4.5 Storm Drainage Systems
Key Takeaways
- Storm water may not discharge into a sanitary sewer, and a storm drain connected to a combined sewer must be trapped.
- Conductors, leaders, and horizontal storm drains are sized from the 100-year hourly rainfall rate for the project's location, which ranges from roughly 2.5 to 4.5 inches per hour across Texas.
- Dome-type roof drain strainers must extend at least 4 inches above the roof with an inlet area of at least 1.5 times the conductor area.
- Secondary (emergency) roof drainage is required where the roof perimeter can impound water; the secondary inlet is set 2 inches above the low point of the roof.
- The secondary system must discharge separately from the primary system, above grade, where building occupants or maintenance personnel will notice it.
4.5 Storm Drainage Systems
Core Principle: Storm drainage is a required subject in the TSBPE 48-hour Journeyman curriculum — Board Rule 363.5(g)(2)(I) mandates classroom time on the basic design, materials, and installation of storm water systems, the hazards of improper installations, and testing procedures. Commercial candidates see it on the written exam even though the practical dollhouse station is sanitary only.
The governing idea is simple: storm water and sanitary sewage stay apart. Rain is clean, arrives in enormous surges, and would flood a sanitary treatment plant; sewage is contaminated and must be treated. Everything else in the chapter follows from keeping the two systems separate.
1. Separation and Prohibited Connections
- Storm water shall not discharge into a sanitary sewer, and sanitary sewage shall not discharge into a storm drain.
- Where a combined sewer is the only legal point of disposal and the AHJ approves it, the building storm drain must be trapped. The trap keeps sewer gas out of open roof conductors and area drains, and it must be sized to the drain it serves and provided with a cleanout.
- Subsoil drains, footing drains, and area drains connect to the storm system or to an approved point of disposal, never to the sanitary system.
- Downspouts, condensate lines, and cooling-tower blowdown are all storm-side or indirect-waste problems, not sanitary connections.
2. Roof Drains, Strainers, and Flashing
| Component | Requirement |
|---|---|
| Dome (basket) strainer | Extends not less than 4 inches above the roof surface immediately adjacent to the drain, with an available inlet area of not less than 1.5 times the area of the conductor served |
| Flat-deck strainer | Permitted on sun decks, parking decks, and similar maintained surfaces; set level with the deck with an inlet area not less than 2 times the conductor area |
| Roof drain size | Roof drains are not smaller than the conductor they serve; a minimum 2-inch drain is the practical floor for small roof areas |
| Flashing | The junction of the drain and the roof membrane must be made watertight with an approved flashing, clamped by the drain's flashing ring |
| Location | Drains are placed at the low points of the roof, and the roof structure must be designed for the ponding depth that develops before the drain flows at capacity |
The 1.5-times rule exists because leaves and gravel blind part of the dome. A strainer with the same area as the conductor is fully blocked long before the conductor is.
3. Sizing on Rainfall Rate
Storm piping is not sized in fixture units. It is sized on projected horizontal roof area and the rainfall rate for the project location.
- Find the design rainfall. The code tables are based on the 100-year, one-hour rainfall rate for the site, read from the code's rainfall map or appendix. Across Texas that value runs from roughly 2.5 inches per hour in the far west to about 4.5 inches per hour or more on the upper Gulf Coast — never assume a statewide number, and never carry a rate from another state's project.
- Measure the horizontal projected area. Roof slope does not increase the volume of rain that lands on the footprint; only the horizontal projection counts. Vertical walls that drain onto the roof add area under the code's wall-adjustment rules.
- Enter the table at the correct rainfall column for vertical conductors and leaders, and at the correct slope and rainfall combination for horizontal storm drains and building storm sewers.
- Adjust published one-inch-per-hour values. Where a table is published for a 1 inch per hour rate, the allowable area is divided by the actual rainfall rate. A conductor good for 4,400 square feet at 1 inch per hour serves only 1,100 square feet at 4 inches per hour.
