16.2 DWV Isometric Takeoff

Takeoffs without a code book

Quick Answer: On a Part II isometric, assign 248 CMR 10.15 Table 1 DFUs, then size the horizontal branch (Table 2 left column), the stack (Table 3 for one or two branch intervals), and the building drain (Table 2 pitch columns). A 4-inch building drain holds 180 DFU at 1/8 in/ft and 216 DFU at 1/4 in/ft. A 3-inch building drain has no 1/8 column (40 DFU at 1/4, 48 at 1/2, max four water closets), and the 10-foot drain through the foundation is 4 inches minimum. Then fail trap arms, wet vents, and the illegal fitting (tee on its back, crown vent, too-long arm).

This is the 20-item question style: one drawing, several code checks, one best answer. The sanitary-drainage chapter already taught Table 1–3 in lecture form. Here you use those numbers on a picture. Do not reopen that lecture. Keep a closed-book memory sheet in your head.

Memory sheet: DFU, pitch, trap arm, stack

Massachusetts does not use the IPC private water-closet value of 3 DFU. If you trained out of state, overwrite that number with 4 for a tank-type toilet.

Pitch from 248 CMR 10.05(2): 3-inch and smaller horizontal drainage ¼ inch per foot minimum; larger than 3 inches ⅛ inch per foot minimum. That is why Table 2 leaves the 3-inch building-drain 1/8 cell blank. Even if a 3-inch drain could carry the DFU at ¼ inch pitch, sanitary piping through an exterior foundation wall is not less than 4 inches except the listed 10.15(6)(b) exceptions (hazardous waste, certain laundry dry-wells, baptistries, grinder-pump discharge, small secondary structures).

248 CMR 10.08 Table 1 sets a common lavatory trap at 1½ inches, so the lavatory arm uses the 5-foot row even though Table 1 DFU describes a 1¼-inch outlet. A water closet’s horizontal run to the vent fitting uses the 3-inch / 8-foot row unless the arm is 4 inches.

Wet vent, one sentence: in a bathroom with a 2-inch horizontal waste, a 2-inch extension serving the lavatory may wet-vent the fixtures it serves; the wet-vent pipe breaks the centerline of the drain; kitchen sinks, disposers, dishwashers, washing machines, and culinary fixtures are not wet-vent piping (10.16(5)).

Takeoff 1 — one-bath dwelling with kitchen

The drawing (described). One story. Bathroom: tank-type WC, lavatory, tub. Kitchen: residential sink and residential dishwasher. Gas water heater in a closet (ignore gas DFU). 3-inch soil stack, one branch interval. 4-inch building drain at ⅛ inch per foot through the foundation, 10 feet developed length, cleanout at the wall. Bathroom uses a 2-inch horizontal waste; the lavatory’s 2-inch continuation is the wet vent and ties into the 3-inch stack vent 6 inches above the flood rim. Kitchen wastes independently into the 4-inch building drain through a combination wye. Lavatory trap arm labeled 4 ft on centerline. WC horizontal labeled 5 ft of 3-inch pipe to the stack fitting.

Step 1 — DFU.

• WC tank type: 4
• Lavatory: 1
• Tub: 2
• Kitchen sink: 2
• Residential dishwasher: 1
• Total: 10 DFU

The water heater has no drainage fixture unit in Table 1 unless a pan drain or relief receptor is drawn as a floor drain — it is not.

Step 2 — bathroom branch. Bathroom load = 4 + 1 + 2 = 7 DFU on a 2-inch wet-vented horizontal. Table 2 allows 6 DFU on a 2-inch horizontal branch. 7 exceeds 6. The 2-inch bath waste is undersized as a horizontal branch even though wet-vent language wants 2 inches. The fix the code wants is a 3-inch horizontal soil for the water closet with the 2-inch wet vent serving the tub and lavatory, or a 3-inch bath branch (34 DFU). On the exam, if the drafter labeled the entire bath run 2 inches including the closet, that is the defect. If the closet is 3 inches and only the lavatory/tub run is 2 inches carrying 3 DFU (lav 1 + tub 2), the 2-inch branch is legal (3 ≤ 6).

