What Is Roof Ponding (Ponding Water)?

What roof ponding is

Every flat roof holds a little water after a storm. Small puddles that dry up in a day are normal. Ponding is when the water does not leave.

The National Roofing Contractors Association defines ponding as "the excessive accumulation of water at low-lying areas on a roof that remains more than 48 hours after precipitation under conditions conducive to drying" ¹.

In plain words, if it is still there two days after the rain ended, and the weather was dry enough to dry it, it counts as ponding.

The Asphalt Roofing Manufacturers Association uses the same line: "Ponding water is defined as water that remains on a roof for 48 hours or longer" ². ARMA notes the water may come from rain, snow or runoff from rooftop equipment ³.

The 48-hour test matters because warranties and inspectors use it. A puddle that is gone by the next afternoon is not a ponding problem. A puddle that is still there on day three is.

How ponding starts and grows

Water on a roof runs to the lowest point it can reach. On a well-built low-slope roof, the low points are drains, scuppers or gutters. Ponding starts when a low point forms somewhere with no way out.

Once water sits there, its weight can make the problem worse. ARMA explains, "As water accumulates, deck deflections can increase, thereby resulting in additional ponding" ³.

Deflection means the roof deck bends down under the load. A deeper dip holds more water, which adds more weight, which bends the deck more.

That loop is why a small pond can grow over the years.

Things also collect in the water. Dirt and leaves settle in it. Algae and plants can start to grow. In winter, the pond can freeze and thaw again and again. Each of these wears on the roof surface under the water.

Kinds of ponding by cause

Ponding is usually sorted by what caused the low spot. ARMA's bulletins point to these causes and fixes ^{3 2}:

Knowing the cause matters. Cleaning a drain fixes one kind; a sagging deck needs a structural answer first.

Worked example: how heavy a pond is

A flat porch roof has one pond about 6 feet by 8 feet, about 1 inch deep at the middle. Here is a rough weight (illustrative math; water weighs about 62.4 pounds per cubic foot):

That is one modest pond, sitting in one spot, every time it rains. The real risk is the loop ARMA describes: weight bends the deck, the dip deepens and holds more ³. A structural engineer, not this example, decides what a roof can carry.

Where ponding happens on houses

Ponding is a low-slope roof problem. On homes, look at flat roofs over porches, garages, additions, carports, dormers with flat tops, and modern houses with flat or nearly flat roofs. Built-up, modified bitumen, single-ply, spray foam and some metal roofs are used on these slopes.

Shingle roofs should not pond. The 2021 IRC says asphalt shingles "shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater" ⁴.

At that pitch, water runs off. If water sits on a shingle roof, the problem is a different one, such as a sag or an ice dam.

On a flat roof, ponds tend to form in the middle of spans between supports, beside rooftop units, near drains that sit a little high, and along parapet walls.

What ponding does to a roof

• Deck sag. ARMA warns the added load "may result in deck deflection" and can put the structure at risk ².
• Dirt and cracking. Debris builds up; ARMA says it can lead to "alligator cracking" as wet areas dry and shrink, and can reduce the roof's reflectivity ².
• Bio-growth. "When water stands for long periods of time, it promotes biological growth, such as algae and vegetation," which can attack the membrane ².
• Ice. "Ice formations develop and move constantly with changes in temperature" and can damage the membrane ².
• Leaks. Once the membrane is damaged, ARMA says the "risk of water infiltration into the building and subsequent interior damage is amplified" ².
• Warranty trouble. Some warranties exclude damage from ponding ⁵.

Benefits of fixing ponding

• Takes repeated weight off the deck and breaks the sag-and-pond loop ³
• Removes the standing water that feeds algae and plant growth ²
• Reduces freeze-thaw wear on the roof surface in cold areas ²
• Keeps the roof in line with warranty terms that exclude ponding ⁵
• Lowers the chance a small surface flaw becomes a leak inside

What a ponding fix cannot do

• Clearing a drain cannot fix a roof that was built with no slope.
• Adding slope cannot fix a deck that is already rotten or overloaded. That comes first.
• Coatings sold as "ponding resistant" do not remove the water or its weight.
• A fix cannot undo membrane damage already done. Worn areas may need repair or replacement.
• No fix lasts without upkeep. Drains clog again if no one clears them ².

What drives the cost of fixing ponding

• Cause. Clearing a clogged drain is small work. Adding slope or drains is a roofing project.
• Size of the area. Tapered insulation or a new slope layer is priced by area.
• Roof condition. If the membrane is worn, adding slope may mean replacing the roof over the new layer.
• Structure. A sagging deck may need an engineer and framing repair before any roofing.
• Drain work. New drains, scuppers or overflow drains can involve plumbing and wall openings ⁴.
• For a full-roof budget, use our roof cost calculator or the roofing cost guide.

How a roofer deals with ponding

This is the order you should expect. It is not a do-it-yourself guide. Roof work is a fall risk, and a licensed roofer should do it.

