bearing heights, or altering load paths should always be designed or reviewed by a structural professional. As roofing specialists at FlatTop Brooklyn, we work with them so the new framing and new fall work together with the membrane system. On a brownstone in Fort Greene two years ago, we collaborated with the owner’s engineer to sister new joists alongside sagging originals, setting the new members 1/4 inch lower per foot to create fall toward new rear scuppers-then laid new plywood and TPO over that corrected frame.

Localized Falls: Crickets, Saddles, and Small-Scale Adjustments

Not all fall work is full-roof. Sometimes you just need to reshape one stubborn area.

Crickets and saddles-small slopes with big impact:

• Cricket: A raised, often triangular slope that splits water around a penetration or pushes it toward a drain. Think of it like a tiny pitched roof sitting on your flat roof, deflecting water left and right instead of letting it dam up behind a pipe or equipment curb.
• Saddle: A shaped build-up between two higher points, like between drains or between a drain and a scupper, that keeps water from getting “stuck” in the middle.

Where we add localized falls on flat roofs:

• Behind rooftop HVAC units where water tends to hide in dead corners
• Between dual drains to keep a shallow trough from forming mid-roof
• Along parapet bases where long, narrow ponds show up after storms
• Upstream of door thresholds to move water away before it reaches the sill

On a Williamsburg triplex last spring, the owner had persistent ponding behind a rooftop compressor. We built a lightweight cricket using tapered EPS foam and extended the membrane over it-cost about $340 in materials and half a day of labor, but it eliminated a problem that had caused two leak calls in the prior winter.

Start with a Drainage Plan, Then Shape the Fall to Match

Fall is meaningless if water has nowhere good to go. Before we talk slope angles, we map outlets.

A practical drainage plan answers:

• Where will primary drains, scuppers, or gutters be?
• How will water move from the farthest point on the roof to those outlets?
• What happens if a primary outlet clogs-where does overflow go?
• How do we avoid sending large volumes of water onto neighbors or vulnerable walls?

Brooklyn constraints we factor in: Tight property lines, shared walls, existing plumbing risers, and sometimes landmark restrictions can all limit where we can put drains or scuppers. On a landmarked building in Brooklyn Heights, we couldn’t add visible scuppers on the street-facing parapet, so we routed all drainage to internal drains and a concealed overflow system at the rear-then designed the tapered insulation to push water gently back toward those outlets, even though it meant a slightly steeper slope than we’d prefer on a shorter run.

Common Mistakes When Trying to “Fix” Ponding by Adding Fall

Things we regularly correct on Brooklyn flat roofs:

• Coating or mopping more material into a low spot, which often makes the depression deeper and heavier. I’ve seen roofers dump five gallons of mastic into a pond, thinking they’re “leveling” it-but all they did was add 60 pounds of dead weight that sags the deck further.
• Adding random shims or boards under the membrane without a continuous plan, creating new trip edges and stress points where the membrane folds or stretches.
• Ignoring structural sag and just building thicker insulation on top, raising loads without solving the underlying weakness.
• Creating fall toward a wall or door because it was “easier,” then relying on sealant to keep water out-sealant always fails; gravity never does.
• Installing new drains without adjusting surrounding slope, so water still never reaches them. We see this constantly: shiny new drain, but the roof still pitches away from it.

What You Can Decide Yourself vs What a Flat Roof Pro Must Design

You control the goals and trade-offs. We handle the engineering and execution.

You can (and should) decide:

• How important it is to fully eliminate all visible ponding vs reducing it significantly
• Whether you’re planning future uses (deck, planters, equipment) that affect fall design
• Which edges or walls are most sensitive (neighbors, doors, delicate facades)
• Budget comfort for full-slope redesign vs strategic localized fixes

We handle the technical “how”:

• Measuring existing slopes and mapping ponding after storms using a level, laser, or even drone footage
• Choosing between structural, deck-level, or insulation-based fall solutions based on your building’s condition and access
• Designing tapered layouts, crickets, and drain locations that work together as a system
• Coordinating fall design with chosen membranes (TPO, EPDM, modified bitumen) and NYC Building Code requirements for drainage and overflow

Our Flat Roof Fall and Drainage Redesign Service in Brooklyn

What we do when you call us about ponding and fall:

• Inspect and document current slope, ponding, and outlet conditions on your roof-we’ll often visit after a rain to see exactly where water sits
• Review structural condition of decking and visible framing where accessible
• Develop a drainage and fall plan (full or phased) that fits your building, budget, and timeline
• Implement the chosen solution-tapered insulation, localized crickets, or, working with an engineer, structural adjustments-along with your new or existing membrane

We redesign fall and drainage on flat roofs across Brooklyn-on brownstones, rowhouses, and small apartment buildings-so water finally goes where it’s supposed to: off your roof, not into your ceiling.

Flat Roof Fall & Drainage – Questions Brooklyn Owners Ask

Is it possible to add fall without tearing off my existing flat roof?
Meaningful, long-term improvements almost always require at least partial tear‑off. We need access to the deck or insulation layers to shape the surface correctly and integrate new slopes with drains and flashings. “Overlay” fixes on top of old, soft layers rarely last more than a season or two.

Will fixing the fall automatically stop my leaks?
Improving fall and drainage reduces stress on your membrane and details, but we also address any existing weak points at seams, penetrations, and parapet bases as part of the work. Fall alone doesn’t fix a torn or poorly detailed roof, but it does help your new system last 15-20 years instead of 6-8.

Can I just cut an extra scupper or drain instead of changing slope?
Additional outlets can help, but if water can’t naturally reach them, ponding remains. I’ve seen roofs with four drains where all four sat in high spots-useless. We prefer to design fall and outlets together so the roof surface and drainage hardware work as a system.

How much slope do I really need on a flat roof?
Many modern systems and NYC code call for at least 1/4″ per foot toward drains, though some allow a minimum of 1/8″ per foot in specific conditions. In practice, we aim for slopes that perform well under real-world Brooklyn conditions-heavy summer downpours, winter freeze-thaw-not just on paper.

Will adding tapered insulation for fall add too much weight?
Tapered insulation is relatively light compared to water-saturated old roofing or heavy screeds. Polyiso foam weighs about 0.2 pounds per square foot per inch of thickness. We still consider structural limits, especially on older joist systems, but in many cases, removing three layers of old built-up roofing and adding tapered boards keeps or even reduces overall load.

Get Expert Help Putting Proper Fall on Your Brooklyn Flat Roof

Stop living with permanent puddles on your roof.

At FlatTop Brooklyn, we offer:

• On-site evaluation of your flat roof’s slope, structure, and drainage
• A practical plan to create proper fall using the right method for your building
• Professional installation tied into a durable, compatible roof system

We work on rowhouses, walk-ups, and extensions across all Brooklyn neighborhoods-from Red Hook to Bushwick-and we’re known for turning problem ponding roofs into clean-draining, long-lasting systems. Call us or request an assessment, and we’ll show you exactly how to put fall on your flat roof the right way.