11.3 Horizontal Branches, Stacks & Offsets

11.3 Horizontal Branches, Stacks & Offsets

Quick Answer: Use Table 2’s horizontal-branch column for laterals (a 4-inch branch is 160 DFU; a 3-inch branch is 34 DFU, not more than four water closets). Use Table 3 for stacks of one or two branch intervals (a 3-inch stack is 48 DFU, still not more than four water closets; a 4-inch stack is 240 DFU). A branch interval is a story-height run on the stack, not less than eight feet (10.03). Offsets ≤ 45° from vertical size as a straight stack; offsets > 45° size the offset from Table 2. Do not install a double sanitary tee on a horizontal. Hydraulic jump at the stack base is why 10.05 wants wyes and long-sweep quarter bends, not a straight T.

This section is the rest of PSI sanitary: the pipe that is not the 10-foot building drain. Independent teaching uses 248 CMR 10.15 Tables 2–5, 10.03 stack and interval definitions, and 10.05 / 10.15(8) fittings. Vent sizing stays in 10.16 (later chapters).

Horizontal branch column versus building-drain columns

248 CMR 10.03 — horizontal branch drain: a drain branch that extends laterally from a soil or waste stack or a building drain, that may have vertical sections, that receives one or more fixture drains, and that conducts the discharge to the stack or to the building drain. Horizontal pipe means any pipe or fitting at less than 45° to the horizontal plane.

Table 2 therefore prints two jobs on one page. The first load column is horizontal branch drain and has no slope sub-columns. The next three columns are building drain at 1/8, 1/4, and 1/2 in/ft. Same 4-inch iron or PVC, different legal loads:

A 170 DFU load is illegal on a 4-inch branch and legal on a 4-inch building drain at 1/8. A 190 DFU load needs a 5-inch branch (360) or it must be the building drain at 1/4 (216) or steeper. Do not “borrow” the 180 or 216 numbers for a second-floor battery waste.

3-inch comparison: branch 34 DFU; building drain 40 / 48 at 1/4 and 1/2; no 1/8 building-drain cell; not more than four water closets on that 3-inch row. 1-1/2 / 2 / 2-1/2 exist only as branches (3 / 6 / 12 DFU).

248 CMR 10.15(4): no soil or waste stack shall be smaller than the largest horizontal waste branch connected to it. A 4-inch branch into a stack makes a 4-inch stack even if Table 3’s load number would have allowed 3 inches. Exception language in older drafts for a 4 × 3 toilet connection is not a license to reduce the stack below the largest branch that actually joins it — read the live 10.15(4) sentence.

Kitchen and laundry arrangement (10.15(8)). Residential kitchen branch not less than 1-1/2 inches, connecting independently. A kitchen sink shall not waste into any horizontal drain smaller than 3 inches that already receives a flat-bottom fixture. Rough high enough for a disposer. Full-size cleanout under all kitchen sinks, except a two-piece trap that can be taken apart. Where laundries are installed on more than three branch intervals, those wastes go to an independent laundry stack and an independent laundry main drain (Example 4).

Table 3 stacks, branch intervals, and Table 4

Branch interval (248 CMR 10.03): a distance along a soil or waste stack corresponding in general to a story height, but not less than eight feet in vertical height, wherein the horizontal branches from one floor connect to the stack (Example 2). Two floors 7 feet apart are not two full intervals. Stack means a vertical soil, waste, vent, or inside conductor that extends beyond at least one branch interval in height. Stack vent begins six inches above the highest flood-level rim on that stack.

248 CMR 10.15 Table 3 — Maximum Loads in Fixture Units for Soil and Waste Stacks Having One or Two Branch Intervals:

Footnote on the 3-inch stack: no more than four water closets. Five tank toilets are 20 DFU, which is under 48, and still illegal on a 3-inch Table 3 stack because of the closet count. Four flush-valve toilets are 24 DFU and four closets — load and footnote both pass Table 3; the horizontal branch that collects them may still fail the 34 DFU / four-closet 3-inch branch cap (Chapter 11.1 example at 36 DFU).

Table 4 is Maximum Loads in Fixture Units for Any One Branch Interval on Multistory Soil and Waste Stacks, for buildings with 15 or fewer branch intervals. It is not Table 3. Verified 12/8/2023 excerpt (per-interval load at 3 intervals → 15 intervals, then maximum total stack load):

Table 4’s 3-inch footnote is no more than three water closets. On a 3-inch stack taller than two intervals, the closet cap tightens from four to three, and at three intervals any one floor may dump only 20 DFU into that stack. Buildings with more than 15 intervals use the printed formula with N = Table 3 load for that diameter and n = interval count — look up the formula on the code page rather than inventing a coefficient. 10.16(2)(a) separately requires buildings with three or more branch intervals to have at least one main vent stack sized from the vent table; that is venting, not Table 3.

