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Flat roof drainage

Flat Roof Drains and Scuppers: How They Work and Why They Clog

Flat roof drains and scuppers are the exits for rain. A drain sends water down a pipe, and a scupper lets it out through a wall. Florida code calls for secondary overflow ones where a parapet could trap water. Clogs, low spots or a blocked opening cause ponding.

By Josh Hildebrand, owner of Coastline Roofing · Florida license CCC1331076 ·

Water standing on a flat roof two days after a storm usually means water has no easy way out. Here is how drains and scuppers work and what to do about it.

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The basics

What are flat roof drains and scuppers?

A drain is an opening in the roof surface that sends water down a pipe. A scupper is an opening through a wall or parapet, which is a short wall around the roof edge, that lets water out the side.

A flat roof is not truly flat. It is built with a slight fall so rain moves toward its exits. Those exits are drains, scuppers, or the roof edge itself. The Florida Building Code, Building volume, Section 1503.4, says roof drainage systems must meet Chapter 15 and the Plumbing code's storm drainage chapter, unless the roof slopes to drain over its edges.

So when does a roof need drains or scuppers? When water has a wall in its way. If a parapet or raised edge runs around the roof, rain can't run off the side. It needs a way out. That describes most commercial buildings, plus many homes with a flat section, a porch roof or an addition tucked behind a short wall.

Most flat roofs use two layers of protection. Primary drains carry normal rain. Secondary drains or scuppers, also called overflow, take over if the primary ones back up. The Building volume, Section 1503.4.1, says secondary emergency overflow drains or scuppers must be provided where roof drains are required and the roof's perimeter construction extends above the roof.

Why the backup? The Plumbing volume, Section 1101.7, tells designers to assume all primary roof drains are blocked when working out the deepest water the roof could hold. Code writers expect drains to clog. Your roof should be built so a clog does not become a collapse or a flood.

Standing water on your flat roof?

Coastline can look at a flat or low-slope roof and give you a written photo report of the drains, scuppers and low spots. Josh reviews every job, so you get a plain answer on whether it is a cleaning job or a repair. Coastline Roofing works all over Florida (license CCC1331076).

Call (941) 896-7793 or text your town and a photo.

Causes

Why do flat roof drains and scuppers clog in Florida?

Most clogs come from a short list of causes.

Leaves and debris

The Florida Roofing and Sheet Metal Contractors Association (FRSA) names debris as one reason a roof stops draining. Leaves and storm debris pile up at drain openings, especially after a windy week.

Rain heavier than the drain can pass

FRSA also lists rainfall that exceeds the system's capacity. A Florida summer storm can drop rain faster than a drain carries it. That is when the secondary scupper has to work.

Missing or damaged domes and grates

The International Code Council commentary on Section 1102.6 says roof drains need a dome or grate over the opening. Without one, trash drops straight into the pipe.

Roofing material over the opening

Plumbing Section 1105.1 says the drain opening must not be obstructed by the roofing membrane. Roofing material that covers part of the opening cuts how much water gets through.

A roof that has sagged or been squashed

Professional Roofing magazine, from the National Roofing Contractors Association (NRCA), says deck or framing deflection can create low areas, and compressed insulation can alter the surface level. Water collects in those low spots.

Equipment in the water's path

NRCA's article says equipment placed in drainage paths can block water flow. A new air conditioner platform or a cable run can act as a dam.

Signs

How can you tell your drains or scuppers are not working?

The clearest sign is standing water that is still there two days after the rain stops.

The NRCA's test for proper drainage is no ponding water 48 hours after a rain, during weather that lets the roof dry. Short-lived puddles are normal. Water that lingers is not, and NRCA says ponding beyond 48 hours can be harmful to the roof assembly.

The FRSA tapered insulation article suggests other clues: discoloration, debris lines, and coating deterioration, plus checking the roof 48 hours after a heavy rain.

  • Water standing around a drain, or a ring of dirt where it stood
  • Water spilling over the top of a parapet or out of the wrong place
  • Stains on the wall below a scupper, or water running down the wall
  • Ceiling stains inside that show up after storms
  • Moss, algae or plants growing in a low spot
  • A soft or spongy feel underfoot near a drain, which a roofer should check

Please do not climb onto a wet flat roof to look. A roofer can check from a ladder or with a drone. If you can see a ceiling stain, our guide to what to do when your roof is leaking and the one on finding where a roof leaks will help.

The fix

How are clogged or poorly draining flat roof drains fixed?

The fix starts with clearing the drain. If water still ponds after that, the roof has a slope problem, and the answer is usually tapered insulation, crickets, or another drain.

  1. Clear the opening and check the dome

    A roofer removes the debris, checks that the dome or grate is in place, and checks that nothing is blocking the line below.

  2. Look at how the roof meets the drain

    JM's TPO installation guide says to taper the insulation at least 36 by 36 inches around drains for proper drainage. Roofing membrane must not choke off the opening, per Plumbing Section 1105.1.

  3. Test for ponding

    The NRCA 48-hour rule tells you whether the problem is a clog or the roof's shape. Water that clears after the drain is cleaned was a clog. Water that stays has a slope problem.

  4. Add slope where the roof has lost it

    NRCA's article says tapered insulation can be used to give a roof slope for drainage. It also recommends sloped saddles and crickets, which are small peaked ridges that steer water to a drain, built with a slope twice that of the roof around them.

