Sewage Disposal System Design

In the course of many thousands of Sewage Disposal System Designs for the replacement of failed existing systems, for the expansion of current systems and for new residential and commercial systems, we at Stamski and McNary, Inc., have experienced situations in which our clients do not have an understanding of the process of designing and constructing a septic system. We offer following information to attempt to clarify the process.


The civil engineer reviews the needs of the client. The approximate size of a system is computed and a primary location of said system is determined. Locations of wells, property lines, wetlands and other utilities are considered in the placement of septic systems.

The civil engineer coordinates the design process with the local Board of Health (BOH) and with the client. Soil evaluations are conducted by the engineer and witnessed by the local BOH. The actual date and time of said evaluations are dictated by the BOH. Soil evaluations consist of digging holes to a depth of approximately ten (10) feet to observe soil types and groundwater depths. A backhoe is utilized to dig these holes. Percolation tests are conducted to determine the permeability of the soil. In certain instances, the engineer may choose to wait for dryer soil conditions to conduct the percolation tests.

Based upon the soil data and the topography of the location, a septic system can be designed. The type and size of the system is determined by estimated sewage flow from the facility being served, by soil conditions and by groundwater depths. Standards for design are set by the Commonwealth and are frequently supplemented by local Boards of Health.

Designs are reviewed and approved by the local BOH and in the instances that require variances from the regulations, the Commonwealth is notified.

In addition to the engineer’s design fee, the local BOH usually requires a fee for the witnessing of soil evaluations and a fee for the review and permitting of designs. The cost of backhoe services is in addition to the design fee.


With the approved design plan and permit to install in hand, the client needs to select an installer of the system. Installers are required to be licensed locally and lists are available at the local BOH. It is recommended that at least three estimates be obtained. The cost of an installation can vary significantly between installers depending mostly upon their workloads.

When selecting an installer, the client should be aware of additional cost that may be required from the designer of the septic system. Some towns require that the location of the septic system be staked by the designer. All towns require that an as-built plan be prepared by the designer certifying the location and elevation of the septic system as constructed. The client should determine who shall be responsible for the cost of these services.

The installation of the septic system is periodically reviewed by a representative of the local BOH for quality of work issues. The designer is usually responsible for the determination of compliance with location and elevation issues. The installer should notify the engineer when certain components are ready to be inspected. We advise that the final grades over the septic system be verified prior to any final landscaping in case some adjustments are necessary.

When construction is completed in accordance with the design plan and an as-built plan has been submitted and approved by the local BOH, the BOH then issues a Certificate of Compliance to the owner of the septic system. We advise that the septic system is not complete until said Certificate of Compliance is issued and contracts with installers should be structured accordingly. A Certificate of Compliance must be obtained from the Board of Health prior to the expiration of the permit to install the septic system.