2.4 Building Drains, Building Sewers & Private Interceptor Main Sewers

Definitions that drive the answer

• Building drain: horizontal piping within or under the fully enclosed part of a building, below the lowest fixture, that drains by gravity to the building sewer (SPS 381.01 (39)).
• Building subdrain: the horizontal portion that does not flow by gravity to the sewer, so it discharges to a sump (SPS 381.01 (47)).
• Building sewer: the part of the drain system outside the fully enclosed building that conveys discharge to a public sewer, private interceptor main sewer, POWTS or other discharge point (SPS 381.01 (44)).
• Private interceptor main sewer: a sewer serving 2 or more buildings that is not part of the municipal system (SPS 381.01 (193)).

Table 382.30-3: building drains, subdrains, sewers and private interceptor main sewers

NP means not permitted. Pipes 3 to 6 inches cannot be laid at 1/16 inch per foot, and pipes 2 inches and smaller need at least 1/4 inch per foot. Building sewers 12 inches and larger, and private interceptor main sewers 8 inches and larger, follow the municipal criteria in NR 110.13.

Worked example. A commercial building drain carries 200 dfu. At 1/8 inch per foot a 4-inch drain is limited to 180, so it needs 5 inches; at 1/4 inch per foot a 4-inch drain (216) works. Pitch can save a pipe size.

Minimum sizes (SPS 382.30 (4) (b) and (c))

Building drain elevation and backwater (SPS 382.30 (11) (b))

• Building drains go below the lowest floor on which fixtures may be installed, if the sewer elevation permits.
• Where an above-ground building drain discharges to a vertical pipe, it must connect to the building sewer at least 30 inches above the basement floor.
• A building drain subject to backflow must be protected by a backwater valve or a sump with pumping equipment. The valve must have full-pipe capacity when open and be readily accessible for cleaning.
• A floor that is entirely below grade and has a plumbing fixture or appliance must have a floor drain.

Depth and frost protection (SPS 382.30 (11) (c))

Frost protection is not required: within 30 feet of the building drain where the sewer feeds a holding tank, POWTS tank or grease interceptor under an uncleared surface; where the predicted frost depth (Figure 382.30-1 and Table 382.30-6) doesn't reach the pipe; or for public facilities closed from November 15 to March 15 that are not places of employment.

The approved insulation methods are:

1. Flat extruded polystyrene: at least 18 inches below grade and at least 6 inches above the pipe. Thickness and width come from Tables 382.30-5 and 382.30-7, and any height above 6 inches is added to the width.
2. Box method: a 3-sided polystyrene box at least 12 inches below grade, 6 inches above and 6 inches to each side of the pipe.
3. Lightweight insulating concrete: placed to the spring line, extending 6 inches each side, with the top at least 12 inches below grade.

Trenching, bedding and backfill (SPS 382.30 (11) (e))

• Use open-trench work unless a local ordinance or inspector allows otherwise.
• Where stone over 1 inch or bedrock is found, excavate 3 inches below grade and bed with material that passes a 3/4-inch sieve, compacted to 90 percent standard Proctor.
• Initial backfill extends 12 inches over the pipe, uses material passing a 1-inch sieve, and is placed in lifts of 6 inches or less, well tamped.
• Provide at least 36 inches of cover before wheel-loading the trench.
• Unstable bottoms: drive sheathing 48 inches below the trench bottom, remove 24 inches of soft material and replace it, or pour a reinforced concrete cradle or slab at least 3 inches thick.

Connections and location rules

• A building sewer may not pass under one building to serve another, except for farm buildings on one property or buildings on the same property with a piping document recorded with the register of deeds within 90 days.
• If no public sewer connection exists within 3 feet of the designated point, use an approved saddle, a cut-in tee or wye, or for concrete or clay mains a connection encased in at least 3 inches of concrete.
• A forced sewer connecting to a pressurized public sewer needs a curb stop, a check valve and an accessible quick disconnect upstream of the check valve.
• Separation from wells follows NR 811 and NR 812.
• Tracer wire for nonmetallic sewers: at least 18-gauge insulated single-conductor copper, within 6 inches above the pipe, green insulation, accessible at 400-foot intervals, and conductivity-tested. Warning tape is not a substitute. Recording GPS data with the municipality is an alternative.
• Storm and clear water may not discharge to a sanitary building drain connected to a publicly owned treatment works, except as SPS 382.36 (3) allows.