Read the sizes. This takeoff assumes the closet is 3 inches into the stack and the 2-inch line carries only lavatory and tub (3 DFU). Legal.

Step 3 — kitchen. 2 + 1 = 3 DFU on a 1½-inch fixture drain into a 3-inch kitchen waste (flat-bottom rule). 3 DFU is within the 3-inch branch allowance of 34. Independent connection: legal.

Step 4 — stack. One branch interval, 10 DFU, largest branch 3 inches. Table 3: 3-inch stack 48 DFU, four WC max. One WC. Stack 3 inches is legal. Stack must not be smaller than the largest horizontal branch — both 3 inches.

Step 5 — building drain. 10 DFU. 4-inch at ⅛ inch per foot allows 180 DFU. The 10-foot foundation piece is 4 inches as required. Do not “save pipe” by calling a 3-inch building drain: Table 2 has no ⅛-inch column for 3 inches, 10.05 requires ¼ inch pitch on 3-inch pipe, and 10.15(6)(b) still wants 4 inches through the wall.

Step 6 — trap arms and wet vent. Lavatory 4 ft < 5 ft. WC 5 ft of 3-inch < 8 ft. Wet vent is 2-inch lavatory extension on a 2-inch bath waste, breaks centerline, no kitchen on it. Kitchen is individually vented. Legal.

Verdict. Sizes work if the closet is 3 inches. The item’s trap is usually elsewhere — a sanitary tee on the horizontal kitchen connection, or a wet vent that never breaks centerline. If both fittings are correct, the answer may be “the installation complies.”

Takeoff 2 — two-story wet-vented baths plus laundry

The drawing. Two stories, two branch intervals (each story ≥ 8 feet). Each floor: tank-type WC, lavatory, shower. First-floor laundry connection (3 DFU) on a 2-inch waste into the stack. First-floor kitchen sink (2 DFU) on its own 3-inch branch to the building drain. Stack labeled 3 inches all the way, long-sweep into a 3-inch building drain at ⅛ inch per foot, 12 feet to the foundation, no increaser.

DFU by floor.

Each bathroom group: 4 + 1 + 2 = 7 DFU. Two groups = 14. Laundry 3. Kitchen 2. Total 19 DFU. Two water closets.

Horizontal branches. Each 3-inch bath branch at 7 DFU < 34, two WCs on the whole system, not four on one branch. Laundry 3 DFU on 2-inch < 6. Kitchen 2 DFU on 3-inch, independent of the bath — legal as a kitchen waste.

Stack. Table 3 (one or two branch intervals): 3-inch = 48 DFU, four WC max. 19 DFU and two WCs — stack DFU is fine. But the stack cannot be smaller than the largest horizontal branch; 3-inch branches into a 3-inch stack is consistent.

Building drain — this is the fail. The drafter kept 3 inches at ⅛ inch per foot through the foundation.

• Table 2: 3-inch building drain, ⅛ in/ft column is blank.
• 10.05(2)(b): 3-inch horizontal drainage minimum ¼ inch per foot.
• 10.03 definition and 10.15(6)(b): the 10-foot building drain through the foundation is minimum 4 inches.

Even though 19 DFU is less than 40 DFU (the 3-inch, ¼-inch-pitch number), the isometric is illegal for three independent reasons. On a four-option item, the credited defect is usually the missing 4-inch foundation drain or the ⅛-inch pitch on 3-inch pipe. Do not pick “19 DFU exceeds Table 2” — it does not, at ¼ inch pitch, and that answer is a trap for people who memorized IPC 42 DFU or UPC 3-inch values.

