Study guide
7.2 Plan-Based Sizing Practice: Branches, Stacks, Building Drain, Vents & Water
Alabama Journeyman Plumber Exam
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1 Chapter 1: Alabama Law, General Regulations, Trade Math, Tools & Safety
1.1 Alabama Exam Format, Licensing Law & Board Rules 1.2 IPC Chapter 3 General Regulations: Protection, Sleeves, Trenching & Structural Safety 1.3 Trade Math I: Pressure, Head, Fall, Volume & Expansion 1.4 Trade Math II: Fractions, Offsets, Areas & Heat Calculations 1.5 Plumbing Tools: Layout, Cutting, Joining, Threading & Test Equipment 1.6 Jobsite Safety: Trenches, Ladders, Confined Spaces, Hot Work & PPE
2 Chapter 2: Plumbing Fixtures, Fittings & Clearances
2.1 Minimum Plumbing Facilities, Fixture Clearances & Accessibility 2.2 Fixture Materials, Prohibited Fixtures, Overflows, Waste Outlets & Fixture Setting 2.3 Flushing Devices, Water Conservation & Temperature-Limiting Controls
3 Chapter 3: Water Supply, Distribution, Backflow & Water Heaters
3.1 Water Service & Distribution Sizing: WSFU, Demand, Pressure & Friction 3.2 Water Piping Materials, Joints, Valves, Supports & Disinfection 3.3 Cross-Connection Control & Backflow Prevention (IPC 608 & 312.10) 3.4 Water Heaters: Installation, Relief Valves, Pans & Thermal Expansion
4 Chapter 4: Sanitary Drainage Systems & DWV Sizing
4.1 Drainage Fixture Units & Sizing Branches, Stacks and Building Drains 4.2 Drainage Slopes, Building Drain & Sewer Rules, Offsets and Backwater Valves 4.3 DWV Materials, Fittings, Joints, Supports & Underground Installation 4.4 Indirect Waste, Special Waste, Sumps and Sewage Ejectors
5 Chapter 5: Traps, Cleanouts, Interceptors & Separators
5.1 Fixture Traps, Trap Seals, Prohibited Traps & Trap Seal Protection 5.2 Cleanouts: Required Locations, Size, Arrangement, Clearance & Access 5.3 Grease Interceptors, Oil Separators & Other Interceptors (IPC 1003)
6 Chapter 6: Vents and Venting Methods
6.1 Vent Purpose, Vent Stacks, Connections, Terminals & Vent Sizing 6.2 Fixture Vents, Individual Vents, Common Vents & Wet Venting 6.3 Circuit Vents, Combination Waste & Vent, Island, Waste Stack & Single-Stack Venting 6.4 Air Admittance Valves: Standards, Where Permitted, Location & Prohibitions
7 Chapter 7: Plan Reading, Sizing Practice, Testing & Fuel Gas
7.1 Reading Plumbing Drawings, Isometrics, Symbols & Schedules 7.2 Plan-Based Sizing Practice: Branches, Stacks, Building Drain, Vents & Water 7.3 Inspections and Tests: DWV, Water, Sewers, Storm, Shower Pans & Backflow 7.4 Fuel Gas Basics, Combustion Air, Venting & Alabama Gas Fitting Scope
8 Chapter 8: Roof Drain & Storm Drainage Systems
8.1 Roof Drains, Rainfall Rates & Storm Drain Sizing (IPC 1101–1106) 8.2 Secondary (Emergency) Roof Drainage, Scuppers, Controlled Flow, Subsoil Drains & Storm Sumps
7.2 Plan-Based Sizing Practice: Branches, Stacks, Building Drain, Vents & Water
Key Takeaways Explain This Topic AI
Size horizontal fixture branches and stacks with Table 710.1(2), and size building drains and sewers with Table 710.1(1) at the installed slope.
A 3-inch stack carries no more than 20 dfu in any one branch interval, even if its total load is under 48 dfu, so a single floor discharging 28 dfu forces a 4-inch stack.
A 4-inch stack with three or fewer branch intervals carries up to 240 dfu total and 90 dfu per interval.
In Table 906.1, a 4-inch stack carrying up to 140 dfu allows a 2-inch vent to 27 feet, a 2-1/2-inch vent to 65 feet and a 3-inch vent to 200 feet.
A public flushometer water closet is 10 wsfu but only 4 dfu (1.6 gpf), so water sizing and drain sizing use different fixture values from different tables.
Last updated: October 2026
Why this section matters
Sizing questions combine several tables in one problem. Work every problem in the same order , and you will avoid the classic mistakes: the wrong table, the wrong column, or skipping the stack's per-interval limit. The exam allows about 2.3 minutes per question (75 questions in 2.92 hours), so a fixed routine matters.
The routine
Read the schedule. Note the fixture types, public or private, flush volume, and flush valve or tank.
Assign values. Use Table 709.1 for dfu, and Appendix E Table E103.3(2) for wsfu.
Add up the load only for the fixtures upstream of the pipe in question.
