Storm drainage: sizing interior conductors, leaders, and storm drains

Interior storm drainage, and where it sits in the building

Interior storm drainage is the piping inside the building that takes the water the roof sheds and gets it to the ground. The roof drain or the scupper is the inlet. Below it runs a vertical conductor, then a horizontal storm drain, then the building storm sewer out to the site system. That whole path is one job: move the roof flow down and out without dumping it where it does not belong.

This guide is the piping. The roof side, the drains, the overflow drains, and the scuppers, and how you size them from the roof area, lives in the roof drainage and scupper sizing guide. The two are one design split across two trades, so size the inlet there and size the pipe here, with the same rainfall and the same flow carrying across the handoff.

The storm system is not the sanitary system, and that separation is the rule the whole chapter is built on. Sanitary carries waste and is sized by drainage fixture units, covered in the DWV venting and pipe sizing guide. Storm carries clean rainwater and is sized by roof area and rainfall. They run as two systems, and you do not cross them.

Where does the roof drainage hand off to the interior piping?

The handoff is at the drain body or the scupper. Above that point the roof drainage guide sizes the inlet for the flow off the roof area. Below it, the same flow becomes your problem, and the names change with the geometry of the pipe.

A leader is the vertical pipe, often the term used outside the building or on the exposed downspout. A conductor is the same vertical pipe running inside the building. A horizontal storm drain is the sloped run that collects the bottoms of several conductors and carries the combined flow toward the building storm sewer. The trade uses leader and conductor loosely, so read the plan and the code for which one the drawing means, but the physics is the same: vertical pipe drains by gravity through an open core, horizontal pipe drains by slope.

The number that crosses the handoff is the flow in gallons per minute. The roof guide computes it from area and rainfall. You take that same gpm and carry it down the conductor, into the horizontal drain, and out the storm sewer, growing the pipe wherever conductors combine. Size each piece for the flow it actually sees at that point, not for the whole building everywhere.

What rainfall rate do you size storm piping for?

You size storm piping for the design rainfall rate at the site, in inches per hour, from the 100-year storm. The International Plumbing Code, Chapter 11, bases conductor, leader, and storm drain sizing on the 100-year, 1-hour rainfall rate shown in its rainfall figures, with the source on those maps being the National Weather Service and NOAA. That single rate drives every flow in the system.

It is the same rainfall basis as the roof drainage guide, because it is the same storm hitting the same roof. Size the drain for one rate and the conductor below it for another and the two halves of the design do not match. Pull one number and carry it through.

Here is the part the old hands check. The rainfall maps printed in the IPC trace back to older NOAA precipitation data and have not been refreshed every cycle, while the storms have gotten heavier. The code allows approved local weather data, and the current source is NOAA Atlas 14, the point-precipitation frequency atlas. On a building that matters, pull the Atlas 14 value for the actual coordinates and compare it to the code map. Where Atlas 14 reads higher, the conservative move is to design the piping to it, and to note which rate you used so the next person knows which storm the pipe was built for.

How do you size a storm leader or conductor?

You size a conductor or leader by the roof area it drains and the design rainfall, converted to a flow, then read the pipe size from the IPC vertical conductor table. The flow is the same equation the IPC and the trade use: gallons per minute equals 0.0104 times the drained area in square feet times the rainfall rate in inches per hour. The 0.0104 is the unit conversion, because one inch of rain per hour on one square foot is about 0.0104 gpm, the same as dividing the area-times-rate product by 96.23.

Run the flow, then go to the table. The IPC sizes vertical conductors and leaders by flow alone, because a vertical pipe carries far more than a horizontal pipe of the same diameter. Gravity does the work and the falling water keeps an open air core down the center. So a 3 in conductor handles a flow that would need a much larger horizontal drain to carry at a shallow slope.

Add the wall that sheds onto the roof. The IPC adds one-half of the area of any vertical wall that diverts rain onto the roof to the projected roof area before you size the pipe. A tall wall draining onto a low roof feeds a conductor more water than the flat area alone suggests. Miss that and the conductor is undersized for the water that actually reaches it.