1. Confirm it is ponding: check the roof about two days after rain in drying weather ¹.
2. Map the ponds: where they are, how big and how deep, often with photos.
3. Check the drains, scuppers and gutters for clogs and height. ARMA calls for regular checks to keep drains clear ².
4. Look for the cause: low slope, a high drain, a sag between supports, or equipment blocking flow ³.
5. If the deck sags, get a structural review before adding any weight.
6. Fix the cause: clear or move drains, add crickets behind equipment, or add tapered insulation to build in slope ³.
7. Repair or replace any worn membrane, and recheck the roof after the next storm.

How to keep a low-slope roof from ponding

• Clear leaves and debris from drains, scuppers and gutters, especially in fall
• Check the roof two days after a big rain and note any water still there
• Keep photos of each pond over time; growing ponds point to deck sag
• Do not store things on the roof or let rooftop gear block the flow
• Have the roof inspected on a schedule; one loaded warranty requires owner inspections at least twice a year ⁵
• Fix small membrane damage in pond areas quickly, before water gets in

Ponding and your warranty

Many low-slope roof warranties treat ponding as the owner's or designer's problem, not the maker's. We only quote a warranty we have read.

The TRI-BUILT asphaltic membrane limited material warranty lists among its exclusions "Lack of positive slope or inadequate drainage, to completely remove water from the roof system to prevent ponding water conditions on the roof system as defined by the NRCA (48 hours)" ⁵.

It also excludes "Clogged drains or lack of adequate drainage that does not promptly and readily remove water from the roof" and says owner inspections, including "unclogging drains and removing standing water," must take place at least twice a year, and that "Lack of regular maintenance shall void this Limited Warranty" ⁵.

That is one warranty. Yours may say something else. Read the exclusions and the maintenance section of the warranty for the roof you have, and keep records of drain cleaning and inspections.

What the code requires

These rules come from the 2021 International Residential Code, Chapter 9, as adopted in Maryland ⁴. States and cities adopt the IRC on their own schedule and can amend it.

The code does not define ponding or set a 48-hour rule. It sets minimum slopes and drain rules that, followed, should prevent it. The 48-hour line comes from the industry, through NRCA and ARMA ^{1 2}.

Sources: [4]

Ponding vs related roof problems

Ponding often comes with sagging. If a pond is growing year by year, ask a pro to check the structure as well as the roof surface.

Common misconceptions

Myth Any water on a flat roof is ponding.

Reality Water that drains or dries within 48 hours in drying weather is not ponding under the NRCA definition ¹.

Myth Flat roofs are truly flat.

Reality Code minimums call for slope, usually ¼ inch per foot for most low-slope roofs ⁴.

Myth A coating will solve ponding.

Reality The water and its weight stay. The fix is slope or drainage.

Myth Ponding is only a looks problem.

Reality ARMA lists deck sag, bio-growth, ice damage and leaks as effects ².

Myth The warranty will cover ponding damage.

Reality At least one loaded membrane warranty excludes it ⁵. Read yours.

Local code and climate

The code sections on this page come from the 2021 IRC as adopted in Maryland ⁴. Your state or city may use another edition or amend it, and commercial buildings fall under a different code. Your building department sets the rule where you live.

Climate changes how fast ponding does harm. In cold places, ponds freeze and thaw and can move and damage the membrane ². In warm, wet places, algae and plants grow faster in standing water ².

Areas with heavy storms put more water on a roof at once, which tests drains and overflow scuppers. Snow load rules also matter where ponds add weight to a roof that already carries snow.

What to ask your roofer about ponding

• Is this ponding by the 48-hour test, or just slow drying?
• What is causing it: slope, drains, a sag or equipment?
• Does the deck need a structural check before any work?
• Will you add slope, add drains, or both? Will you add crickets behind rooftop gear?
• Does my roof's warranty exclude ponding, and what maintenance does it require?
• Should we repair the roof or replace it while we fix the slope?

Keep going

• Roof repairFixing drains and pond damage
• Roof inspectionFind the cause of standing water
• Repair or replaceDecide if a ponding roof needs replacing
• Roof flashingEdges and penetrations on a flat roof

Related terms

Low-Slope Roof

A low-slope roof, often called a flat roof, is a roof with only a slight fall, usually less than 2 inches of rise per foot, so it needs a sealed covering such as a membrane instead of overlapping shingles.

Roof Sagging

A visible dip in a roof line or plane. It can mean a wet or failed deck or a structural problem and needs a professional look.

Roof Leak

Water getting past the roof covering into the deck, attic or house. Most leaks start at flashing, penetrations or damaged shingles, not open field.

Roof Blistering

Raised bubbles in shingles or a membrane where trapped moisture or gas lifts the surface. Blisters that break expose the material below.

Roof Inspection

A check of the covering, flashing, penetrations, ventilation and attic to report the roof's condition and any damage.

Roof Warranty

A roof warranty is a written promise to repair or pay for certain roof failures for a set time.

Manufacturer Warranty

A manufacturer warranty is the shingle maker's written promise to repair, replace or pay toward its products if they fail from a manufacturing defect within a set period.

Also referenced from: Low-Slope Roof