248 C.M.R. 10.17 – Storm Drains

(1) Storm Water Drainage. Any pipe receiving the discharge from rain or surface water which at any point enters the building or structure shall be considered storm water drainage piping including, but not limited to, area drainage and clear water waste.
(2) Storm Water Drainage to Sewer Prohibited. Storm water shall not be drained into sewers intended for sewage only.
(3) Size of Building Storm Drain. The size of the building storm drainage system including all horizontal branches shall be based upon the maximum projected surface area to be handled according to 248 CMR 10.17(2): Table 1: Size of Horizontal Storm Drains.
Exception: Siphonic Roof Drainage Systems. (a) A Massachusetts professional engineer is, per 248 CMR, responsible for the design of the symphonic roof drainage system. (b) As part of the design process, the Massachusetts professional engineer shall be responsible for assuring that the piping installation, including pipe sizing, dimension, and other aspects, meet the requirements for proper functioning as designed. (c) The Local or State plumbing Inspector shall be responsible for all other aspects of the installation, as required by 248 CMR but is under no obligation to approve or otherwise involve themselves in the design process or ensuring the System meets the design specifications.
Table 1
Size of Horizontal Storm Drains
Maximum Projected Area for Storm Drains of Various Slopes
Diameter of
c Slope ¼ inch Slope ½ inch Slope
Drain in Inches
Square Feet Square Feet Square Feet
3 1,160 1,644 4 1,880 2,650 3,760 5 3,340 4,720 6,680 6 5,350 7,550 10,700 8 11,500 16,300 23,000
10 20,700 29,200 41,400 12 33,300 47,000 66,600 15 59,500 84,000 119,000

Note 1: Table 1 is based upon a maximum rate of rainfall four inches per hour.

(4) Vertical Storm Conductor. A vertical storm conductor shall be based upon the maximum projected area to be drained according to 248 CMR 10.17(2): Table 2: Size of Vertical Storm Drain Conductors and Outside Leaders.
Table 2
Size of Vertical Storm Drain Conductors and Outside Leaders
Maximum Diameter of Storm Maximum Diameter of Storm Projected Conductor or Projected Conductor or Area Outside Leader Area Outside Leader
(Square Feet) (Inches) (Square Feet) (Inches) 720 2 8,650 5 1,300 2½ 13,500 6 2,200 3 29,000 8 4,600 4
(5) Values for Continuous Flow. Where there is a continuous or semi-continuous discharge into the building storm drain or building storm sewer, as from a condensate pump, ejector, air conditioning equipment, or similar device discharging clear water waste, each gallon per minute of such discharge shall be computed as being equivalent to 24 square feet of roof area, (based upon a four-inch rainfall.)
(6) Building Sub-drains.
1. (a) Building sub-drains located inside the building below the gravity storm water drainage piping level shall discharge into a sump or receiving tank.
2. (b) The contents of the sump or receiving tank shall be automatically lifted and discharged into the storm drainage system as required for building sumps.
(7) Sub-soil Drains.
1. (a) When a subsoil drain for a building is subject to backwater:
  1. 1. An accessibly located backwater valve shall protect the subsoil drain.
  2. 2. Sub-soil drains may discharge into a properly trapped area drain or sump.
  3. 3. Such sumps do not require vents.
  4. 4. Piping used for sub-soil drains shall not be less than four inches in diameter.
2. (b) Materials for sub-soil drains shall comply with 248 CMR 10.06 and the following requirements shall be satisfied.
  1. 1. Piping may be either perforated PVC or installed with open joints.
  2. 2. Spigot end lengths shall have joints protected with screens securely fastened to pipes.
  3. 3. Screens and fastenings shall be non-ferrous or other approved corrosion resisting material.
  4. 4. Perforated piping shall be installed with sealed joints.
  5. 5. All sub-soil drain piping shall be installed with sufficient pea stone or similar aggregate to permit the flow of ground water to the piping.
(8) Storm Drainage Subject to Backflow.
1. (a) All roof, area and clear water waste piping subject to backflow shall be provided with a backwater valve with a trap installed in an accessible location that is not subject to freezing. See 248 CMR 10.17: Example 1.
2. (b) A backwater valve may be installed on area drains prior to connecting to the storm system. Refer for 248 CMR 10.15(10)(b), (c) and (d) for materials, diameter, and location.
  Example 1

Backwater Valves for Roof, Area & Clear Water Waste Subject to Backflow

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(9) Traps on Storm Drains and Leaders.
1. (a) Where Required. Conductors and storm drains serving low roofs when connected to a combined storm and sanitary system shall be trapped.
2. (b) Where Not Required. No traps shall be required for storm water drains that are connected to a system carrying storm water exclusively.
3. (c) Trap Size. Traps for individual conductors shall be the same size as the horizontal drain
to which they are connected.
1. (d) Method of Installation.
  1. 1. Individual storm water traps shall be installed on the storm water drain branch serving each conductor, or a single trap shall be installed in the main storm drain just before its connection with the combined building, sewer main, drain, or public sewer.
  2. 2. Conductor traps shall be located so that an accessible cleanout may be installed on the building side of the trap.
(10) Conductors/Leaders and Connections.
1. (a) Combining Storm with Sanitary Drainage.
  1. 1. The sanitary and storm drainage system of a building shall be entirely separate.
  2. 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 fitting to form a combined building sewer at least ten feet outside the inside face of the foundation wall.
2. (b) Offsets.
  1. 1. Offsets of 45E or less from the vertical, and offsets of more than 45E from the vertical that do not exceed ten feet in length, shall be sized according to 248 CMR 10.17(2): Table 2: Size of Vertical Storm Drain Conductors and Outside Leaders.
  2. 2. Offsets of more than 45° from the vertical in excess of ten feet shall be sized according to 248 CMR 10.17(2): Table 1 : Size of Horizontal Storm Drains.
(11) Roof Drains.
1. (a) All roof drains shall be product-accepted.
2. (b) Roof Drain Assemblies.
  1. 1. General Use. Roof drain assemblies that serve vehicle parking decks or that serve the outside top level of open parking garages shall convey storm discharge to an independent gas, oil, and sand interceptor/separator in accordance with 248 CMR 10.09(1)(b) and shall discharge to the storm drainage system or other approved method of disposal.
  2. 2. Flat Decks. Roof drain assemblies for use on sun decks, parking decks, and similar areas, normally serviced and maintained, may be of the flat surface type, level with the deck and shall have an available inlet area not less than two times the area of the conductor to which the drain is connected.
  3. 3. Roof Drain Flashings Required. The connection between roofs and roof drains which pass into the interior of the building shall be made watertight using proper flashing methods and material.
3. (c) When a secondary roof drainage system is installed, it shall discharge independent of the primary building storm system and shall terminate the building at a minimum of 18” and a maximum of 60” above grade in an area that will be visible to the people who occupy the building. This system shall be compliant with all federal, state, and local codes.