Plumbing Mathematics, Plan Reading, and Isometric Drawings
The four calculations that show up most
- Slope. Drainage slope is given in inches per foot. Convert to percent by dividing by 12 and multiplying by 100: 1/4 in/ft = 0.0208 = about 2.1%; 1/8 in/ft = about 1.0%.
- Fall over a run. Fall = slope × run. Keep units consistent: convert feet to inches first.
- 45° offsets. True length (travel) = offset × 1.414; the horizontal/vertical advance = offset × 1.0. For a 22.5° offset the multiplier is about 2.61.
- Fixture units. Never mix drainage fixture units (dfu) with water supply fixture units (wsfu); they come from different tables and different loads.
Reading plans
Locate the plan view, riser/isometric, and schedule. On an isometric, vertical lines are risers, 45° lines are horizontal runs in plan, and the drawing is read from the viewer's position. Use the scale given on the sheet - never measure with an assumed scale.
Traps
- Forgetting to add fitting lengths to measured pipe when ordering material.
- Using plan-view dimensions for a sloped run without adjusting for the slope.
- Treating 1/4 in/ft as 1/4%.
- Confusing pipe size with outside diameter; schedule 40 and schedule 80 differ in wall thickness and inside diameter.
Rule of thumb
1/4 in per foot = 1 in per 4 ft ≈ 2.1%. If asked for the drop over a run, convert the run to inches and divide by 48.
Worked example: A 30 ft run at 1/4 in/ft falls 30 × 0.25 = 7.5 in. A 12 in offset on 45° fittings needs 12 × 1.414 = 17 in of travel pipe.