11.2 Building Drains & Building Sewers

11.2 Building Drains & Building Sewers

Quick Answer: The building drain is the lowest sanitary piping, measured ten feet in developed length from the inside face of the foundation; that ten-foot section is minimum 4 inch (248 CMR 10.03, sized with 10.15(6)(b) and Table 2). The building sewer starts where that drain ends and runs to a public sewer, septic tank, or other disposal point. Table 2 loads a 4-inch building drain at 180 DFU @ 1/8 in/ft, 216 @ 1/4, and 250 @ 1/2. 216 DFU is not a 1/8-inch grade number. A 3-inch building-drain row has no 1/8 column (40 @ 1/4, 48 @ 1/2). No public sewer means an independent 310 CMR 15.00 Title 5 connection, not a shared house-to-house tap.

This is the second sanitary section in the PSI DWV cluster. Candidates lose items by quoting an older “ten feet outside the wall” building-sewer sentence from a withdrawn PDF, by putting 216 DFU on a 4-inch drain at 1/8, or by running a 3-inch building drain through the foundation because Table 2 prints a 3-inch row. Independent OpenExamPrep teaching uses the 12/8/2023 definitions and tables.

Definitions you must not mix

Building drain (248 CMR 10.03). The lowest piping in a drainage system receiving discharge in fixture units from soil, waste, and other drainage piping, conveying that waste to a building sewer, measured ten feet in developed length from the inside face of the foundation wall. The ten-foot section defined as the building drain shall be minimum four inch and sized in accordance with 248 CMR 10.15(6)(b). 10.03 Example 3 is the Board’s picture. A building drain — sanitary conveys plumbing-fixture discharge. A building drain — storm is a different pipe (Chapter 10.3).

Building sewer (248 CMR 10.03). The pipe that begins at the end of the building drain and extends to a public sewer, septic tank, or other place of sewage disposal (Example 4). Combined, sanitary, and storm building sewers are named separately; a sanitary sewer does not take storm water.

Older Mass.gov extracts said the sewer began ten feet outside the inner face. Current 10.03 puts the ten-foot piece on the building drain, measured from the inside face, and starts the sewer at the end of that drain. On the exam, use the current pair: drain = lowest piping including the 10-foot / 4-inch piece; sewer = from that point to disposal.

248 CMR 10.15(6)(a) — no portion of the drainage system installed underground shall be less than two inches. 10.15(6)(b) — sanitary piping that passes through an exterior foundation wall shall be no less than four inches, with listed exceptions: a hazardous-waste system under 10.13; a residential laundry to a Board-of-Health-authorized dry well (minimum 2 inches); baptistries or similar fixtures (minimum 2 inches, see 10.10(11)); and secondary structures / cabanas on residential property discharging into the main building drain or sewer, sized from 10.15 but in no case less than 2 inches. A house building drain through the wall is still 4 inches, even when Table 1 only added 13 DFU.

Invert (248 CMR 10.03). The lowest point inside a pipe upon which water can flow (Example 8). Grade is invert to invert, not trench-bottom to trench-bottom and not crown to crown. If 80 feet of 4-inch building sewer runs at 1/8 in/ft, the invert must fall 10 inches from the end of the building drain to the public-sewer or septic connection. If the street-sewer invert is higher than that math allows, gravity is gone and 10.15(9) ejectors apply (next section).

Table 2 — horizontal branch versus building drain, and the 216 trap

248 CMR 10.15 Table 2 — Maximum Loads in Fixture Units for Horizontal Drains (12/8/2023):

The 3-inch row (and the 3-inch horizontal-branch 34) is footnoted not more than four water closets. 1-1/2, 2, and 2-1/2 inch pipes have no building-drain columns — they are not building drains.

Memorize the 4-inch building-drain triple: 180 / 216 / 250. A load of 216 DFU on a 4-inch building drain belongs at 1/4 in/ft, not 1/8. At 1/8 in/ft the same 4-inch drain stops at 180. 250 is the 1/2 in/ft column. Practice items that call 216 DFU “4-inch at 1/8” contradict Table 2. Teach the table.

Worked example. A Lowell two-family totals 200 DFU on the building drain. At 1/8 in/ft, 4-inch allows 180 — 200 exceeds 180, so 4-inch at 1/8 fails. At 1/4 in/ft, 4-inch allows 216 — 200 fits. A 5-inch drain at 1/8 allows 390 if you keep the flat grade. The ten-foot foundation piece is still minimum 4 inch; the question is whether 4 inch at the chosen slope can carry 200, not whether you may drop to 3 inch.

3-inch building-drain columns exist (40 / 48) but there is no 1/8 in/ft cell. 248 CMR 10.05(2) is why: horizontal drainage 3 inches or smaller gets a minimum uniform pitch of 1/4 in/ft; piping larger than 3 inches may go 1/8 in/ft. 10.05(2)(e) then points back to 10.15 Table 2 for building-drain pitch. A 3-inch building drain at 1/8 is not a Table 2 option. Independently, the 10-foot foundation section is still minimum 4 inch. Do not punch a 3-inch building drain through the wall because 40 DFU “fits” the 1/4 column.

248 CMR 10.05(2)(a)–(d) — run horizontal drainage in straight practical runs at a consistent uniform pitch. Storm or sanitary piping may leave those pitches only when a Massachusetts professional engineer designs a computed discharge velocity of not less than 2 feet per second.

Independent sewer, Title 5, and cleanouts that see the sewer

248 CMR 10.05(15) and 10.02 Principle 16: each building shall have an independent connection to a public sanitary sewer or to a sewage-disposal system that complies with 310 CMR 15.00 (State Environmental Code, Title 5). If there is no public sewer within a reasonable distance, you still do not tee two dwellings onto one private sewer as a field convenience. Title 5 siting, construction, and inspection of on-site systems sit with MassDEP / the local Board of Health; the plumber still provides an independent building sewer to that tank or to the public main.

Cleanouts to the sewer (248 CMR 10.08(3), taught in full in Chapter 7.2). Put an accessible cleanout on the building drain so you have a direct-line shot through the drain to the building sewer. On a slab or with less than 36 inches of crawl, that cleanout may sit outside, not more than 5 feet beyond the foundation wall. Cleanouts on 4-inch or smaller horizontals are not more than 50 feet apart. Every cleanout opens in the direction of flow or at right angles. Size matches the pipe up to 4 inches. The stack-base cleanout is a different trigger; the foundation cleanout is the one that looks at the sewer.

Scenario. A New Bedford slab ranch has a 4-inch building drain turning 90° just inside the wall, then 12 feet to the stack, then the 10-foot section to the sewer. You still owe: 4-inch through the wall, Table 2 slope for the house DFU, invert fall on the sewer to the city tap or Title 5 tank, a foundation cleanout with a straight shot to the sewer (outside within 5 feet is available on a slab), and a cleanout at that 90 on the building drain. Do not size that 4-inch piece from the horizontal-branch column (160). Once it is the building drain, the columns are 180 / 216 / 250.

Official resources

• 248 CMR 10.03 Definitions (Cornell) — building drain, building sewer, invert
• 248 CMR 10.15 Table 2 (Cornell) — 180 / 216 / 250 and the blank 3-inch 1/8 cell
• Selecting the Size of Drainage Piping (UpCodes) — HTML Table 2
• 248 CMR 10.05(2) and 10.05(15) (Cornell) — pitch and independent sewer / Title 5
• 310 CMR 15.00 Title 5 (Mass.gov)