Q

Storm flow in gallons per minute carried by the pipe at the point you are sizing

A

Drained roof area in square feet, plus one-half of any vertical wall that sheds onto the roof

i

Design rainfall rate in inches per hour, from the code map or NOAA Atlas 14

Field example: piping a 9,000 ft² roof at 3 in/hr

Take a 9,000 ft² roof section in a location with a design rainfall of 3 in per hour, drained by three roof drains laid out so each carries about 3,000 ft². The roof guide sizes the drains; here we size the pipe below them.

Flow per conductor: 0.0104 times 3,000 ft² times 3 in per hour gives about 93.6 gpm at each drain. From the IPC vertical conductor table, that flow falls within a 4 in conductor on most editions, so each drain drops into a 4 in leader. The total reaching the horizontal main is three times 93.6, about 281 gpm, which is the number the main has to carry once all three conductors have tied in.

Now the horizontal drain, and the slope changes the answer. At 1/8 in per ft, 281 gpm needs a larger pipe than the same flow at 1/4 in per ft, because the flatter pipe runs slower and carries less. Read the horizontal storm drain table at the slope you can actually build under the structure, and confirm the size against the adopted edition. Change one input, the rainfall, the drain count, or the slope you can fit, and every pipe size downstream moves with it.

How do you size the horizontal storm drain?

You size a horizontal storm drain by the flow it carries and the slope you can build, both together, because a horizontal pipe does not carry a fixed amount. The IPC horizontal storm drain table lists capacity by pipe diameter across a range of slopes, commonly from 1/16 in per ft up to 1/2 in per ft. The same diameter carries more as the slope steepens, so the pipe size and the slope are one decision, not two.

The reason is velocity. A horizontal storm drain runs partly full and relies on slope to keep the water moving. Flatten it and the water slows, the pipe backs up sooner, and the capacity drops. Steepen it and the same pipe swallows more flow. So a 6 in drain at 1/8 in per ft handles a smaller roof area than a 6 in drain at 1/4 in per ft, and you read the column that matches the slope you can actually fit.

On a combining run the flow grows downstream. A main collecting four conductors carries the sum of all four below the last tie-in, not the flow of one, and it is sized for that total at its slope. Size each branch for the conductors above it and the main for everything downstream of the last connection. The pipe steps up as the running total crosses the table capacity, the same way a sanitary building drain grows by fixture units.

How do you adjust the storm table for your local rainfall?

The older IPC storm drain tables are printed for a rainfall of 1 in per hour, and you adjust them for the local rate by ratio. The rule the code gives is direct: for a rainfall rate other than 1 in per hour, the allowable roof area for a given pipe size is the area shown in the 1 in per hour column divided by the design rainfall rate. A pipe good for 10,000 ft² at 1 in per hour carries 5,000 ft² at 2 in per hour and about 3,300 ft² at 3 in per hour.

It is the same physics as the flow equation, run from the other end. Double the rainfall and you double the flow on the same roof, so the pipe that handled a given area at 1 in per hour handles half that area when twice as much water falls on it. Divide the table area by the rate and you land on the area the pipe can actually take.

The trap is grabbing a table area without checking what rainfall it assumes. Read a 1 in per hour table as if it were your site's 3 in per hour rate and you have undersized every conductor and drain by a factor of three. Newer editions list capacity directly in gpm across rainfall rates, which sidesteps the conversion, but plenty of plans and references still run on the area-at-1-in/hr tables. Know which table you are holding before you trust the number.

Secondary and overflow drainage runs on its own piping

The secondary, or overflow, drainage is the independent backup that carries water off the roof when the primary clogs or is overwhelmed, and on the piping side the rule is one word: separate. The IPC requires the secondary roof drainage system to be a separate system of piping, independent of the primary, discharging above grade in a spot the building occupants or maintenance people would normally see. The roof guide sizes the overflow drains and scuppers; here the point is that they get their own pipe.

Separate means separate all the way down. The overflow conductor does not tie into the primary conductor. The overflow horizontal drain does not merge into the primary storm drain. They run as two systems to two discharge points, and the secondary is sized in accordance with the same Section 1106 method for the same rainfall the primary uses. Confirm whether the adopted edition pairs a heavier short-duration rate with the secondary.

