1. How are trap sizes selected for plumbing fixtures?

Trap sizes are based on the fixture type and its discharge rate. For example, a lavatory typically requires a 1¼ in. trap, a bathtub 1½ in., and a floor drain 2 in. Traps must be large enough to handle fixture flow but not oversized, to maintain a proper water seal.

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2. What is the minimum trap seal depth?

The minimum trap seal is 1½ inches, and maximum is 2 inches. This water seal prevents sewer gases from entering occupied spaces.

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3. What is a Drainage Fixture Unit (DFU)?

A DFU is a unit of measure representing the probable discharge into the drainage system by various plumbing fixtures. Each fixture is assigned a DFU value; the sum determines pipe sizing and venting.

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4. How are DFUs converted into flow rates?

Codes provide conversion tables. For example:

• 1 DFU ≈ 7.5 GPM (0.47 L/s) intermittent flow.

• 2 DFU ≈ 10 GPM (0.63 L/s)

• 4 DFU ≈ 16 GPM (1.0 L/s)
  This relationship allows engineers to calculate required pipe sizes.

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5. How do you determine maximum fixture load on a pipe?

Tables define how many DFUs can be connected:

• A 2 in. vertical pipe can carry 32 DFUs, while horizontal is limited to 26 DFUs.

• A 4 in. vertical stack can handle 500 DFUs.
  These limits prevent overload and ensure self-cleansing velocity.

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6. What is the maximum length of a drainage pipe?

Maximum lengths vary with diameter and slope. For example:

• 2 in. vertical pipe → 65 ft max.

• 3 in. vertical pipe → 212 ft max.

• Horizontal runs are unlimited if properly vented, but branch lengths are restricted.

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7. How are horizontal fixture branches sized?

The number of DFUs that may connect depends on pipe size and number of branch intervals:

• 2 in. branch → 10 DFUs max.

• 3 in. branch → 20–35 DFUs.

• 4 in. branch → 90–500 DFUs depending on intervals.

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8. How are vents sized and limited in length?

Vent pipe sizing depends on DFU load and developed length. For example:

• 2 in. vent supports up to 24 DFUs with 50 ft max length.

• 3 in. vent supports up to 84 DFUs with 212 ft max length.
  Vents longer than this must be upsized.

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9. How do you size stormwater drains and roof leaders?

Storm drainage is sized based on rainfall intensity and roof area:

• A 3 in. drain handles ~3,200 sq. ft at 2 in./hr rainfall.

• A 6 in. drain handles ~13,600 sq. ft at same rainfall.
  Larger leaders are used for bigger roof areas or higher rainfall zones.

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10. What slope is required for horizontal drainage piping?

Typical minimum slopes:

• 2½ in. or smaller pipes → ¼ in. per foot (2%).

• 3–6 in. pipes → ⅛ in. per foot (1%).
  Slopes ensure self-cleansing velocities (~2 ft/s).

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11. How do offsets affect stack design?

When a vertical stack offsets horizontally, capacity is reduced. Additional venting may be required. Codes restrict offsets within branch intervals to prevent overload.

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12. How are bathroom groups counted in calculations?

A typical bathroom group (toilet, lavatory, bathtub/shower) is assigned a combined DFU value (usually 6–8 DFUs). This simplifies branch sizing.

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13. What are the maximum DFUs allowed on a building drain or sewer?

• 6 in. building drain → up to 3,500 DFUs.

• 8 in. building drain → up to 10,000 DFUs.
  Exceeding this requires parallel or larger mains.

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14. How do vent connections protect trap seals?

Each trap must be vented within a certain distance (usually 2–5 ft depending on pipe size). Vents prevent siphonage and maintain the water seal depth.

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15. Why are DFU tables different for vertical vs. horizontal pipes?

Vertical stacks can carry more DFUs because gravity assists flow. Horizontal pipes are limited due to greater risk of surcharge, slower velocity, and partial pipe filling.

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