10.3 Storm Drainage

10.3 Storm Drainage

Quick Answer: Storm work on this exam is 248 CMR 10.17, not 10.15. 10.15 sizes sanitary drains in drainage fixture units. 10.17 sizes storm pipe from maximum projected area on Table 1 (horizontal) and Table 2 (vertical conductors and outside leaders). Those tables already assume 4 inches of rainfall per hour — do not invent a second rainfall chart. Keep sanitary and storm entirely separate in the building. If the street is a combined sewer, join them with a single wye in the same horizontal plane at least 10 feet outside the inside face of the foundation. Trap conductors that serve low roofs on that combined system; do not trap an exclusive storm system. A secondary roof drain, when installed, dumps independently and daylight 18 to 60 inches above grade where people can see it.

PSI lists Storm Drain under DWV. Candidates who grew up on IPC Chapter 11 still have the right ideas (conductors, overflow, combined sewers) and the wrong section number. Some UPC-based codes tuck storm into the sanitary chapter. Massachusetts does not. The 12/8/2023 outline is 10.15 Sanitary Drainage System, 10.16 Vents, 10.17 Storm Drains. Independent OpenExamPrep teaching uses that live numbering. It is not a Board publication and it does not claim official approval.

Names: conductor, leader, building storm drain

248 CMR 10.03 (and 10.03 Example 5 / Example 9):

248 CMR 10.17(1) pulls a pipe into the storm chapter when it receives rain or surface water and at any point enters the building or structure, including area drainage and clear-water waste. A leader from an outside scupper that never enters the building is treated as outside that interior storm system (see 10.03 jurisdiction notes on exterior scupper leaders). Once the rainwater enters the structure, you are in 10.17.

Separate inside; combined only where the sewer is combined

248 CMR 10.17(2): storm water shall not be drained into sewers intended for sewage only. That is the separate-storm-sewer town.

248 CMR 10.17(10)(a):

1. The sanitary and storm drainage system of a building shall be entirely separate.
2. Where a combined sewer is available, the building storm sewer may be connected to the building sanitary sewer in the same horizontal plane through a single wye to form a combined building sewer at least 10 feet outside the inside face of the foundation wall.

The 10-foot dimension matches the building drain definition in 10.03 (developed length from the inside face of the foundation). Do not comb a roof conductor into a 3-inch sanitary stack in the basement because “the street is combined.” Combine outside, with a wye, in one horizontal plane.

Offsets (10.17(10)(b)). Offsets of 45° or less from vertical, and offsets of more than 45° that do not exceed 10 feet, use Table 2 (vertical conductor sizing). Offsets of more than 45° that exceed 10 feet use Table 1 (horizontal storm sizing).

Traps (and vents) on combined systems — not on exclusive storm

248 CMR 10.17(9):

A combined building sewer still carries sewage. The storm trap is a sewer-gas device, not a courtesy P-trap. 248 CMR 10.08(2)(e) and 10.02 Principle 9 still require a maintained liquid seal and protection from siphonage, back pressure, and the other listed failures, with venting under 248 CMR 10.16 so a trap is not subjected to more than 1 inch of water pressure differential. Do not skip the storm trap on a combined system because “it is only rain.” Do not add storm traps on a storm-only public system that 10.17(9)(b) says shall not be trapped.

Backwater (10.17(8)). Roof, area, and clear-water piping subject to backflow gets a backwater valve with a trap in an accessible location not subject to freezing. Area drains may get a backwater valve before the storm system; valve materials, diameter, and location follow 10.15(10)(b)–(d).

Sizing method — use the Massachusetts tables, not a homemade rainfall map

248 CMR 10.17(3). Size the building storm drainage system, including all horizontal branches, from maximum projected surface area using 10.17 Table 1. Exception: siphonic roof drainage (below).

248 CMR 10.17(4). Size a vertical storm conductor from maximum projected area using Table 2.

Note 1 on the tables: they are based on a maximum rainfall rate of 4 inches per hour. That is the Massachusetts published method. Do not invent NOAA maps, 100-year charts, or IPC rainfall figures for this closed-book exam. If a later Board policy or PE design uses a different rate, that is an engineered exception — it is not the Table 1/Table 2 journeyman method.

Table 1 — Size of Horizontal Storm Drains (projected area, square feet; 12/8/2023):

A 3-inch horizontal storm drain has no 1/8 inch per foot column — the same kind of gap 10.15 Table 2 shows for a 3-inch sanitary building drain. Do not invent a 3-inch / 1/8-inch storm load.

