PASEO Training Tutorial #1  — Building Sewers

1.0   Introduction:

The building sewer is an integral, yet often overlooked, component of a sewage disposal system.  An improperly constructed building sewer can result in clogging and sewage back-up into the structure.

The purpose of this tutorial is to inform the reader …

  • of the regulations concerning building sewers,
  • how to inspect building sewers, and
  • how to improve the construction of building sewers.

At the conclusion there will be an inspection field sheet to assist in conducting field inspections.

Important:  The plumbing within the structure falls under the jurisdiction of the municipality’s building code program.  Sewage Enforcement Officers are not authorized to inspect the plumbing of the structure.

2.0   Building Sewers

When water is disposed in a structure, it flows through the structure’s plumbing.  At a certain point the plumbing exits the structure.  From this point forward this the pipe is referred to as a “building sewer”.  The building sewer conveys the sewage from the structure to a treatment tank, lift tank, or holding tank. 

Note: For purposes of this tutorial, we will ignore holding tanks and lift tanks and assume all building sewers flow into treatment tanks.

The building sewer is highlighted in red.

3.0  What Do the Regulations Say:

 § 73.21 (a)  Building sewers shall be constructed of a durable material acceptable to the Department or the local agency.

Typically, the building sewer will be made of polyvinyl chloride (PVC) pipe.  However, an SEO may also encounter acrylonitrile butadiene styrene (ABS) or ductile iron pipe. 

 § 73.21 (b)  The local agency may restrict the type of materials used by code, ordinance or resolution and shall notify the applicant when restrictions are imposed.

The local agency SEO should be versed in the local agency’s sewage ordinance and aware of any special requirements.  However, with the advent of the statewide building code, there may be piping requirements that exceed those found in Chapter 73.  SEOs should request that the municipal building inspector inform the SEO of any changes to the sewage piping requirements.  If there are requirements that a specific material or sized pipe is required, the SEO must note such on the issued sewage permit.

 § 73.21 (c ) When the average daily flow of sewage from an establishment is 1,000 gallons or less, building sewers shall be at least 3 inches in diameter unless otherwise specified by local plumbing or building codes. When the average daily flow exceeds 1,000 gpd, all building sewers shall be at least 6 inches in diameter unless otherwise specified by local plumbing or building codes.

§ 73.21 (d)  Cleanouts shall be provided at the junction of the building drain and building sewer.

The cleanouts are typically installed inside the structure just before the plumbing exits the structure.  However, some municipalities may require the cleanout to be sited just after pipe exits the foundation, or in both locations.  The premise for use of an outside cleanout is that if used, any spillage would be outside the structure.

Outside Cleanout
Inside Cleanout

 § 73.21 (e)  Cleanouts shall be provided at intervals of not more than 100 feet.

Cleanout on a 100+ foot run.

Generally speaking, to avoid blockages, treatment tanks should be kept as close to the structure as practical (but at least 10 feet away).  There may be situations in which the tank may need to be located at a greater distance and cleanouts along the pipe will facilitate removing any blockages.

 § 73.21 (f)  Bends ahead of the treatment tank shall be limited to 45° or less where possible. If 90° bends cannot be avoided, they shall be made with two 45° bends.

Bends in the building sewer encourage blockage, the more gentle the bend, the less chance of blockage.  While two 45° bends are the maximum specified in the regulations, four 22.5° bends are even better.

 § 73.21 (g)  The grade of the building sewer shall be at least 1/8 inch per foot; however, the grade of the 10 feet of building sewer immediately preceding the treatment tank may not exceed 1/4 inch per foot.

1/8 inch per foot is the minimum necessary to convey sewage.  ¼ inch to the foot is preferred.  Greater slopes should be avoided when possible.  The requirement for the last 10 feet of pipe not to exceed ¼ inch to the foot is to slow the rate of the sewage flow to prevent short circuiting the treatment tank.

 § 73.21 (h)  Building sewers shall be constructed with watertight joints, shall be of sufficient strength to withstand imposed loads and installed on material suitable for preventing damage from settling.

The pipes of a building sewer must be sealed by couplings.  Pipes in which the “male” end is inserted into a “female” end do not seal well enough to prevent leakage or groundwater infiltration.  While schedule 40 is not specifically called for, thinner walled pipes may flatten with the weight of the backfill.  Building sewers that run under driveways should be at least schedule 60 or encased in an iron pipe.

 § 73.21 (i)  The building sewer shall be installed to allow continuous venting of the treatment tank through the main building stack unless otherwise specified by local plumbing or building codes.

§ 73.21 (j)  Building sewers shall be connected to treatment tanks by means of watertight mechanical seals or hydraulic grouting. Use of Portland cement grouting is not permitted.

A mechanical seal is typically a Fernco coupling molded into the tank at the time of manufacture.  The building sewer is inserted into the coupling which is tightened onto the pipe by a large hose clamp.

Hydraulic Grout is cement that does not shrink after drying.  Hydraulic grout is manufactured by many companies including Dry-Lok ®, Quikrete ®, and Sakrete ®.  Although not specifically prohibited until 1997, portland cement is not an acceptable tank sealant as it shrinks upon drying leaving gaps and will often crack and fall out of place.  The result can be voids that allow groundwater to enter the tank.

