7.1 Reading Plumbing Drawings, Isometrics, Symbols & Schedules

Why this section matters

The exam gives 5 questions to Drawings and Plan Reading. Those questions, plus many Sizing and Design items, show a floor plan or an isometric and ask you to read sizes, find a load, or spot a violation. Reading drawings quickly saves minutes for the harder calculations.

The sheets in a plumbing set

Where a drawing and the written specifications conflict, the contract documents usually say which governs. On a code exam, the code governs over a drawing that shows a violation.

Isometric drawings

An isometric riser diagram shows piping in three dimensions on a flat sheet:

• Vertical pipe is drawn as a true vertical line.
• Horizontal pipe runs along two axes drawn at 30 degrees above horizontal, to the left and to the right.
• Offsets at 45 degrees are often drawn with a shaded or hatched triangle that shows which plane the offset lies in.

Isometrics are usually not to scale. They show connection order, fittings, venting and flow direction. Take lengths from the dimensioned plans or from the problem statement.

            vertical (true plumb)
                   |
                   |
     30 deg  ______|______  30 deg
            /             \
   left horizontal     right horizontal

Line types and symbols

Offices use different conventions, so always read the legend first. Common conventions:

Schedules: where the numbers live

A plumbing fixture schedule gives each fixture a mark (for example P-1 water closet, P-2 lavatory) and lists its manufacturer and model, rough-in dimensions, trap size, cold, hot and waste connection sizes and often its dfu and wsfu. Read the schedule before you start adding:

• Is the water closet a public 1.6-gpf unit (4 dfu) or a private one (3 dfu)?
• Is it on a flushometer valve (10 wsfu public) or a tank (5 wsfu public)?
• Is the urinal 1 gallon per flush or less (2 dfu)?
• Is the shower head rated 5.7 gpm or less (2 dfu, 1-1/2-inch trap)?

Equipment schedules list water heaters (Btu/h, gallons, recovery), pumps (gpm at feet of head), and backflow assemblies (type and size).

Tracing systems on a drawing

Water:

1. Public main, corporation stop and service pipe (at least 3/4 inch).
2. Curb valve and meter, then a full-open valve on the meter discharge.
3. Full-open valve at the building entrance.
4. Containment backflow assembly, if the supplier requires it.
5. PRV with a strainer if static pressure is over 80 psi.
6. Mains and risers, with valves at riser bases where required, and the water heater's cold-water valve.
7. Branches and fixture supplies, each ending within 30 inches of the fixture.

Drainage:

1. Fixture and trap (2- to 4-inch seal).
2. Fixture drain, within the Table 909.1 distance to the vent.
3. Horizontal branch, sized by Table 710.1(2).
4. Stack, sized by Table 710.1(2) for both total and per-interval load.
5. Stack base, using Table 706.3 fittings.
6. Building drain, sized by Table 710.1(1) at its slope, ending 30 inches outside the wall.
7. Building sewer to the public sewer.

Vents:

1. Vent takeoff above the drain centerline.
2. Vertical rise to 6 inches above the highest flood rim.
3. Branch vent graded to drain back.
4. Vent stack or stack vent (sized by Table 906.1).
5. Terminal: away from openings, at the local roof height.

A plan-review checklist (common exam "violations")

Exam traps

• Legend first. Symbols vary between offices.
• Isometrics are not to scale. Use the stated dimensions.
• Schedule first. Public versus private and flush valve versus tank change both dfu and wsfu.
• Blueprint weight: Drawings and Plan Reading is 5 of 75 questions.