Offsets, hydraulic jump, stack-base fittings, no horizontal double tee

248 CMR 10.15(7) offsets.

1. 45° or less from vertical. Size as a straight vertical stack. If a horizontal branch hits the stack within two feet above or below that offset, install a relief vent per 10.16(2)(d) (Example 1).
2. More than 45° from vertical. Size the offset as a horizontal drain from Table 2. The stack above the offset uses Table 3 for the DFU above the offset. The stack below uses the entire stack load. In buildings of five stories or more, a relief vent is required per 10.16(2)(d). No horizontal branch may connect to the offset or to the stack within two feet above or below that offset (Example 2).
3. Offset above the highest branch is a stack-vent offset; treat it only as it affects vent developed length.
4. Offset below the lowest branch: less than 45° — no diameter change required; greater than 45° — size the offset and the stack below from Table 2.

Worked offset. A two-story stack carries 40 DFU, all above a 60° offset, then drops to the building drain. The piece above is Table 3: 40 < 48, so 3-inch could carry the vertical load, but not more than four closets. The offset itself is a horizontal drain. Table 2’s 3-inch branch is 34 DFU — 40 exceeds 34, so the offset is 4-inch (branch 160, or building-drain columns if that run is truly the building drain). Do not keep 3-inch through a more-than-45° offset just because Table 3 said 48.

Hydraulic jump. When a vertical stack dumps into the horizontal building drain, the fast stream hits the invert, goes from supercritical to slower flow, and the water surface rises — a hydraulic jump — so the drain can run full for a short distance and shove air toward nearby traps. Massachusetts does not print a “ten pipe diameters” jump table. It does print the fittings that keep that jump from becoming a dry trap: 248 CMR 10.15(8)(a) sends pitch, direction-change fittings, prohibited fittings, heel inlets, and obstructions to 10.05. 10.05(3) allows wyes, long-sweep quarter bends, fifth, sixth, eighth, or sixteenth bends for drainage direction change. A quarter bend may turn flow from horizontal to vertical. Tees and crosses are vent fittings only. 10.05(4)(a)1. — no fitting that incorporates a straight T branch shall be used as a drainage fitting. 10.08 still wants a cleanout at or near the base of each soil or waste stack so you can rod the jump zone. Stack-base practice that matches those rules is a combination wye and 1/8 bend, two 45s, or a long-sweep 90 — not a short sanitary tee laid as the stack heel.

Back-to-back single fitting (10.15(8)(b)). Horizontal: install with double sanitary wye fittings only, to eliminate throw over. Vertical: double sanitary drainage-pattern tee fittings (Example 2). Exception: water closets may use double wye fittings if the closet manufacturer’s instructions require it (Example 3). A double sanitary tee on its back on a horizontal soil branch is the classic throw-over failure: one closet’s discharge washes into the opposite closet. Use the wye on the horizontal. 10.16 common-vent language that mentions a double sanitary tee is the vertical same-level pattern, and it points back to 10.15(8)(b).

Ejectors — Table 5 in brief

248 CMR 10.05(13) sends drainage below sewer level to 10.15(9). The drain discharges into a tightly covered and vented sump; automatic pumping lifts it into the gravity system. Single-family toilet sumps: full-size discharge, minimum 20 GPM. Table 5 (GPM versus toilets per ejector): 1 toilet → 20; 2–3 → 75; 4–5 → 100; then 125 / 150 / 200 / 250 / 300 / 350 / 375 up through 31–35 toilets. If fixtures other than toilets exceed four times the toilet count, add 3 GPM per extra fixture. Installations that do not meet Table 5 are Massachusetts PE designs. The 2 DFU per gpm rule in 11.1 still applies to sewage pumps except Table 5 pumps at a maximum of 20 GPM. That is enough ejector for this sizing chapter; do not treat Table 5 as a substitute for Table 1 fixture values on a gravity stack.

Official resources

• 248 CMR 10.15 (Cornell) — Tables 2–5, offsets, 10.15(8) fittings, ejectors
• Selecting the Size of Drainage Piping (UpCodes) — Table 2 branch column, Table 3
• 248 CMR 10.03 (Cornell) — branch interval, stack, horizontal branch, invert
• 248 CMR 10.05(3)–(4) (Cornell) — wyes, long-sweep bends, no straight-T drainage fittings