[!NOTE] Continuous flow. Where a continuous or semi-continuous discharge — condensate, pump discharge, or process water — enters the storm system, each gallon per minute is converted to an equivalent roof area and added to the drainage load before sizing.
4. Conductors, Leaders, and Horizontal Storm Drains
- Conductor = the vertical piping inside the building. Leader = the vertical piping outside the building. Both are sized the same way.
- Interior conductors must be of approved materials for the location and may not be reduced in size in the direction of flow.
- Horizontal storm drains follow the same minimum slope logic as sanitary drains: pipes 2-1/2 inches and smaller at 1/4 inch per foot, 3 through 6 inches at 1/8 inch per foot, and 8 inches and larger at 1/16 inch per foot.
- Cleanouts are required at the base of conductors and at changes of direction, on the same interval basis as sanitary drainage.
- Where a conductor is subject to physical damage in a parking area or loading dock, it must be protected by a guard or by a run of ferrous pipe.
5. Secondary (Emergency) Roof Drainage
This is the highest-consequence topic in the chapter. Ponded water weighs about 5.2 pounds per square foot per inch of depth; a blocked primary drain on a flat commercial roof with a parapet can collapse the structure.
| Requirement | Rule |
|---|---|
| When required | Where the roof perimeter construction — a parapet, wall, or raised edge — extends above the roof so water can be impounded |
| Secondary drain inlet elevation | Set 2 inches above the low point of the roof, so it stays dry until the primary drain fails |
| Scupper option | Overflow scuppers are permitted in place of secondary drains; a scupper opening is not less than 4 inches in height and wide enough to pass the flow of the roof drain it backs up |
| Sizing | The secondary system is sized on the same rainfall rate as the primary system, and is calculated as if the primary system were entirely blocked |
| Discharge point | Separate from the primary system, discharging above grade in a location that building occupants or maintenance personnel would normally observe |
That last rule is deliberate. An overflow that quietly dumps into the same underground storm main gives no warning; an overflow that gushes onto the sidewalk above the entrance gets the roof drain cleaned that afternoon.
6. Subsoil Drains, Building Subdrains, and Sumps
- Subsoil drains are open-jointed, horizontally split, or perforated pipe placed in the ground to collect groundwater; they discharge to a sump or to an approved point of disposal, never to the sanitary sewer.
- Building subdrains that cannot drain by gravity discharge into a watertight sump and are lifted by an automatic pump or ejector to the building's gravity storm drain.
- A storm sump serving only clear water does not need the gas-tight cover and independent vent that a sewage sump requires — the distinction between a sump pump and a sewage ejector is heavily tested.
- Sump discharge piping needs a check valve and a full-way shutoff valve, in that order downstream of the pump, exactly as in sanitary applications.
7. Rainwater Harvesting in Texas
Rainwater harvesting is explicitly part of the Texas curriculum: the Board's 48-hour Journeyman program devotes classroom time to new technology promoting water and energy conservation, including rainwater harvesting, and the Water Supply Protection Specialist (WSPS) endorsement course covers rainwater harvesting per the Texas Water Development Board's Rainwater Harvesting Manual.
The rules that matter to a journeyman:
- A harvested-rainwater system that is connected to or has an auxiliary connection from a public water supply must be protected against cross-connection by an approved backflow prevention assembly or an air gap.
- Non-potable rainwater and reclaimed-water piping must be clearly identified — purple pipe, marking, or approved labeling — and must not be connected to potable outlets.
- Storage tanks require debris exclusion, overflow, and mosquito-proof screening, and any overflow discharges like storm water, not to the sanitary system.
A building storm drain must be connected to a municipal combined sewer with the approval of the authority having jurisdiction. What additional requirement applies?
What is the minimum height and inlet area required for a dome-type roof drain strainer?
A commercial flat roof is surrounded by a parapet wall. Where must the inlet of the secondary (emergency) roof drain be set, and where may it discharge?
A vertical conductor table lists a maximum roof area of 4,400 square feet at a rainfall rate of 1 inch per hour. On a Houston project designed for a 4 inch per hour rainfall rate, what roof area may that same conductor serve?