Invert elevation is the most critical measurement in sewer and storm drain construction. Get it wrong and the pipe won't drain — and the city engineer will send you back to fix it. This guide covers what invert elevation is, how to measure it correctly, how to document manhole records, and how to verify self-cleaning velocity before backfill.
The invert elevation of a sewer pipe is the elevation of the inside bottom of the pipe at a specific point. At a manhole, this is measured at the inside face of the manhole wall where the pipe connects.
Invert elevation is distinct from:
Why invert elevation matters: The slope of a sewer pipe run is calculated as the difference between upstream and downstream invert elevations divided by the pipe run length. This slope controls whether the pipe drains at adequate self-cleaning velocity. A pipe installed flat — or with reverse grade — will accumulate solids, back up, and eventually overflow.
Set up an auto level or dumpy level on a stable tripod with a clear line of sight to your benchmark and to the manhole(s) you're measuring. The instrument must be level — check the circular bubble and, if equipped, the optical plumb. Choose a setup location that minimizes the number of moves required to cover the run of manholes you're documenting.
For sewer work in traffic or on live roads, follow your safety plan — cone off the instrument area and use a flagger if required. Keep the tripod on solid, undisturbed pavement or ground, not on loose backfill adjacent to the manhole.
Hold a grade rod on your benchmark (a survey monument, benchmark disk, or established elevation hub). Read the rod through the instrument. Record the backsight (BS) reading.
Record the benchmark ID, benchmark elevation, and BS reading in your field notes. This is the foundation of all subsequent elevation shots from this setup.
Lower the grade rod into the manhole and rest it on the pipe invert — the inside bottom of the pipe at the manhole wall. Hold the rod plumb (use a rod level or bubble on the rod holder). Read the rod through the instrument. Record the foresight (FS) reading.
Measure invert in and invert out separately for each pipe at the manhole. If the pipes are close in elevation, the difference may be small — read the rod carefully to 0.01 ft. For drop manholes where the incoming pipe is significantly higher than the outgoing pipe, document both elevations and calculate the drop.
While your instrument is set up, shoot the rim elevation (top of the manhole frame). Hold or tape the grade rod on the manhole rim and read the foresight. Rim elevation is required in the as-built package and allows calculation of manhole depth (Rim Elevation − Invert Out).
Some municipalities also require the sump elevation (bottom of the manhole cone below the lowest pipe invert). Lower the rod to the manhole floor and shoot a foresight. Sump elevation is important for manhole inspection records and rehabilitation design.
Every manhole in the as-built package needs a complete record. City engineers will reject an as-built that's missing fields. Here's what goes on each manhole record:
| Field | Description | Notes |
|---|---|---|
| MH ID | Manhole identifier from approved plans | Match plan numbers exactly |
| GPS Coordinates | Northing, easting, GPS datum (NAD83, WGS84) | Required by most municipalities |
| Rim Elevation | Top of manhole frame/cover | To 0.01 ft |
| Invert In | Inside bottom elevation for each incoming pipe | One entry per incoming pipe |
| Invert Out | Inside bottom elevation of outlet pipe | This controls downstream grade |
| Pipe Size | Inside diameter of each pipe | Confirm actual installed, not design |
| Pipe Material | PVC, RCP, DIP, HDPE, etc. | Confirm from field, not just plans |
| Slope to Next MH | Calculated pipe grade to downstream MH | (Inv In − Inv Out) ÷ Length |
| Depth of Cover | At road crossings or per plan requirements | Check against min. cover spec |
| Surveyor/Inspector | Name and date of measurement | Required for certification |
Before backfilling and certifying the pipe run, verify that your as-installed slope produces at least 2.0 fps of flow velocity at full pipe flow. Use Manning's equation:
If your as-installed slope falls below the minimum for self-cleaning velocity, flag the run for correction before backfilling. Corrective action options: lower the downstream manhole, raise the upstream pipe connection, or add manholes to break the run into shorter segments with more achievable slope.
The invert elevation is the elevation of the inside bottom of the pipe at a specific location — typically measured at the inside wall of a manhole. It controls the pipe slope between manholes and determines whether the pipe drains at adequate self-cleaning velocity. It's different from the pipe centerline, crown, or rim elevation.
Set up a level and shoot a backsight to your benchmark to calculate HI (Height of Instrument). Lower your grade rod into the manhole to the pipe invert and read the foresight. Invert Elevation = HI − Foresight Reading. Measure both invert in and invert out for each pipe at the manhole.
Per IPC 704.1: 4-inch = 1/4 inch per foot (2.08%); 6-inch = 1/8 inch per foot (1.04%); 8-inch = 1/16 inch per foot (0.52%). These minimums ensure 2.0 fps self-cleaning velocity at full flow. Local municipalities may have stricter requirements — always verify with the project engineer.
MH ID, GPS coordinates, rim elevation, invert in (each pipe), invert out, pipe size and material for each run, slope to next manhole, depth of cover at road crossings, and surveyor/inspector name and date. Missing any of these is the most common reason as-builts get rejected by city engineers.
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