How the stack is built
- Height inside = rim − lowest invert, the way precasters and owners measure a manhole's depth.
- From the top: frame and cover, grade rings, the cone, the risers, and the base. The base's floor sits under the invert, so only its wall above the floor counts toward the height.
- The base has to rise far enough above the highest pipe: its crown (its invert above the outlet plus its outside diameter) plus the clearance the owner requires, 9 inches on Charlotte Water's detail.
- The search. Every base and cone the precaster makes is tried; the risers are picked for the fewest pieces; and the rest is made up with grade rings, within the owner's limit from the top of the cone to the top of the frame. The stack with the fewest joints wins, then the one with the least left to mortar.
- Weights are the precaster's published figures where there are any. Otherwise risers and bases are estimated from their shape at 150 lb/ft³, and marked so.
Site Bid carries structures from rim to invert in its pipe networks and prices the trench to them; the trench calculator does the run between them.
Charlotte Water's rules
- Manhole size by pipe: 8 to 16 in pipe in a 4 ft manhole, 18 to 36 in in a 5 ft, 42 to 54 in in a 6 ft.
- Walls at least 1/12 of the inside diameter and never under 5 in; the top of the cone at least 7 in.
- Eccentric cones: at least 32 in tall on a 4 ft manhole, 40 in on a 5 ft, sloped for their full height.
- Risers in 1 ft increments with 12 in step spacing, or 16 in increments with 16 in spacing.
- No more than three grade rings, and no more than 21 in from the top of the cone to the top of the frame, frame included. Rings come 2, 4, 6 or 8 in. Brick and mortar over a precast ring is allowed to adjust grade, except on manholes set above grade.
- Pipes 16 in and smaller get 0.2 ft of fall through the manhole. A drop under 2.5 ft gets a sloped trough; 2.5 ft or more an inside drop, which needs at least a 5 ft manhole.
Other owners differ: Concord, for one, allows up to 12 in of grade rings. Choose "My own limits" for theirs.
A worked example
Rim 712.40, outlet invert 701.15, 8-inch PVC (8.4 in O.D.) with the inlet 0.2 ft above the outlet, a 4 ft Tindall manhole under Charlotte Water's rules. Rim to invert is 11.25 ft, 135 in. The stack:
- 7.5 in frame and cover, 310 lb
- 4 in grade ring, 170 lb
- 64 in eccentric cone, 4,540 lb
- 64 in base, 6,689 lb
- 1.5 in made up with mortar or shims
4 pieces, 11,709 lb in all; the heaviest is 6,689 lb.
Sources
- Charlotte Water Design and Construction Standards, Ch. 11A Gravity Sewer Design (Jan 2026). Checked October 4, 2026.
- Charlotte Water, Ch. 11B Sewer Specifications (Jan 2026). Checked October 4, 2026.
- Charlotte Water detail 11.2.1, 4' precast manhole (v2.0, 01/2026). Checked October 4, 2026.
- Charlotte Water detail 11.2.2, 5' precast manhole (v2.0, 01/2026). Checked October 4, 2026.
- Charlotte Water detail 11.2.4, sewer manhole grade ring (v1.0, 04/2024). Checked October 4, 2026.
- Tindall Corp., Underground Products Catalog, G Series (heights and weights). Checked October 4, 2026.
- Foley Products, manhole drawing sheets (heights). Checked October 4, 2026.
- ACPA Design Data 20, Circular Precast Concrete Manholes. Checked October 4, 2026.
- Foley publishes weights in tons without saying which, so this page estimates Foley weights instead of converting them.
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