The independence is the whole warning system, and it is the part that gets violated to save pipe. The overflow that pours out of a wall scupper or an above-grade leader in a storm is the signal that the primary is plugged. Pipe the overflow into the primary and you have thrown that warning away, and now both systems clog together with no one the wiser until the roof is loaded. Run the second pipe.

Can storm and sanitary share a pipe?

No. The IPC requires the sanitary and storm drainage systems of a building to be entirely separate, except where a combined public sewer is the only thing available. Storm water is not drained into a sewer intended for sewage, and the clean roof water does not run into the same building drain as the waste. Cross-connect the two and you have built a code violation and a backup path for sewage into the storm system or rainwater into the sanitary.

The combined sewer is mostly history. Older cities built combined sewers that carried storm and sanitary in one pipe to the treatment plant, and in a hard rain those overflow raw sewage to the waterway, which is exactly why jurisdictions have spent decades separating them. New work runs two systems. You only deal with a combined sewer where the public main downstream is still combined.

Where the public sewer genuinely is combined, the code does not let you simply tee the two inside the building. The storm drain connects to the combined sewer through its own connection, commonly through a single wye a set distance downstream of any soil stack, and the storm sewer is connected independently to the public sewer. Confirm the exact arrangement and distances against the adopted edition. Even then, keep the systems separate inside the building and join them only at the point the code allows, outside.

Pipe, fittings, and joints

Storm piping inside the building uses the same material families as sanitary drainage, and the IPC points the storm conductors and drains to the same material tables. Aboveground conductors are commonly cast iron, hubless or hub-and-spigot, or Schedule 40 PVC or ABS, with copper, galvanized steel, and stainless also listed. Underground building storm drain is the buried-rated material from the underground table. Match the material to the location, the temperature, and what the spec and the AHJ accept.

Cast iron earns its place on the riser. It is quiet, which matters when a conductor full of falling water runs down a wall behind an occupied space, and it takes the heat and the abuse better than plastic. PVC is cheaper, lighter, and faster to install, and it is the default on a lot of commercial storm. The trade-off the spec usually decides is noise and fire rating versus cost. A plastic conductor in a finished space can be loud enough that the owner notices the rain, so check what the spec calls for before you assume PVC.

The joints follow the material. Hubless cast iron joins with the no-hub coupling and the band torqued to the maker's value. PVC is solvent-welded with primer and the right cement for the diameter. The same workmanship rules as any drainage pipe apply, and the storm system gets the same test, so a sloppy joint shows up before the pipe is buried.

Why does a storm leader sweat, and where to insulate

A storm leader sweats because cold rainwater runs down a pipe in warm, humid indoor air, and the pipe surface drops below the dew point of that air. Water condenses on the outside of the conductor the same way it beads on a cold glass in summer. On a long interior conductor in a hot, humid building, that sweat can be enough to drip, stain a ceiling, rot a finish, and get blamed on a roof leak that does not exist.

The fix is insulation on the cold pipe, and which pipe needs it depends on the building. A conductor running through a conditioned, humid space, a kitchen, a pool building, a warehouse with high indoor moisture, is the candidate. The insulation has to carry a continuous vapor barrier, because if humid air reaches the cold pipe through a gap in the jacket, it sweats there and you have hidden the problem instead of solving it. How thick and which type is set by the indoor design conditions, and the mechanical insulation by topic is its own coordination item with the spec.

Cast iron sweats less than copper or plastic at the same conditions, partly because its surface stays a little warmer and partly because the trade often insulates it for noise anyway. The point is to decide it on purpose. A bare conductor in a humid hall is a callback waiting for the first cold storm, and the time to insulate it is before the ceiling goes up, not after the stains appear.

Cleanouts and access on the storm system

A storm system gets cleanouts the same way a sanitary system does, because a horizontal storm drain catches leaves, grit, and roofing debris that wash down the conductor, and eventually something has to clear it. Code requires cleanouts at the same kinds of points: where the building storm drain meets the building storm sewer, at changes of direction past a set angle, at the base of conductors, and at intervals along long horizontal runs. Confirm the spacing against the adopted edition, commonly on the order of one every hundred feet on the larger runs.