Choose the table:
Horizontal branch or stack: Table 710.1(2) .
Building drain or sewer: Table 710.1(1) at the slope.
Stack vent or vent stack: Table 906.1 .
Other vents: 906.2 (half the drain size, plus one size over 40 feet).
Water: E103.3(3) demand, then the friction charts.
Apply minimums: 3 inches for any building drain serving a water closet, trap sizes from Table 709.1, and 3/4 inch for a water service.
The building
Two-story office. Public fixtures, 1.6-gpf water closets on flushometer valves, 1-gpf urinals on 3/4-inch flushometer valves.
- Floor Fixtures dfu
- 2, men 2 water closets (4 each), 1 urinal (2), 2 lavatories (1 each) 12
- 2, women 3 water closets (4 each), 2 lavatories 14
- 2, break room 1 kitchen sink (2) 2
- Floor 2 total 28
- 1, men 2 water closets, 2 urinals, 2 lavatories 14
- 1, women 3 water closets, 2 lavatories 14
- 1, break room 1 kitchen sink (2), 1 drinking fountain (1/2) 2.5
- Floor 1 total 30.5
- Building total 58.5
- Horizontal branches (Table 710.1(2))
- Branch Load Smallest size that works Size
- Floor 2 men 12 dfu, water closets 3 in (20 dfu) 3 in
- Floor 2 women 14 dfu, water closets 3 in (20 dfu) 3 in
- Break room sink 2 dfu 1-1/2 in (3 dfu) 1-1/2 in
- Floor 1 men 14 dfu 3 in 3 in
- Floor 1 women 14 dfu 3 in 3 in
- Break room and fountain 2.5 dfu 1-1/2 in (3 dfu) 1-1/2 in
The 2021 IPC does not limit how many water closets a 3-inch branch may carry. The 20-dfu limit governs. If the restrooms on each floor joined into a single floor branch before reaching the stack, that branch would carry 28 or 30.5 dfu and would need 4 inches (160 dfu).
The stack (Table 710.1(2))
The stack has two branch intervals.
Total: 58.5 dfu. A 3-inch stack with three or fewer intervals allows 48, so it fails .
Per interval: Floor 2 discharges 28 dfu and Floor 1 discharges 30.5 dfu. A 3-inch stack allows 20 per interval, so it fails on that test too.
4 inches: 240 total and 90 per interval. Both pass. Use a 4-inch stack .
Variation. Suppose the total were only 38 dfu, all entering from one floor. The total fits a 3-inch stack (48), but one interval at 38 exceeds 20, so the stack would still be 4 inches . Always check both columns.
Building drain (Table 710.1(1))
The drain carries 58.5 dfu.
- Slope 3-inch capacity 4-inch capacity Size
- 1/8 in/ft 36 180 4 in
- 1/4 in/ft 42 216 4 in
Remember not to use the horizontal-branch column for a building drain. A 3-inch building drain at 1/4 inch per foot carries 42 dfu, not 20.
Stack vent (Table 906.1)
The 4-inch stack carries 58.5 dfu, and the stack vent's developed length is 45 feet. Use the 4-inch, 140-dfu row, the first row at or above 58.5 dfu.
Vent size Maximum length (4-in stack, 140 dfu) 45 ft OK?
- 2 in 27 ft No
- 2-1/2 in 65 ft Yes
- 3 in 200 ft Yes
The minimum is 2-1/2 inches , which is also at least half of 4 inches. Many contractors use 3 inches because 2-1/2-inch DWV fittings are scarce. The building has only two branch intervals, so no separate vent stack is required (904.2).
Water supply (Appendix E)
Floor 2 water load, using public values from Table E103.3(2):
- Fixture Count × wsfu Total
- Water closet, flushometer valve 5 × 10 50
- Urinal, 3/4-in flushometer valve 1 × 5 5
- Lavatory, public 4 × 2.0 8
- Kitchen sink (private-type break room) 1 × 1.4 1.4
- Floor 2 64.4
Demand (Table E103.3(3), flushometer column): 60 wsfu = 54 gpm and 70 wsfu = 58 gpm, so about 55.8 gpm . The flush-tank column would give only about 33 gpm for the same wsfu, which shows why using the wrong column undersizes flush-valve systems. Each flush valve still needs a 1-inch fixture supply (Table 604.5) and 35 psi of flow pressure (siphonic, Table 604.3).
Time management
Answer only what is asked. If the question is about one branch, total only its upstream fixtures.
Tab the four tables you use most: 709.1, 710.1(1), 710.1(2) and 906.1. Add E103.3(2) and E103.3(3) for water.
Write the load at each node on scratch paper as you trace the drawing.
Check both stack columns every time.
Exam traps
Horizontal branches use Table 710.1(2) , not Table 710.1(1).
A public 1.6-gpf water closet is 4 dfu but 10 wsfu on a flush valve.
Per-interval stack limits often decide the stack size.
Read vent rows at or above the actual dfu, then the first length that covers the run.