Sanitary drainage: Minnesota Journeyworker Plumber Practice Questions

Under the UPC, what is the standard minimum slope for a 3-inch horizontal drain?

The UPC sets 1/4 inch per foot (2%) as the standard slope and allows a flatter 1/8 inch per foot only for pipe 4 inches and larger where the AHJ approves it; its sizing table assumes 1/4 inch per foot.

A 40-ft horizontal drain is laid at 1/4 inch per foot. How much total fall does it have?

Multiply the slope per foot by the length. Using 1/8 inch per foot by mistake gives 5 inches.

A 4-inch building drain runs 66 ft at 1/8 inch per foot. What is the total fall in feet and inches?

One-eighth inch per foot over 66 ft is 8.25 inches. Using 1/4 inch per foot gives 16.5 inches (1 ft 4.5 in).

A slope of 1/4 inch per foot is about what percent grade?

A foot is 12 inches, so 1/4 inch per foot is 0.25 ÷ 12, about 2%. Codes often write it as 2 percent.

The invert of a building drain where it leaves the building is at elevation 96.50 ft. The drain runs 80 ft to the sewer tap at 1/8 inch per foot. What is the invert elevation at the tap?

Convert the fall from inches to feet before subtracting. Subtracting 10 feet instead of 10 inches gives 86.50.

A sump pump discharges 15 gpm continuously into a gravity drain. Use the code rule of 2 drainage fixture units per gpm of continuous flow. How many DFU does it add?

Both codes assign 2 DFU for each gallon per minute of continuous or semicontinuous flow, such as pumps and air-conditioning condensate.

Use these values for this problem: lavatory 1 DFU, bathtub 2 DFU, water closet 3 DFU, kitchen sink 2 DFU, clothes washer 3 DFU. A horizontal branch serves 2 lavatories, 1 bathtub and 1 water closet. What is its DFU load?

Add each fixture's DFU, counting both lavatories. Counting one lavatory gives 6.

Use these values for this problem: a 2-inch horizontal branch may carry up to 6 DFU, a 2-1/2-inch up to 12 DFU, a 3-inch up to 20 DFU (no more than 2 water closets on 3-inch, for this problem). A branch carries 1 water closet (3 DFU), 2 lavatories (1 each), 1 shower (2) and 1 laundry tray (2). What is the smallest branch size?

By DFU alone 2-1/2 inch would carry 9 DFU, but no water closet may discharge into a drain smaller than 3 inches.

Use these values for this problem: at 1/4 inch per foot, a building drain of 3 inches carries up to 42 DFU, 4 inches up to 216 DFU and 5 inches up to 480 DFU. A house has 2 bathroom groups at 6 DFU each, a kitchen sink at 2, a dishwasher at 2, a clothes washer at 3 and a laundry tray at 2. What is the smallest building drain allowed?

The total of 21 DFU fits within the 3-inch capacity given, and 3 inches also meets the water closet minimum.

What is the smallest drain a water closet may discharge into?

Both codes require at least a 3-inch drain for a water closet.

May a horizontal drain be reduced in size in the direction of flow?

Reducing a drain downstream creates a choke point that catches solids. The only common exception is the 4 x 3 closet bend or flange.

How must a drainage stack compare in size with the horizontal branches connected to it?

A stack smaller than a branch feeding it would choke. The stack is sized by its total DFU and must be at least as large as the largest branch.

A sanitary tee is used to change direction in a drain. In which direction is this allowed?

A sanitary tee turns flow from a horizontal branch down into a vertical stack. It may not turn vertical flow to horizontal or be used flat (horizontal to horizontal).

Which fitting is acceptable at the base of a stack where flow changes from vertical to horizontal?

Vertical-to-horizontal changes need easy turns: long-sweep bends, a wye with an eighth bend, or other combinations allowed by the code.

A horizontal drain must turn 90 degrees while staying horizontal. Which fitting choice is correct?

Horizontal-to-horizontal turns use 45-degree wyes, long-sweep bends or combinations of eighth and sixteenth bends. A sanitary tee on its back is prohibited.