Sealed with hydraulic grout
Sealed with a mechanical seal.

         2.2   The Duties of the Sewage Enforcement Officer

The building sewer, like all other components of the sewage disposal system, must be inspected by the sewage enforcement officer.

First, and foremost, the sewage enforcement officer should determine that the building sewer is pitched with the structure at the high end and the treatment tank at the low end.  The pitch must be at least 1/8 inch to the foot.  The final ten feet before the treatment tank must be between 1/8 and ¼ inch to the foot.  The pitch of the pipe may be determined with a laser level or contractor’s (dumpy) level, if one is on site.  Otherwise, a bubble level (also known as a spirit level) may be placed on the top of the pipe.

Typical bubble level.

Observe the length of the building sewer to ensure that it does not have any bends that exceed 45° per joint.

Observe the pipe size to determine that it is at least 3 inches in diameter for flows under 1000 gallons per day and 6 inches in diameter for flows 1000 gallons per day, or greater.  (This information should be on the sewage permit application and construction plans.)

If the building sewer is over 100 feet long, make sure that there are cleanouts no more than 100 feet apart.

Check to see that there is a cleanout just inside, or just outside the structure.  When inside the structure, also note if there is a U-trap before, or as part of, the cleanout.  The U-trap will prevent the treatment tank from venting through the structure’s plumbing and result in a violation of § 73.21 (i).

U-trap with a cleanout before and after.

4.0   How to Do a Better Job

Unless more greatly regulated by local ordinance, a Sewage Enforcement Officer is supposed to inspect the system to ensure that it meets the requirements of Chapter 73.  This means that the SEO can only enforce the regulations to the “minimum standards” and can not impose greater requirements.  For the sake of the property owner and the environment, the SEO should educate installers in methods that would improve the quality of their work.

4.1  Bedding the Building Sewer

The regulations require nothing more than a trench being dug, the building sewer laid on the bottom, and the trench backfilled.  But seasoned installers know that bedding a pipe can increase its structural integrity.

As seen in these drawings, a pipe has been laid in a trench.  When backfilled, the soil pushes down on the pipe.  This weight can be excessive should the soil depth be great, the soil become saturated, or outside forces are exerted on the soil (e.g. a person walks over or a car drives over the cover soil).  Such weight may deform and/or crack the pipe.

Pipe in trench
Force vectors show pipe deformation

By bedding the pipe, the downward force is the same.  However, the bedding does not allow the pipe to deform.  This greatly increases the structural integrity of the pipe and lessens the possibility of cracking or collapsing.

Pipe bedded in sand
Bedding prevents deformation

The bedding material may be gravel, sand or clean soil.  The bedding material should be tamped to ensure it gets under the curvature of the pipe and packs well against the pipe.

Unless pipe bedding is required in the local sewage ordinance, an SEO can not require a building sewer to be bedded.  However, the SEO should educate the local installers on how properly bedding the building sewer is an inexpensive means of protecting the pipe.

Building sewer bedded in sand.

4.2  Prevent over-dig sagging

Building sewers placed over areas of excavation such as crossing trenches or the over-dig outside a tank can result in the pipe sagging over time.  This can be avoided by encouraging the installer to …

  • Use a more rigid pipe (e.g. Schedule – 40 or ductile iron).
  • Compacting the fill soil that will be placed below the pipe so as to prevent it from settling.
  • Minimizing the amount over-dig so the tank is closer to the excavation wall.
This drawing shows a tank with a large “over-dig”. That is. the area excavated that is larger than the tank.
By tamping the soil in the area shown in red, the building sewer will be less prone to sagging.

Unless required in the local sewage ordinance, an SEO can not require a building sewer to be constructed of stronger material or bedded on tamped soil.  However, the SEO should educate the local installers on how such actions will better protect the pipe.

4.3  Case the building sewer under a driveway

The average American automobile weighs in at slightly over 2 tons.  Many weigh even more.  That can be a lot of weight to place on a building sewer should it run under a driveway.  While it is best to avoid running a building sewer under a driveway, the use of extra strong pipe (such as Schedule-60) or protecting the pipe by placing it inside a larger steel or iron pipe is recommended.

A 4 inch building sewer cased in a stronger 8 inch pipe.

4.4  When possible, avoid bends in the building sewer

Whenever possible, it is best to align the tank so that the building sewer flows in a straight line into the tank.  This will greatly reduce the chances of clogging.

While this bend is an acceptable 45°, it still produces a potential spot for clogging to occur.

4.5 Require Pipe Labeling to face Upwards

When installed, most pipe looks the same.  By having installers place the labeling on top of the pipe, the SEO can better determine if the appropriate pipe is used.

With the label up, the SEO can determine if the pipe is appropriate for the usage.

Inspection Field Sheet – Building Sewer – Use this check sheet when inspecting a building sewer.  After the inspection attach it to the local agency copy of the sewage permit and keep as a record of the inspection.