Wet vents. Each bathroom’s 2-inch lavatory wet vent is on its own floor. 248 CMR 10.03 defines a wet vent as serving fixtures on the same floor level. Do not wet-vent the second-floor water closet through the first-floor lavatory. If the isometric shows a 2-inch pipe rising from the first-floor lavatory all the way to the second-floor WC with no dry vent on floor two, that is a second defect. Takeoff 2’s intended miss is the 3-inch, ⅛-inch building drain.

Takeoff 3 — find the illegal fitting (the 20-item style)

The drawing. Same one-bath-plus-kitchen load as Takeoff 1 (10 DFU), 3-inch stack, 4-inch building drain at ⅛ inch per foot. Three callouts the stem asks you to judge:

1. Kitchen 3-inch horizontal joins the 4-inch building drain with a sanitary tee laid on its back (run and branch both horizontal).
2. Lavatory vent takes off the top of the 1½-inch trap, dimensioned 2 inches from the weir (the trap is 1½-inch pipe, so two diameters = 3 inches; 2 inches is inside two diameters).
3. Tub trap arm is 6 feet of 1½-inch pipe to the wet-vent fitting.

Fitting law (248 CMR 10.05). Drainage changes direction with wyes, long sweeps, and similar. No fitting with a straight T branch is a drainage fitting. Quarter bends may turn horizontal to vertical. A sanitary tee is a drainage-pattern tee meant to take a horizontal branch into a vertical stack (flow turning down). Rotate that same tee so it lies on its back in a horizontal-to-horizontal join and you have a sharp 90-degree horizontal turn — illegal. Horizontal kitchen into horizontal building drain needs a wye, combination wye, or wye plus eighth bend.

Back-to-back fixtures: horizontal pairs use double sanitary wyes to kill throw-over; vertical pairs use double sanitary drainage tees (10.15(8)(b)). That is the opposite of “any tee is fine if it says sanitary.”

Crown vent (248 CMR 10.16(1)(b) and 10.08(2)(c)). No vent within two pipe diameters of the trap weir. Crown-vented traps are a prohibited trap type. The lavatory takeoff 2 inches from the weir on 1½-inch pipe is a crown vent. The wet vent should come in on the horizontal drain, breaking the centerline, not off the crown of the P-trap.

Too-long trap arm. 1½-inch maximum developed length is 5 feet. The tub arm at 6 feet fails Table 1. (A 2-inch tub arm would be allowed 6 feet — read the size. Here it is 1½ inches.)

How PSI writes the question. The stem often asks for the defect, or which statement is correct, not “list all three.” If the options isolate one defect, pick the option that names a real 248 CMR violation. If the options are “tee on back,” “4-inch drain too small,” “10 DFU needs a 4-inch stack,” and “kitchen needs a 2-inch vent through the roof,” then tee on its back is the only true statement. The 4-inch drain at 180 DFU capacity is not too small. The 3-inch stack at 48 DFU is not too small. Kitchen vent size is a different check.

When two defects are real, read the question: “Which fitting is prohibited?” points at the tee. “Which trap-arm dimension fails?” points at the 6-foot 1½-inch tub. “Which vent is prohibited?” points at the crown vent. The drawing is the same; the stem picks the scoring target.

Stack offsets and cleanouts you will still see

If the stack doglegs more than 45 degrees, size the offset as a horizontal drain (Table 2) and the stack above from Table 3 (10.15(7)). A 45-degree or less offset stays on Table 3, with a relief vent if a horizontal branch lands within 2 feet of the offset.

Cleanouts: 50 feet on horizontal drainage 4 inches or less, 100 feet over 4 inches; at or near the base of each stack; building-drain cleanout looking in a direct line through the drain to the sewer (10.08). If the isometric omits the stack-base cleanout, that can be the entire item.

Work every remaining isometric the same way: DFU, branch, stack, building drain, trap arms, wet vent, then the dirty fitting. That is the closed-book takeoff.