Storm drainage and special systems problems

Storm drainage is only about one F25 question, but storm problems also turn up inside plan-reading questions, and the special systems in this lesson, such as sewage ejectors, sumps, grease interceptors and indirect waste, add several more. Luis grouped them together on one Saturday because they share a habit: each starts with a flow, not a fixture unit count, and each ends with a size or a volume.

Maya's practice building returned once more, this time with its flat roof, a parapet wall on one side, and a café kitchen that needed a grease interceptor.

How storm loads are calculated

Rain falling on a roof becomes a flow at the drains. The flow depends on the roof area and on the rainfall intensity, the rate at which rain falls during a short, heavy storm. The IPC gives design rainfall rates in inches per hour for each location, from a map or table based on climate records. The flow in gallons per minute is the area in square feet times the intensity in inches per hour, times 0.0104.

The constant comes straight from the units: one inch of rain on one square foot is 1/12 cubic foot, or about 0.623 gallons, and an hour has 60 minutes, so 0.623 divided by 60 is 0.0104. Knowing where a constant comes from means not having to memorize it.

Walls add load. Rain driven by wind strikes a vertical wall above a roof and runs down onto the roof. Codes add a share of the wall area to the roof area that drains it, commonly half the wall area for a single wall. Where two or more walls meet, the rules for how much to add get more detailed, and a question will say which applies.

The practice tables

Vertical leaders carry far more than horizontal drains of the same size, because water falling vertically fills the pipe as an annular sheet at high speed, while a horizontal drain flows at a modest velocity set by the slope. The horizontal drain is almost always the governing size.

Worked problem: roof drains, leaders and the storm drain

Now add the wall. A parapet 6 ft high runs along the 120 ft edge, beside the two drains on that side. Half its area is 360 ft² of extra roof, so each of those two drains now serves 2,700 + 180 = 2,880 ft², and its flow rises to 120 gpm. The leaders stay at 4 in, and the total rises to 464 gpm, which still fits 8 in at 1/8 in per ft. The wall did not change the answer here, but on a small roof with a tall wall it often does.

Secondary drainage and controlled flow

Flat roofs with parapets need a way for water to escape when the primary drains are blocked, so that the roof does not fill and collapse. Codes require secondary or emergency drainage: overflow scuppers through the parapet, or a separate set of overflow drains set above the roof surface and piped independently to a visible point of discharge. A question about secondary drainage usually tests one of three points: it must be independent of the primary system, it must discharge where it will be noticed, and it is sized for the same rainfall as the primary system.

Controlled-flow roof drainage, covered in PLB-330, deliberately holds water on the roof for a short time and releases it slowly, which reduces the peak flow to the storm sewer. Its questions turn on the structural design of the roof, the depth of water allowed and the drain's rated flow. On the exam, they are rare, and they usually ask what controlled flow is for rather than for a calculation.

Sumps and sewage ejectors

Fixtures below the level of the building sewer, such as a basement washroom, drain to a sewage ejector: a sealed, vented pit with a pump that lifts waste to the building drain. Subsoil drainage, the water collected by foundation drains, goes to a separate sump. A sewage ejector discharges to the sanitary system; a sump discharges to storm or to grade, as the local rules require. Mixing them is one of the most common code violations in older houses and one of the most common distractors on the exam.

Ejector pits are sized so that the pump does not start and stop too often, which wears it out. The working volume between the start and stop levels must hold enough to give a reasonable run time and a minimum time between starts. A practical rule many designers use is that the pump should start no more than about six to ten times an hour at peak inflow. The pump must also handle solids of a specified size, commonly 2 in spheres for sewage ejectors serving water closets.

Ejector discharge piping has its own rules: a check valve and a full-port shutoff valve on the discharge, a connection to the building drain from the top or side rather than into a stack, and a pit vent sized by the pump capacity. The discharge enters the drainage system as a pumped flow, which converts into fixture units for sizing the building drain, as in lesson 4.

Grease interceptors

A café kitchen sends grease to the drain whenever pots and pans are washed. A hydromechanical grease interceptor slows the flow so that grease rises and is held back, and it is rated by the flow it can handle. One common sizing method works from the volume of the sinks it serves: add the volumes of the compartments, take three quarters of that as the actual water in them, and divide by the time allowed to drain them, usually one minute. The result is the interceptor's flow rating, and the next standard size up is selected.

The calculation shows why the drain time assumption matters so much: it doubles or halves the answer.

Indirect waste and air gaps

Some equipment must not connect directly to the drainage system, because a backup could contaminate food, drinks or sterile equipment: ice machines, food preparation sinks in some codes, commercial dishwashers in some arrangements, sterilizers and drinking fountains in certain cases. These drain indirectly, through an air gap or an air break, into a receptor such as a floor sink or a hub drain. An air gap is an open vertical distance between the end of the indirect waste pipe and the flood level rim of the receptor, commonly at least twice the diameter of the pipe and never less than 1 in.

The receptor must be accessible, sized for the flow it receives, and trapped and vented like any fixture. Questions about indirect waste usually ask which equipment needs it, how big the air gap must be, or what the difference is between an air gap, where the pipe ends above the rim, and an air break, where the pipe ends below the rim but above the receptor's trap seal.

A mixed calculator for flow-based problems

Maya's calculator for this lesson combines the storm sizing, the ejector cycle and the interceptor rating. Each part is short, and running them together reminded her that all three start from a flow.

# Flow-based sizing: storm drains, ejector cycles, grease interceptors
import math
SIZES = ["3 in", "4 in", "6 in", "8 in"]
LEADER = [92, 192, 563, 1208]                      # gpm
HORIZ = {"1/8 in/ft": [34, 78, 222, 478], "1/4 in/ft": [48, 110, 314, 677]}

def size(table, q):
    return next(SIZES[i] for i, cap in enumerate(table) if q <= cap)

area, rain, drains = 120 * 90, 4, 4               # ft2, in/h
Q = 0.0104 * area * rain
print(f"Roof flow {Q:.0f} gpm; each leader {Q / drains:.0f} gpm -> {size(LEADER, Q / drains)}")
for n in range(1, drains + 1):
    q = n * Q / drains
    print(f"  storm drain after {n} drain(s): {q:5.0f} gpm -> {size(HORIZ['1/8 in/ft'], q)} at 1/8 in/ft")

inflow, pump, vol = 8, 40, 40                     # gpm, gpm, gal
fill, run = vol / inflow, vol / (pump - inflow)
print(f"Ejector: fill {fill:.2f} min, run {run:.2f} min, {60 / (fill + run):.1f} starts/h")

compartments, l, w, d = 3, 24, 20, 14            # in
water = 0.75 * compartments * l * w * d / 231    # gal
for t in (1, 2):
    print(f"Interceptor, {t}-minute drain: {water / t:.1f} gpm")

Roof flow 449 gpm; each leader 112 gpm -> 4 in
  storm drain after 1 drain(s):   112 gpm -> 6 in at 1/8 in/ft
  storm drain after 2 drain(s):   225 gpm -> 8 in at 1/8 in/ft
  storm drain after 3 drain(s):   337 gpm -> 8 in at 1/8 in/ft
  storm drain after 4 drain(s):   449 gpm -> 8 in at 1/8 in/ft
Ejector: fill 5.00 min, run 1.25 min, 9.6 starts/h
Interceptor, 1-minute drain: 65.5 gpm
Interceptor, 2-minute drain: 32.7 gpm

The storm section prints the size of each section of horizontal drain as the drains join, which is exactly the kind of question that a riser or roof plan on the exam asks: what size is the drain between the second and third leaders?