11.1 Part 2 Explained: Reading the Waste and Vent Drawing

What L&I and PSI Say About Part 2

You will have Table 702.1 on screen, but you need Table 703.2, the Table 703.2 notes, the half-drain vent rule, Table 1002.2 trap arm limits, and the aggregate venting rule from memory. The exam is based on the 2015 UPC with Washington amendments.

The Drawing

The drawing is a riser or isometric-style view. The piping is drawn so that vertical pipes look vertical and horizontal runs are drawn at angles, letting you see every floor's fixtures and piping at once. Common conventions:

• Solid lines are drainage (waste and soil). Dashed or lighter lines are vents. Always check the drawing's legend.
• Fixtures are labeled with abbreviations: WC (water closet), LAV (lavatory), UR (urinal), FD (floor drain), MS or SS (mop or service sink), KS (kitchen sink), DW (dishwasher), CW (clothes washer), BT (bathtub), SH (shower), DF (drinking fountain).
• Pipe segments are usually marked with letters or numbers at points where fixtures join. Questions ask for the fixture units carried by, or the minimum size of, a named segment.
• Cleanouts (CO) appear at the ends of horizontal runs, at the stack base, and near the building drain's exit.
• The drawing states whether fixtures are private, public, or assembly use. That choice changes the DFU values in Table 702.1.

A Reading Method That Prevents Mistakes

1. Read the title block and notes. Find the occupancy type (private or public), the flush volumes (1.6 gpf or more), any slope other than 1/4 inch per foot, and the building sewer or point of disposal.
2. Identify every fixture and write its Table 702.1 DFU value and minimum trap size next to it.
3. Trace the flow. Start at the most remote fixture on the top floor and follow the water downhill to the building drain. Waste always flows from trap to trap arm to branch to stack to building drain.
4. Label each segment as H (horizontal) or V (vertical). Table 703.2 has different limits for each.
5. Count water closets in each segment. Note 3 bars water closets from 2-inch pipe. Note 4 allows four water closets on a 3-inch vertical and three on a 3-inch horizontal.
6. Trace the vents separately. Find each fixture's vent, where vents join, and every terminal through the roof.

Example Drawing Used in Sections 11.2 and 11.3

The next two sections work one drawing all the way through. A two-story office building has an identical public restroom group on each floor, and the first floor also has a mop sink and a drinking fountain.

• Each restroom group: 2 flushometer water closets (1.6 gpf), 2 lavatories, 1 integral-trap urinal (1.0 gpf), and 1 floor drain.
• Second floor: the branch runs from LAV-1 (segment A) past LAV-2 (B), the urinal (C), the floor drain (D), WC-1 (E), and WC-2 (F), then into the stack.
• Stack: vertical from the second floor down to the first-floor branch (segment G).
• First floor: the same restroom group, plus a mop sink and a drinking fountain on a branch (H) that joins at the stack base. The combined load leaves in the horizontal building drain (J).
• Vents: individual vents from each fixture join a branch vent on each floor. The branch vents tie into a vent stack that runs through the roof beside the stack vent.

Trap Arms Are Different

Table 703.2 note 1 says unit loadings exclude the trap arm. The trap arm, from the trap weir to the vent, is sized from Table 702.1 (minimum trap and trap arm size). Table 702.2(1) caps the DFU a trap arm can carry: 1 for 1-1/4 inch, 3 for 1-1/2 inch, 4 for 2 inch, 6 for 3 inch, and 8 for 4 inch. The trap arm's length is limited by Table 1002.2 (Section 9.2). On the drawing, the short run from a single fixture's trap to its vent is its trap arm, sized at the fixture's minimum trap size.

Time Management

Twenty-five items in 90 minutes gives you about 3.5 minutes per item. Spend the first 10 to 15 minutes building a DFU and size ledger: one line per segment, recording the running DFU total, H or V, the number of water closets, and the minimum size. Most questions then become lookups from your own ledger instead of new calculations. Section 14.1 covers pacing across all three parts.