How to Document Sewer Invert Elevations

Understanding Sewer Inverts and Why They Are Critical

The invert elevation is the hydraulic foundation of a gravity sewer system. Every pipe must slope continuously downhill at sufficient grade to keep solids in suspension and prevent sewer stoppages. Install a pipe even slightly uphill and you have created a future maintenance problem that will generate complaints from homeowners and repair work orders for the municipality — often traceable directly to the installing contractor years later.

Invert documentation serves two purposes. During construction, it confirms the pipe was installed at the correct grade before backfill covers it forever. After construction, the as-built invert log is submitted to the municipality as part of the system acceptance package. Without accurate invert documentation, the municipality will not accept the system — no acceptance means no final payment for the contractor.

Equipment Setup for Invert Surveys

The standard setup for manhole invert surveys is an automatic level or digital level with a grade rod that has an extension section for reaching down into deep manholes. A standard 13-foot rod with a 4-foot extension handles manholes up to approximately 15–16 feet deep. For deeper manholes, use a total station with a reflective target on the rod and stand the rod base on the invert.

Set up the level at a location with a clear line of sight to multiple manholes. A single instrument setup on a flat street can typically cover 4–6 manholes before needing to move, which significantly speeds up the survey. Establish the height of instrument (HI) carefully — any error here propagates to every invert shot taken from that setup. Calculate HI = benchmark elevation + rod reading on benchmark.

Measuring Invert Elevations at Each Manhole

At each manhole, remove the cover and lower the grade rod to the pipe invert. The invert is the bottom inside surface of the pipe at the point where it passes through the manhole wall. For manholes with a concrete channel (bench), the invert is the lowest point of the channel flow line, not the top of the concrete bench.

When the manhole has both an incoming and an outgoing pipe, measure both inverts separately. The difference between the incoming and outgoing invert at the same manhole is the manhole drop — a hydraulic feature that compensates for head losses at the manhole junction. Design drops are shown on the sewer profile. Document the incoming invert, outgoing invert, and calculated drop for every manhole with multiple connections.

Calculating MH-to-MH Grade

After measuring inverts at two consecutive manholes, calculate the pipe grade between them:

Where pipe run length is the horizontal distance between manholes from the approved plan. A positive result means the pipe slopes correctly (downhill from upstream to downstream). A negative result or zero indicates a flat or reverse-sloping pipe — a serious defect requiring the pipe to be excavated and re-laid.

Compare the calculated as-built grade to the design grade from the approved sewer profile. Flag any run where the as-built grade is below the minimum self-cleansing velocity slope for the pipe diameter, even if the invert elevations are individually within tolerance. A pipe can have inverts within ±0.05 ft of design but still have a grade problem if both ends are high or both are low.

Minimum Sewer Slopes Reference Table

Per ASCE/WEF MOP FD-5. Verify minimum slope requirements with local municipal standards — some jurisdictions require higher minimums.

What to Include in the Invert Documentation Submittal

The invert documentation package submitted for municipal acceptance typically includes: the benchmark used (ID, elevation, datum), the date of the survey, the surveyor name, a table of all manhole IDs with design invert, as-built invert, deviation, incoming and outgoing inverts, manhole drop, MH-to-MH grade, and pass/fail status. Photographs of each manhole structure are increasingly required by municipalities as part of the as-built package.

Some municipalities require a separate video inspection (CCTV) of the sewer mains in addition to the invert documentation. The invert shots document the vertical alignment; the video inspection documents the pipe interior condition and confirms the alignment is correct. If your project requires CCTV, coordinate the invert survey and video to happen on the same pass through the system to minimize traffic management costs.

Common Sewer Invert Documentation Mistakes

• Reading to the pipe crown instead of invert — The invert is the bottom inside surface of the pipe, not the crown (top). In concrete manholes with benching, confirm you are reading the flow channel, not the top of the bench.
• Not documenting incoming vs. outgoing inverts — At manholes with multiple pipes, each connection has a separate invert. Document all connections, not just the primary flow direction.
• Skipping the grade verification — Individual invert elevations can be within tolerance while the pipe grade still fails. Always calculate and report MH-to-MH grades, not just individual inverts.
• Using an unverified benchmark — The sewer design datum may differ from the street or grading plan datum on the same project. Confirm the benchmark datum matches the sewer plan before starting the survey.