Table 2 — Size of Vertical Storm Drain Conductors and Outside Leaders (projected area → diameter):

Continuous or semi-continuous flow (10.17(5)). Condensate pumps, ejectors, air-conditioning, or similar clear-water discharges into the building storm drain or storm sewer: each gallon per minute equals 24 square feet of roof area, based on a 4-inch rainfall. Add that equivalent area to the projected roof before you enter Table 1 or Table 2.

Worked example (table values only). Roof 80 ft × 50 ft = 4,000 sq ft. No extra vertical-wall area is published as a 10.17 multiplier — do not import another state’s “half the wall” rule. One 4-inch vertical conductor covers 4,600 sq ft on Table 2, so it is large enough. A 4-inch horizontal storm at ¼ in./ft covers only 2,650 sq ft, so it is too small for 4,000. At ½ in./ft a 4-inch covers 3,760 — still short. A 5-inch at ¼ in./ft covers 4,720 and works. A 20 gpm condensate stream adds 20 × 24 = 480 sq ft; size as 4,480 sq ft instead of 4,000.

Pitch reminder (10.05(2)). Horizontal drainage 3 inches or smaller: minimum ¼ in./ft. Larger than 3 inches: minimum ⅛ in./ft, unless a Massachusetts PE designs a pitch that still produces at least 2 feet per second.

Roof drains, secondary overflow, siphonic, sub-soil

248 CMR 10.17(11). All roof drains shall be Product-accepted. Connection of interior roof drains to the roof shall be watertight with proper flashing. Assemblies serving vehicle parking decks or the outside top level of open parking garages discharge through an independent gas, oil, and sand interceptor under 248 CMR 10.09(1)(b), then to storm or another approved disposal. Flat-deck drains on sun decks, parking decks, and similar maintained areas may be flat and level with the deck, with inlet area not less than two times the conductor area. General-use strainers (from the 12/8/2023 storm text) extend not less than 4 inches above the adjacent roof and provide inlet area at roof level not less than 1½ times the conductor area, except areas that drain to hanging scuppers and gutters.

Secondary roof drainage — code and policy together. 248 CMR 10.17(11)(c) and the Mass.gov Secondary Roof Drainage Systems interpretation (enacted 05-04-2011, amended 01-07-2020) say the same operational rule: when a secondary system is installed, it shall discharge independent of the primary building storm, terminate to daylight not less than 18 inches and not more than 60 inches above grade, in an area visible to occupants, and comply with other federal, state, and local codes. The Board tied that policy to 10.02 Principle 7 (adequate drainage). Secondary is not a second wye into the primary conductor inside the building.

Siphonic roof drainage. 10.17(3) exception and the Mass.gov Siphonic Roof Drain Systems policy (Board vote 10-29-2008, posted 12/07/2017): a Massachusetts licensed professional engineer is responsible for design, including pipe size and dimensions for proper function. The local or State plumbing Inspector inspects the rest of 248 CMR installation and is under no obligation to approve or certify the design. Do not size a siphonic network from Table 1/Table 2 as if it were a gravity conductor.

Building sub-drains (10.17(6)) below gravity storm level discharge into a sump or receiving tank and are automatically lifted into the storm system.

Sub-soil drains (10.17(7)). If subject to backwater, protect with an accessible backwater valve. They may discharge into a properly trapped area drain or sump; those sumps do not require vents. Sub-soil piping is not less than 4 inches. Materials follow 10.06; perforated PVC or open joints are listed; perforated pipe uses sealed joints; use pea stone or similar aggregate so groundwater can reach the pipe; screens on open-joint spigots shall be non-ferrous or other corrosion-resisting material.

Storm hangers still follow 10.11. Storm materials still follow 10.06 Table 1 (including the storm-only fiberglass footnote). Sanitary DFU tables in 10.15 do not size a roof.

Official resources

• 248 CMR 10.17 Storm Drains (Cornell) — Tables 1–2, traps, combined wye, secondary termination
• 248 CMR 10.03 Definitions (Cornell) — conductor, leader, building drain/sewer — storm
• Secondary Roof Drainage Systems (Mass.gov) — 18–60 inches, independent, visible
• Siphonic Roof Drain Systems (Mass.gov) — Massachusetts PE designs; Inspector does not certify design
• 248 CMR 10.15 Sanitary Drainage System (Cornell) — contrast: DFU sanitary sizing, not storm