The base of the conductor is the cleanout that matters most. That is where the vertical pipe turns horizontal, where the velocity drops, and where debris that rode the falling water settles out. A cleanout at that elbow lets you rod the horizontal drain and clear the turn without opening a wall. Leave it out and the first blockage at the base of the riser is a demolition job.

A cleanout has to be full size up to a limit and has to face a direction a cable can actually enter. The one built wrong is the one pointed into a corner where no one can get a rod on it. Set them where a person standing in the space can reach them with a machine, not where they are tidy on the drawing.

Tying into the site storm, detention, and the backwater valve

The building storm drain leaves the building and becomes the building storm sewer, which ties into the site storm system, a detention pond or vault, or a daylight discharge to grade. Where the flow goes is a civil decision, and the plumbing job is to deliver the design flow to that point at the right invert and slope without necking the pipe down going downstream. The storm sewer does not get smaller than the building drain feeding it.

Detention changes the downstream side, not the building piping. In many jurisdictions the site cannot dump its peak storm flow straight into the public system, so the water is held in a detention vault or pond and metered out slowly. That is a civil structure downstream of your connection, and it can hold water at the connection point in a big storm, which is the reason the backwater question comes up.

Where any part of the storm drainage sits below the level it could back up from, it gets a backwater valve, the same as sanitary. The IPC calls for storm drainage systems to be protected by backwater valves the same way sanitary systems are, on the piping that drains fixtures or areas below the next upstream manhole cover or the public sewer level. Below-grade area drains, a loading dock, a below-grade garage, all need the check so a surcharged storm main does not push water back up into the low spot. Put the valve where it is accessible, because a backwater valve no one can service is a future flood.

Conductor heads and leader heads at the parapet

A conductor head, also called a leader head or a rainwater head, is the open box at the top of a leader that catches the flow from a scupper or a gutter and funnels it into the vertical pipe. It sits at the parapet where a through-wall scupper discharges, and it gives the water a place to drop into the leader with some air around it instead of choking the pipe inlet.

The head does two things worth knowing. It provides an air gap and a visible overflow point, so if the leader below it plugs, the head overflows where you can see it rather than backing water onto the roof unseen. And it gives the falling water room to enter the pipe without the inlet running full and gurgling. Size the head to the flow it takes and to the leader below it, and keep the outlet from the head no smaller than the leader.

On a scupper-drained roof the conductor head is the handoff from the roof guide's scupper to this guide's leader. The scupper is sized as a weir over on the roof side. The head catches what the scupper throws and the leader carries it down. Coordinate the three so the leader is not the bottleneck that backs water up through the scupper and onto the roof.

Controlled-flow and siphonic storm piping

Two systems change how the interior piping is sized, and both are engineered, not pulled off the standard table. Controlled-flow drainage, the basis of many blue-roof designs, uses restricting drains that meter water off the roof slowly and let the roof hold a shallow planned pond during the storm. The win is a lower peak flow into the site storm and the detention, which is a stormwater-management move in dense areas. The interior piping is sized for the reduced, metered flow rather than the full instantaneous storm, so the conductors and drains can be smaller, but the roof and the structure have to carry the held water on purpose. The roof drainage guide covers the held-load side.

Siphonic drainage is the high-flow approach for large roofs. Special outlets exclude air so the piping runs full-bore, and the full pipe creates negative pressure that pulls water out far faster than gravity. The horizontal mains run nearly level instead of sloped, the pipe is smaller, and there are fewer downspouts, which is why it shows up on warehouses and distribution centers with vast roofs. It is designed as a system by the supplier against the full pipe condition, not sized off the gravity tables in this guide.

Both are specialty designs with the same caveat. A controlled-flow or siphonic primary still needs a secondary overflow path for the case it clogs or is overwhelmed, and that backup is usually a conventional gravity system. Do not let the engineered primary talk anyone out of the independent overflow.

