16.6 Gutters, Subsoil Drains, Building Subdrains, and Storm Sumps
Key Takeaways
- IPC Table 1106.6 sizes horizontal gutters by dimensions and slope, rating a 6-inch semicircular gutter at 110 gpm at 1/8 inch per foot and 157 gpm at 1/4 inch per foot.
- IPC 1111.1 requires subsoil drains to be not less than 4 inches in diameter, protected by an accessibly located backwater valve where subject to backwater.
- IPC 1111.1 states the subsoil sump shall not be required to have either a gastight cover or a vent, unlike a sanitary sump.
- IPC 1113.1.2 requires a storm sump pit not less than 18 inches in diameter and not less than 24 inches in depth, with access and a removable cover.
- IPC 1113.1.4 requires storm sump discharge piping to include a gate valve and a full flow check valve, with an exception permitting only a check valve in one- and two-family dwellings.
Gutters — IPC 1106.6 and Table 1106.6
Horizontal gutters shall be sized based on the flow rate from the roof surface. The flow rate in horizontal gutters shall not exceed that specified in Table 1106.6.
Table 1106.6 is organized by gutter dimensions — width by depth for rectangular shapes, single dimensions being diameters of a semicircle — and by slope.
| Gutter dimensions (in) | Slope (in/ft) | Capacity (gpm) |
|---|---|---|
| 1-1/2 x 2-1/2 | 1/4 | 26 |
| 1-1/2 x 2-1/2 | 1/2 | 40 |
| 4 (semicircular) | 1/8 | 39 |
| 2-1/4 x 3 | 1/4 | 55 |
| 2-1/4 x 3 | 1/2 | 87 |
| 5 (semicircular) | 1/8 | 74 |
| 4 x 2-1/2 | 1/4 | 106 |
| 3 x 3-1/2 | 1/2 | 156 |
| 6 (semicircular) | 1/8 | 110 |
| 3 x 5 | 1/4 | 157 |
| 3 x 5 | 1/2 | 225 |
| 8 (semicircular) | 1/16 | 172 |
| 8 (semicircular) | 1/8 | 247 |
| 4-1/2 x 6 | 1/4 | 348 |
| 4-1/2 x 6 | 1/2 | 494 |
| 10 (semicircular) | 1/16 | 331 |
| 10 (semicircular) | 1/8 | 472 |
| 5 x 8 | 1/4 | 651 |
| 4 x 10-1/2 | 1/2 | 1,055 |
Footnote a: dimensions are width by depth for rectangular shapes; single dimensions are diameters of a semicircle.
Two useful readings:
- Slope buys a great deal of gutter capacity. A 1-1/2 x 2-1/2 gutter carries 26 gpm at 1/4 inch per foot and 40 gpm at 1/2 — a 54 percent gain.
- A "6-inch gutter" in the trade is a 6-inch semicircular gutter at 110 gpm at 1/8 inch per foot. That is the everyday residential size.
Worked gutter sizing
A 1,200 square foot roof section in Durango (R = 1.8) drains to one gutter run.
- Q = 0.0104 x 1,200 x 1.8 = 22.5 gpm.
- Table 1106.6: a 4-inch semicircular gutter at 1/8 inch per foot carries 39 gpm. Sufficient.
- The leader from that gutter: Table 1106.3, 2-inch round = 30 gpm. Sufficient at 22.5 gpm.
Move it to Lamar (R = 3.0): Q = 0.0104 x 1,200 x 3.0 = 37.4 gpm. The 4-inch gutter at 39 gpm still works, barely; the 2-inch leader at 30 gpm does not — go to 2-1/2 inch (54 gpm).
Subsoil drains — IPC 1111
Subsoil drains shall be open-jointed, horizontally split or perforated pipe conforming to one of the standards listed in Table 1102.5. Such drains shall be not less than 4 inches in diameter. Where the building is subject to backwater, the subsoil drain shall be protected by an accessibly located backwater valve. Subsoil drains shall discharge to a trapped area drain, sump, dry well or approved location above ground. The subsoil sump shall not be required to have either a gastight cover or a vent. The sump and pumping system shall comply with Section 1113.1.
Five requirements worth separating:
- Open-jointed, horizontally split or perforated — the pipe must admit ground water.
- Not less than 4 inches in diameter.
- Backwater valve, accessibly located, where subject to backwater.
- Discharge to a trapped area drain, sump, dry well or approved above-ground location.
- The subsoil sump needs neither a gastight cover nor a vent — a direct contrast with the sanitary sump of IPC 712.3.2, which requires both.
That contrast is a favorite exam item. A subsoil sump handles ground water, not sewage; there is no gas to contain.
