8.1 Storm Drainage Scope, Materials, Traps & Prohibited Connections
Key Takeaways
- UPC Chapter 11 governs storm drainage and carries 10 percent of the Iowa Journeyperson Plumber with Gas examination — the same weight as water supply and distribution.
- Roofs, paved areas, yards, courts, and courtyards must drain to an approved storm system or other approved point of disposal, not to the sanitary drainage system.
- Roof drains are constructed of aluminum, cast iron, copper alloy of not more than 15 percent zinc, leaded nickel bronze, stainless steel, ABS, PVC, or other approved materials.
- Where a storm drain connects to a combined sewer, or where storm piping could admit sewer gas to the building, the storm drain is trapped and the trap is protected.
- Cleanouts for building storm drains follow the same cleanout rules as sanitary building sewers.
Storm Drainage Scope, Materials, Traps & Prohibited Connections
Why this chapter deserves a full tenth of your study time
Look at the official Iowa content outline for the Journeyperson Plumber with Gas examination and you will find ten subject areas, each tied to a chapter of the Uniform Plumbing Code. Chapter 11 — Storm Drainage carries 10 percent. On a 75-question exam that is roughly seven or eight questions, the same weight as Chapter 6, Water Supply and Distribution, and double the weight of General Regulations, Indirect Wastes, or Traps and Interceptors.
Storm drainage is also the subject candidates most often skip, because a residential service plumber can go years without sizing a roof drain. That combination — heavily weighted, rarely practiced — makes Chapter 11 the highest-yield block of studying in this entire guide. Three of the four sections that follow are pure table work, and table work is exactly what an open-book exam rewards.
1. What a storm drainage system is, and what it is not
A storm drainage system conveys rainwater, snowmelt, subsurface water, and other clear-water drainage away from a building and its site. A sanitary drainage system conveys sewage. Chapter 11 exists to keep those two flows in the correct pipes.
Key vocabulary, because the exam uses these words precisely:
| Term | What it means |
|---|---|
| Roof drain | The fitting installed at a low point of a roof to receive rainwater and discharge it into a leader or conductor |
| Leader / conductor | The vertical pipe carrying rainwater from the roof drain down to the building storm drain |
| Building storm drain | The horizontal piping inside the building that receives the leaders and conveys the flow outside |
| Building storm sewer | The continuation of the building storm drain from outside the building wall to the point of disposal |
| Gutter | The channel at a roof edge that collects runoff and delivers it to a leader |
| Scupper | An opening through a parapet wall that lets water leave a roof |
| Subsoil drain | Perforated or open-joint piping installed below grade to collect groundwater |
| Areaway drain | The drain in a below-grade window well, stairwell, or light well |
Roofs, paved areas, yards, courts, and courtyards must be drained to an approved storm system or other approved point of disposal. Where a public storm sewer is available, that is normally the point of disposal; where it is not, the AHJ approves an alternative such as a drywell, a detention basin, or discharge to grade at an approved location.
2. Why storm and sanitary flows are separated
Two independent reasons, and the exam tests both.
Hydraulic capacity. A one-inch-per-hour rainfall on a 10,000 square foot roof produces roughly 104 gallons per minute — more than the entire peak sanitary flow of a small office building, sustained for as long as the storm lasts. Dumping that into a sanitary sewer surcharges the main, backs sewage into basements through the lowest fixtures, and, at the far end, overwhelms the treatment plant.
Treatment cost and permit compliance. Every gallon of stormwater sent to a sanitary sewer is a gallon that a municipal plant must pump, treat, and discharge under its permit. Communities on combined systems spend heavily to separate them, which is why an illegal downspout connection or a sump pump tied into a floor drain is a serious violation rather than a technicality.
Prohibited connections
- Do not connect roof drains, leaders, conductors, area drains, or subsoil drains to a sanitary drainage system.
- Do not connect sanitary fixtures to a storm drainage system.
- Where a combined sewer is the legal point of disposal, the storm drain connects to it only as the AHJ permits, and the combination is made outside the building or in the manner the code and the local ordinance prescribe.
3. Materials, strainers, and traps
Roof drain materials
UPC Section 1102.0 lists the acceptable materials for roof drains: aluminum, cast iron, copper alloy of not more than 15 percent zinc, leaded nickel bronze, stainless steel, ABS, PVC, and other approved materials. The 15 percent zinc limit matters — high-zinc brasses dezincify in wet, aerated, occasionally acidic roof runoff and fail from the inside out.
Strainers
A roof drain gets a dome strainer that projects above the roof surface so leaves and debris cannot seal the inlet flat. Deck drains and area drains in walking surfaces use flat strainers, which are permitted because there is no ponding leaf load and a projecting dome would be a trip hazard.
Where storm piping must be trapped
Storm piping does not carry sewage, so it does not normally need a trap. It needs one where sewer gas could otherwise reach the building or the outdoor air at an unacceptable point — most commonly where the building storm drain connects to a combined sewer, and at area drains, areaway drains, and similar inlets that are close to doors, windows, or occupied space. A trap serving a storm inlet still needs its seal maintained, which is why a trap primer or a barrier-type trap seal device shows up on a parking-garage area drain the same way it does on an interior floor drain.
Cleanouts
Cleanouts for building storm drains comply with the same cleanout requirements as the sanitary building sewer. Everything you learned in Section 3.5 — upper terminal, junction with the building sewer, aggregate changes of direction exceeding 135 degrees, spacing on long runs, direction of flow, clearance for rodding — applies here. The code does not write a second set of cleanout rules for storm piping; it points back to the sanitary rules.
4. How to work Chapter 11 on exam day
Storm questions come in three shapes, and the fastest route through each is different:
- "Which of these may connect to what?" — a scope and prohibited-connections question. Answer it from the separation principle above.
- "What size pipe for this roof area at this rainfall rate?" — a table lookup. Sections 8.2 and 8.3 drill the three tables: vertical piping, horizontal piping, and gutters.
- "What do I add for this wall, this overflow, this subsoil drain?" — a rule lookup. Sections 8.4 and 8.5 cover side walls draining onto a roof, secondary drainage, and subsoil systems.
Tab Chapter 11 before you sit down. On a 3-hour, 75-question exam you have about 2 minutes 24 seconds per question, and a storm sizing question you can find is a 30-second question while a storm sizing question you have to hunt for is a 4-minute question.
What percentage of the Iowa Journeyperson Plumber with Gas examination is drawn from UPC Chapter 11, Storm Drainage?
Which material is NOT listed by UPC Section 1102.0 as acceptable for roof drains?
What does the UPC require for cleanouts on a building storm drain?