Area drains, trench drains, and subsoil drainage

The roof is not the only thing that drains to the storm system. Paved areas, plazas, ramps, and loading docks drain through area drains and trench drains, and those tie into the storm piping, not the sanitary, because they carry rainwater. Size them the same way, by the area they collect and the design rainfall, and trap them where the code requires so the storm system does not breathe odor or vermin up through an open grate, especially if the downstream sewer is combined. The floor and trench drain detailing by topic is its own coordination item.

Subsoil and foundation drainage is the other tie-in, and it is groundwater, not rain off a surface. Foundation drains, footing drains, and underslab drainage collect the water in the soil around and under the building and carry it off to keep the structure dry. The IPC routes subsoil drains, not less than 4 in, to a trapped area drain, a sump, a dry well, or an approved point above ground.

Where the building sits below the sewer it would drain to, the subsoil and the low storm drainage cannot run by gravity, so they discharge to a sump and the contents are pumped up into the storm system. That sump and pump are a maintenance item the owner inherits, and a failed sump pump in a wet spring is how a basement floods. Where backup is possible, the sump or the low drainage gets the backwater valve from the connection section above.

Support and expansion on a tall conductor

A vertical conductor carries weight and moves with temperature, and both have to be held. The pipe full of water is heavy, so the riser is supported at each floor and at the spacing the material calls for, with the load carried at the base so the whole column is not hanging on the upper hangers. Undersupport a cast iron riser and the no-hub joints take a load they were not meant to carry.

Temperature moves plastic more than people expect. PVC and ABS expand and contract with the water and air temperature, and a long conductor or a long horizontal run can grow and shrink enough to matter over a tall building or a big floor plate. The fix is the manufacturer's expansion guidance, expansion joints or expansion offsets where the run is long, and supports that guide the pipe along its movement rather than pinning it so it buckles or pulls a joint. Cast iron moves far less and is usually held solid.

The horizontal storm drain hung from the structure gets the same two checks: enough hangers to hold the water weight without sagging the slope out of the pipe, and room to move with temperature. A sag between hangers becomes a low spot that holds water and silt, and the slope you carefully sized disappears into the belly. Hang it to hold the grade.

How is a storm system tested?

The interior storm piping gets tested the same way the sanitary drainage does, because the IPC routes the conductors and the building storm drain to the same Section 312 test. The water test fills the system, or a section of it, with water to a head above the highest fitting in that section, commonly at least 10 ft of head, and holds it while you walk every joint looking for a drop or a weep. The air test puts the system under a gauge pressure and holds it, watching the gauge.

Test the storm system before it is buried or closed in, the same as any drainage. A conductor inside a wall and a horizontal drain above a hard ceiling are both pipes you do not want to chase a leak in after the finishes are on. The test is the proof the joints hold while you can still see them.

Plastic gets tested with water in many jurisdictions, not air, because a pressurized air failure in plastic pipe is a hazard and water gives a truer leak picture. Confirm the method, the head or pressure, and the hold time against the adopted code and the inspector. And record what it held, because a test with no recorded head and duration is a claim, not a record.

Large flat roofs and the data center case

On a big flat roof the interior storm piping scales with the area, and the data center is the case where it gets designed with margin the warehouse next door does not get. The roof is enormous and sits over equipment a single leak can take down, so the storm system runs more conductors, generous pipe, and often a design rainfall above the code minimum, because the storm that overwhelms a code-minimum design costs downtime, not roof repair.

The piping decisions follow the stakes. More, smaller conductors spread the flow and the risk instead of betting the roof on a few big risers. The secondary overflow is sized and routed as a true equal backup on fully independent pipe, and its discharge is somewhere watched. Siphonic systems show up here because the roof area justifies the engineering and the smaller, near-level mains route better through a congested ceiling full of cable tray and mechanical.

The physics is the same as any roof. The difference is the margin and the consequence. Pipe a small commercial roof to the code and you are fine. Pipe a data center roof to the bare minimum and you have ignored what is under it. Match the redundancy and the rainfall to what a flooded roof would cost, and on the critical building that number is the reason the building exists.

What to document

Sized against a design storm, the leaders and horizontal runs are only shown to meet it through the flows, the rainfall figure, and the built slopes on file, which is exactly what a backup investigation reaches for. The flows, the rainfall, the pipe sizes, and the slopes are the evidence the system met the storm, and they are what the next plumber, the civil engineer, and the inspector all need when a drain surcharges or a ceiling stains.

