Critical Zones for Water & Sewer on Private Property

How to identify critical zones for water and sewer on private property

A Critical Zone is an area on private property where underground utility lines may exist but cannot be confirmed using standard locating methods. In these high-risk areas, extra caution, such as hand digging or vacuum excavation, is essential to prevent damage.

This guide explains how to identify Critical Zones for water and sewer on private property. The focus is on simple properties where certain assumptions can be made about the location of water and sewer service lines. Knowing these zones before you dig helps homeowners, contractors, and landscapers avoid costly utility strikes.

These scenarios do not apply to large commercial sites, multi-unit dwellings, malls, townhouse complexes, or any property with multiple service entry points. Separate guidelines are available for those situations.

Key Assumptions for the Scenarios Below

These diagrams and instructions apply only if all of the following are true:

• There is only one building on the property.

• The water and/or sewer service lines are assumed to exit the front of the building unless confirmed otherwise.

• There is a visible water valve or meter near the property line.

• The sewer cleanout or exit point is known, or location can be reasonably estimated.

• There are no side or rear exits for the water or sewer lines.

• The property is not a corner lot, and there are no roads, easements, or utility corridors to the left or right of the work area that could alter the utility path. If any of these conditions exist, the Critical Zone layout will be different and requires a separate assessment.

Why Assumptions Matter

If the exit point changes, so does the risk area.

For example:

• Side Exit: The Critical Zone shifts toward the side yard and may overlap driveways, landscaping, or neighboring properties.

• Multiple Exits: Each service line requires its own Critical Zone, sometimes overlapping.

• Unknown Exit: The zone must expand to account for multiple possibilities.

Three Common Single-Building Scenarios

Scenario 1 – Water (Valve Known, Line Untraceable)

If the water line cannot be located but you know it exits the front of the building and you can see the water valve at the property line:

• Draw a straight line from each front building corner to the valve.

• The shaded area between those lines is the Critical Zone for the water line as it can be located anywhere within the shaded area.

Excavation Rule: Any digging within the zone, and within 6 feet (2 meters) of its edges, must be done by hand or with vacuum excavation.

Scenario 2 – Sewer (Exit Known)

If the sewer line cannot be located but you know where it exits the building:

• Draw a straight line from each front property corner to the sewer exit point at the building.

• This shaded triangle is your Critical Zone for sewer excavation.

Scenario 3 – Water & Sewer (Exit Unknown)

If neither line can be located and you don’t know their exact exit points:

• Assume both services exit at the front of the building.

• Draw straight lines from each front property corner to a central estimated exit point.

• If there’s a chance they exit from the sides, expand the zone to cover those possible paths.

When These Scenarios Do Not Apply

DO NOT use these layouts for:

• Townhouse complexes

• Malls or shopping plazas

• Industrial units with multiple tenants

• Large commercial sites with multiple entry points

• Properties where water/sewer may exit from sides or rear

Each of these situations requires a custom Critical Zone assessment.

Final Safety Note

When a Critical Zone is identified, mechanical excavation should never begin until the area has been exposed using hand digging or vacuum excavation. This precaution applies within the shaded area and 6 feet or 2 meters beyond its outer edges.

For more on why certain utilities can’t be located, and how to handle them, see our related post: Understanding the Unlocatable: Navigating the Limits of Utility Locates.