  5. Adjust or add drainage

    NRCA says existing drains or scuppers may need to be raised or lowered during a reroof. Adding drains or scuppers can be done but may be a challenge.

A roof's design slope matters here. The FRSA article on tapered insulation says most low-slope roof types need a design slope of at least one-quarter inch per foot, or 2 percent. The code also defines positive drainage as drainage within 48 hours of rain. The article adds that Section 706.1 exempts reroofing from that minimum when the roof provides positive drainage, so the exact answer for your roof is something a roofer should verify.

The pooled water has its own set of problems. Read ponding water on a flat or commercial roof, alligatoring and flat roof splitting and cracking for what long-term standing water does to a membrane.

Florida conditions

What makes drains and scuppers matter more in Florida?

Florida gets heavy rain in short bursts, plus storms that fill drains with debris. That is the worst case for a flat roof.

The FRSA's article on roof drains notes that debris, rainfall beyond capacity, and flooded outlets can each stop drainage, and that a hurricane can combine all three. The same article says drain pipes must be clean and relatively smooth inside for the seals to work.

In Miami-Dade County's High-Velocity Hurricane Zone, Building Code Section 1514.4.2.2 says the existing number or size of required scuppers and drains cannot be reduced when reroofing unless a registered design professional designs a new system. Other counties follow their own local rules, so ask your building department.

A flat roof in Manatee County also faces heat. The Department of Energy and Lawrence Berkeley National Laboratory say a conventional roof can reach 150 degrees Fahrenheit or more on a sunny summer afternoon. So clear drains before the June rainy season starts, and again after any big storm.

After a storm, do a quick check. Look for water still standing, debris around the drain, and new stains inside. If a tarp is needed to keep water out until a repair, see Coastline's temporary repair and tarping page.

Where this fits

This page is for roofs with drains and scuppers: low-slope sections, porch and carport roofs with parapets, and commercial buildings. Our flat roofing page explains what Coastline does on them.

Reviews

What homeowners say about working with Coastline

★★★★★
"Josh was very thorough diagnosing a hard-to-find leak on a tile roof, after 2 other roofing companies tried but failed to catch the problem. ... Great work at a fair price, great service."
Mark R.Leak repair, tile roof
★★★★★
"Loved how Josh used his drone to identify cracked and slipped roof tiles and sent us the photos. Photos also taken of completed work."
Felicity K.Tile roof inspection
★★★★★
"They did a good job on the repair so I would definitely recommend them."
Duke H.Roof repair, Florida Gulf Coast
Keep reading

Related guides

Flat Roofing

TPO and low-slope repair and replacement from Coastline.

See our flat roofing →

Ponding Water on a Flat Roof

Why water pools and what long-term standing water does.

Read about ponding →

How Long a Flat Roof Lasts in Florida

Lifespans by material, and what shortens them.

See the lifespans →

Also see flat roof cost in Florida and flat roof coatings.

FAQ

More questions

What is the difference between a roof drain and a scupper?

A drain is an opening in the roof surface that sends water into a pipe inside the building. A scupper is an opening through a wall or parapet that lets water flow out the side. The Florida Building Code treats both as ways to provide secondary or overflow drainage where a parapet could trap water.

Does a flat roof need an overflow scupper?

The Florida Building Code, Building volume, Section 1503.4.1, says secondary emergency overflow drains or scuppers are required where roof drains are required and the roof perimeter extends above the roof. A roofer or building official can check your roof against that rule.

How often should flat roof drains be cleaned?

No single schedule fits every roof. The NRCA and FRSA articles describe debris and heavy rain as common causes of blocked drainage, so a check before the rainy season and after major storms makes sense. If water stays on the roof past 48 hours after rain, check sooner.

How long is too long for water to sit on a flat roof?

NRCA's test for proper slope is no ponding water 48 hours after a rain, during conditions that let the roof dry. NRCA says ponding beyond that can be harmful to the roof assembly. Manufacturers set their own limits, which differ by roof system.

Can you add a drain to an existing flat roof?

Sometimes. NRCA's article says additional drains or scuppers can be added but may be a challenge. The work involves the deck, the drain line and the roof layers around it, so a roofer should inspect it before anyone promises it.

Can blocked drains cause a roof collapse?

Standing water adds weight, and the Florida Building Code's Plumbing volume tells designers to assume primary drains are blocked when working out the deepest water a roof must hold. That is why secondary drains and scuppers are required. A roof with sagging or deep ponding needs a professional look soon.

Sources

  1. FRSA: Where's the Roof Drain? (quoting 2023 Florida Building Code Section 1503.4 and Plumbing Chapter 11)
  2. Professional Roofing (NRCA): Still water runs deep (July 2012)
  3. FRSA: Tapered insulation considerations
  4. Johns Manville: TPO application guide and installation instructions
  5. Lawrence Berkeley National Laboratory and U.S. DOE: Guidelines for Selecting Cool Roofs
  6. Miami-Dade County Board of Rules and Appeals: Interpretation on existing roof drainage requirements (June 20, 2024)

Have a roofer look at the drains

Send us the address and a photo of the low spot. We will inspect the roof and send a written photo report that shows what is blocked and what has lost its slope.

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