Capture, for each conductor and each horizontal run, the area it drains, the design rainfall and its source, the computed flow, the pipe size, and for the horizontal drains the slope you built. Note whether you sized to the code map or to a higher NOAA Atlas 14 rate, the overflow piping and its independent discharge point, the backwater valves and where they are, and the test result. The record that says which storm the pipe was built for is the one that answers the hard question later.

Common mistakes

• Sizing the piping at the wrong rainfall, or reading a 1 in per hour table as if it were the site rate without the conversion.
• Not adjusting the horizontal storm drain for the slope you can actually build, so the flat run is undersized for its flow.
• Leaving out the secondary or overflow piping, or running it instead of an independent system to its own visible discharge.
• Tying the overflow piping into the primary conductor or storm drain, so both clog together and the warning is lost.
• Cross-connecting storm into sanitary, or sanitary into storm, instead of running two entirely separate systems.
• Running a cold interior conductor bare in a humid space so it sweats, drips, and gets blamed on a roof leak.
• Forgetting the half of the vertical wall area that sheds onto the roof when computing the area for a conductor.
• Omitting the cleanout at the base of the conductor, so the first blockage at the turn is a wall-opening job.
• Skipping the backwater valve on below-grade area or subsoil drainage that a surcharged storm main can flood.

Field checklist

Standards and references

The sizing lives in the plumbing code. The International Plumbing Code, Chapter 11 on storm drainage, sizes the vertical conductors, leaders, building storm drains, and horizontal branches from the roof area and the design rainfall, with the conductor and storm-drain sizing tables in Section 1106 and the secondary and separate-system requirements in the sections that follow. The Uniform Plumbing Code covers the same ground in its own storm drainage chapter where it is the adopted code. Section and table numbers shift between cycles, so confirm them against the edition the jurisdiction adopted and any local amendments before citing one on a submittal.

The design rainfall comes from the code rainfall figures, sourced from the National Weather Service and NOAA, or better from NOAA Atlas 14 point-precipitation frequency data, which the code allows as approved data. The structural rain-load side, the ponding check and the weight of held water, lives with the IBC and ASCE 7 and the roof drainage guide, not in the pipe sizing here. Materials follow the IPC material tables, with the pipe standards published by ASTM, and the rough-in test follows the general test provisions in Chapter 3.

ASPE design references give the engineering background behind the code tables for larger and engineered systems, including siphonic and controlled-flow designs that the standard gravity tables do not cover. The standard that controls any given call is the one the AHJ has adopted and enforces. The model code and the tables are the starting point. The adopted edition, the local amendments, and the inspector's interpretation govern the work.

Units, terms, and conversions

Storm drainage spans the plumbing plan, the civil site drawing, and the product sheet, so the same quantity shows up in different units and under different names.

Flow is in gallons per minute on the plumbing side and sometimes cubic feet per second on the civil side, where 1 cfs is about 449 gpm. Rainfall rate is in inches per hour. Roof area is in square feet. A leader is the vertical drain pipe, often the term used outside; a conductor is the same pipe inside the building; a horizontal storm drain is the sloped collecting run. The terms below are the ones a plumber, a designer, and an inspector use for the same parts.

Conductor / leader

The vertical storm pipe carrying flow from a roof drain or scupper down through the building; conductor inside, leader often outside

Horizontal storm drain

The sloped horizontal pipe that collects conductors and carries the combined flow toward the building storm sewer

Building storm sewer

The storm pipe from the building wall to the site storm system, detention, or daylight

Design rainfall rate

The 100-year rainfall in inches per hour for the site, from the code map or NOAA Atlas 14

Secondary / overflow piping

The independent storm piping for the overflow drains or scuppers, separate from the primary and discharging above grade

Conductor head

The open box at the top of a leader that catches scupper or gutter flow and feeds it into the pipe with an air gap

Backwater valve

A check on below-grade drainage that stops a surcharged storm main from backing water up into a low area

Combined sewer

An older public sewer carrying both storm and sanitary in one pipe, now largely replaced by separate systems

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