16.2 Storm Drainage Materials, Roof Drains, and Conductors

Four material tables

Storm drainage borrows two tables from Chapter 7 and adds two of its own.

IPC 1102.1 sets the general rule: materials and methods for storm drainage comply with Section 1102 and the applicable provisions of Chapter 7.

Table 1102.4 Building storm sewer pipe

The listed materials:

• ABS plastic pipe in IPS diameters, including Schedule 40, DR 22 (PS 200) and DR 24 (PS 140), solid, cellular core or composite wall
• Cast-iron pipe
• Concrete pipe
• Copper or copper-alloy tubing (Type K, L, M or DWV)
• Polyethylene (PE) plastic pipe
• Polypropylene (PP) pipe
• PVC plastic pipe (Type DWV, SDR26, SDR35, SDR41, PS50 or PS100) in IPS diameters, including Schedule 40, DR 22 and DR 24
• Stainless steel drainage systems, Type 316L
• Vitrified clay pipe

This list closely parallels Table 702.3 for sanitary building sewers — the same heavy-duty materials, including concrete, vitrified clay and PE, that are not permitted above ground.

Table 1102.5 Subsoil drain pipe

A much shorter list, because subsoil drains must admit ground water:

• Cast-iron pipe
• Polyethylene (PE) plastic pipe
• PVC plastic pipe (type sewer pipe, SDR35, PS25, PS50 or PS100)
• Stainless steel drainage systems, Type 316L
• Vitrified clay pipe

IPC 1102.5 requires subsoil drains to be open-jointed, horizontally split or perforated pipe — the defining characteristic. A solid-wall pipe is not a subsoil drain.

Roof drains — IPC 1102.6 and Section 1105

1102.6 Materials: roof drains shall conform to ASME A112.3.1 or ASME A112.6.4. Roof drains other than siphonic roof drains shall be tested and rated in accordance with ASME A112.6.4 or ASPE/IAPMO Z1034.

1105.1 General: roof drains shall be installed in accordance with the manufacturer's instructions. The inside opening for the roof drain shall not be obstructed by the roofing membrane material.

That second sentence describes the single most common roof drain defect: a roofer laps membrane over the drain throat, cutting the effective opening in half. The drain was rated at a published flow; the installed assembly delivers far less.

1105.2 Roof drain flow rate:

The published roof drain flow rate, based on the head of water above the roof drain, shall be used to size the storm drainage system in accordance with Section 1106. The flow rate used for sizing the storm drainage piping shall be based on the maximum anticipated ponding at the roof drain.

This is the 2021 IPC's key methodological point. A roof drain does not have a single capacity; it has a flow rate that varies with the depth of water standing over it. A drain that passes 40 gpm at 1 inch of head may pass 90 gpm at 3 inches. You size the piping to the flow the drain will actually deliver at the ponding depth the roof will actually reach.

And 1106.2 closes the loop from the other side: the flow rate calculated from rainfall and area must be checked against the roof drain manufacturer's published flow rate for the specific model and size to verify the selected drain will handle the anticipated flow.

Conductors and leaders — IPC 1104

The vocabulary distinction matters on exam items:

• A conductor is the interior vertical pipe carrying rainwater from the roof drain down through the building.
• A leader is the exterior vertical pipe — the downspout — carrying water from a gutter or scupper.

Both are sized by the same tables, but 1106.2 covers storm drain piping (vertical and horizontal) and 1106.3 covers vertical leaders specifically.

Fittings, cleanouts and change of direction

• 1101.6: connections and changes in direction use approved drainage-type fittings in accordance with Table 706.3; fittings shall not obstruct or retard flow.
• 1102.7 Fittings: approved for the piping material installed, conforming to the respective pipe standards or Table 1102.7; no ledges, shoulders or reductions capable of retarding or obstructing flow; threaded drainage pipe fittings shall be of the recessed drainage type.
• 1101.8 Cleanouts required: cleanouts in the storm drainage system comply with the sanitary drainage cleanout provisions of Section 708 — 100-foot intervals, at horizontal direction changes greater than 45 degrees, sized per 708.1.5, with the clearances of 708.1.10. Exception: subsurface drainage system.
• 1101.5 Change in size: the size of a drainage pipe shall not be reduced in the direction of flow.

Testing

1101.4 Tests: the conductors and the building storm drain shall be tested in accordance with Section 312.

312.8 Storm drainage system test: storm drain systems within a building shall be tested by water or air in accordance with Section 312.2 or 312.3 — that is, a 10-foot head of water or 5 psi of air, held 15 minutes. Recall that under the model code plastic piping shall not be tested using air (312.3), a sentence Colorado deletes.

Installation checklist for a commercial roof