IPC 308.5 · 1106.2 · Supports & storm drainage

Pipe hanger spacing and storm drain sizing, explained

How to read the hanger-spacing table by material, and how to turn roof area and rainfall rate into a storm drain size — with worked examples and a free link to the official tables.

Need the actual tables? Read IPC Table 308.5 and Table 1106.2 free at codes.iccsafe.org →

How to read Table 308.5 (hanger spacing), step by step

Step	What you do	Watch out for
1	Find the row for the pipe material (and, for some materials, the size range)	Copper, steel, cast iron, PVC/ABS, CPVC, PEX, and PE each have their own row — plastic needs far closer support than metal
2	Read the maximum horizontal spacing	It is a maximum — add supports at changes of direction, valves, and heavy fittings
3	Read the maximum vertical spacing	Some small plastic pipe also needs a mid-story guide
4	Check the table notes and the manufacturer’s instructions	Where the listing or manufacturer calls for closer spacing, follow it
5	Protect the pipe: compatible hanger material and isolation for dissimilar metals	Bare steel hangers on copper cause galvanic corrosion

Our own summary, not the code text. Read IPC Table 308.5 and Section 1106 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

Worked example — two 40-ft horizontal runs

Run	Max horizontal spacing	Supports needed
1-in. copper tube, 40 ft	6 ft (copper tubing, 1-1/4 in. and smaller)	40 ÷ 6 = 6.7 → 7 spaces, 8 supports counting both ends
3-in. PVC DWV, 40 ft	4 ft (PVC pipe)	40 ÷ 4 = 10 spaces, 11 supports counting both ends — plus extra at fittings

Worked example values only. Read the full table free at codes.iccsafe.org — other materials and vertical spacing differ.

How to size a storm drain under IPC 1106, step by step

Step	What you do	Watch out for
1	Get the design rainfall rate — the 100-year, 1-hour rate from the IPC rainfall maps (Figure 1106.1) or approved local data	Many jurisdictions publish their own design rate; use it when they do
2	Measure the horizontally projected roof area draining to each roof drain	Walls that shed water onto the roof add area — see the vertical-wall provision in Section 1106
3	Convert area and rainfall to flow: gpm = area (sq ft) × rainfall (in./hr) × 0.0104	The IPC gives this as its storm-flow equation in Section 1106; 0.0104 is simply 1 in./hr on 1 sq ft expressed in gpm
4	Total the flow on each section of storm drain, working downstream	Each section carries everything upstream of it
5	Read Table 1106.2 in the vertical column (leaders and conductors) or the horizontal column for your installed slope	Flatter slope = less capacity
6	Size the secondary (emergency) roof drainage separately (Section 1108)	Secondary drains discharge above grade where they can be seen, or to a separate system — never into the primary piping

Our own summary, not the code text. Read IPC Table 308.5 and Section 1106 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

Worked example — flat roof draining to one horizontal storm drain

Input / step	Value	Source
Horizontally projected roof area	12,000 sq ft	Roof plan
Design rainfall rate	3 in./hr (assumed from the local map)	Figure 1106.1 or local data
Flow	12,000 × 3 × 0.0104 = 374 gpm	IPC storm-flow equation (Section 1106)
Installed slope	1/8 in. per ft	Design
Result	Choose the smallest pipe whose horizontal capacity at 1/8 in. per ft is at least 374 gpm	Table 1106.2 — read it free at codes.iccsafe.org

Worked example with assumed rainfall. Your local rate may be very different — check the map or your AHJ’s design rate.

Common mistakes

Mistake	What to do instead
Using metal-pipe spacing on plastic pipe	Plastic pipe needs much closer support — read the material row
Spacing supports only by the table on long vertical plastic risers	Add the mid-story guides the table calls for on small plastic pipe
Using a 1-in./hr capacity table without adjusting for your rainfall	Convert your actual area and rainfall to gpm and compare with the table
Using sloped roof surface area	Use the horizontally projected (plan) area
Tying secondary roof drains into the primary storm drain	Keep the secondary system separate so a blocked primary is visible
Reading the vertical column for a horizontal drain	Leaders and horizontal drains have separate capacity columns

Our own summary, not the code text. Read IPC Table 308.5 and Section 1106 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

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Holding pipe up and getting water off the roof

Read Table 308.5 and Section 1106 free on codes.iccsafe.org. ICC offers free read-only access to the 2021 and 2024 International Plumbing Code. This page explains how to use the tables; it does not reproduce them.

Hanger spacing is set by material. Table 308.5 gives a maximum horizontal and vertical spacing for each pipe type; stiff metal pipe can span farther than plastic, which sags as it warms. For example, copper tubing 1-1/4 inch and smaller is limited to 6 ft horizontally and PVC pipe to 4 ft, so a 40-ft run of 3-inch PVC DWV takes 11 supports before you add any at fittings. The table is a maximum; manufacturer instructions can be stricter.

Storm drains are sized by flow. Start with the design rainfall rate — the 100-year, 1-hour rate from the IPC maps or approved local data — and the plan area of roof feeding each drain. Multiply area × rainfall × 0.0104 to get gpm, then read Table 1106.2 in the vertical or horizontal column for your installed slope. A 12,000 sq ft roof at 3 in./hr works out to about 374 gpm.

Western jurisdictions on the Uniform Plumbing Code (IAPMO) use the UPC’s own hanger table and storm-drainage chapter; values differ for some materials.

Related: sanitary drain sizing with DFUs, copper tube sizes, and steel pipe schedules. Field PM keeps hanger inspections, test records, and photos with the job.

FAQ

How far apart should pipe hangers be?+

It depends on the pipe material and size. IPC Table 308.5 lists a maximum horizontal and vertical spacing for each material — copper tubing 1-1/4 inch and smaller, for example, is limited to 6 ft horizontally, and PVC pipe to 4 ft. Read the full table free at codes.iccsafe.org and follow closer manufacturer spacing where it applies.

Why does plastic pipe need more hangers than metal?+

Plastic pipe is less stiff and softens as it warms, so it sags between supports. Sagging drain pipe holds water and solids and can lose its slope, which is why plastic rows in the table call for much tighter spacing.

How do I size a storm drain?+

Find the design rainfall rate (IPC Figure 1106.1 or approved local data), measure the plan roof area draining to each drain, convert to gpm with area × rainfall × 0.0104, and pick the smallest pipe whose capacity in IPC Table 1106.2 meets that flow at your installed slope.

Do I need secondary roof drains?+

Where roof parapets or other construction would trap water if the primary drains clog, the IPC requires secondary (emergency) roof drainage under Section 1108, sized for the same rainfall rate and kept separate from the primary system. Confirm with your structural engineer and AHJ.

How does the UPC differ?+

The Uniform Plumbing Code (IAPMO, common in western states) has its own hanger table and storm-drainage chapter, with different spacing for some materials and its own sizing tables. Use the code your jurisdiction adopted.

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