1101.8 Exception: the storm cleanout requirement does not apply to a subsurface drainage system.
Building subdrains — IPC 1112
Building subdrains located below the public sewer level shall discharge into a sump or receiving tank, the contents of which shall be automatically lifted and discharged into the drainage system as required for building sumps. The sump and pumping equipment shall comply with Section 1113.1.
Compare IPC 712.1, the sanitary equivalent, which adds the restriction that in other than existing structures, the sump shall not receive drainage from any piping within the building capable of being discharged by gravity to the building sewer. The storm section does not repeat that sentence, but the same engineering logic applies: do not pump what gravity can carry.
Storm sumps and pumping systems — IPC 1113
1113.1: the sump pump, pit and discharge piping shall conform to Sections 1113.1.1 through 1113.1.4.
| Section | Requirement |
|---|---|
| 1113.1.1 Pump capacity and head | Of a capacity and head appropriate to anticipated use requirements |
| 1113.1.2 Sump pit | Not less than 18 inches in diameter and not less than 24 inches in depth, unless otherwise approved. Provided with access, located so all drainage flows into the pit by gravity. Constructed of tile, steel, plastic, cast iron, concrete or other approved material, with a removable cover adequate to support anticipated loads. The pit floor shall be solid and provide permanent support for the pump |
| 1113.1.3 Electrical | Electrical service outlets, where required, meet NFPA 70 |
| 1113.1.4 Piping | Discharge piping meets 1102.2, 1102.3 or 1102.4 and shall include a gate valve and a full flow check valve. Pipe and fittings the same size as, or larger than, the pump discharge tapping. Exception: in one- and two-family dwellings, only a check valve shall be required, located on the discharge piping from the pump or ejector |
Sanitary sump versus storm sump
| Feature | Sanitary (712.3.2, 712.2) | Storm (1113.1.2, 1113.1.4) |
|---|---|---|
| Pit diameter | 18 inches minimum | 18 inches minimum |
| Pit depth | 24 inches minimum | 24 inches minimum |
| Cover | Gastight, removable, not more than 2 inches below grade or floor level | Removable, adequate to support loads; not required to be gastight for subsoil sumps (1111.1) |
| Vent | Required, per Chapter 9 | Not required for a subsoil sump (1111.1) |
| Valves | Check valve then a full open valve on the discharge side, with access | Gate valve and a full flow check valve; one- and two-family dwellings need only a check valve |
| Solids handling | 2-inch spherical solids for water closet discharge; 1/2 inch otherwise | Not specified — clear water |
| Capacity table | Table 712.4.2 | Not tabulated; "appropriate to anticipated use" |
The pit dimensions are the same. Everything about gas containment and solids differs, because one carries sewage and the other carries rain and ground water.
Combined sanitary and storm public sewer — IPC 1109
Where the public sewer is a combined system for both sanitary and storm water, the storm sewer shall be connected independently to the public sewer.
The storm and sanitary systems remain separate inside and up to the public sewer, joining only at the public main. This mirrors IPC 703.6 on the sanitary side.
And recall 1103.1: a storm system connected to a combined sewer must be trapped, either per conductor branch or with a single main trap just before the connection, with a cleanout on the building side of the trap (1103.4).
A residential storm package
A house in Grand Junction (R = 1.7) with a 1,800 square foot roof, gutters on two sides, and a perimeter foundation drain.
| Item | Result | Basis |
|---|---|---|
| Flow per gutter run (900 sq ft each) | 0.0104 x 900 x 1.7 = 15.9 gpm | Equation 11-1 |
| Gutter | 4-inch semicircular at 1/8 in/ft = 39 gpm | Table 1106.6 |
| Leader | 2-inch round = 30 gpm | Table 1106.3 |
| Discharge | Permitted onto flat areas such as streets or lawns where approved, flowing away from the building | 1101.2 |
| Foundation drain | Not less than 4 inches, perforated or open-jointed, Table 1102.5 material | 1102.5, 1111.1 |
| Foundation drain discharge | Trapped area drain, sump, dry well or approved above-ground location | 1111.1 |
| Subsoil sump | 18 inches diameter, 24 inches deep, no gastight cover, no vent | 1111.1, 1113.1.2 |
| Sump discharge valves | Only a check valve required in a one- or two-family dwelling | 1113.1.4 Exception |
| Backwater protection | Accessibly located backwater valve if subject to backwater | 1111.1 |
What minimum diameter does IPC 1111.1 require for a subsoil drain?
How does a subsoil sump differ from a sanitary sump under the IPC?
What valves does IPC 1113.1.4 require in storm sump discharge piping, and what relief applies to dwellings?
A 6-inch semicircular gutter is installed at a slope of 1/8 inch per foot. What is its capacity under Table 1106.6?