What this code requires
CPC 1101.12 is the California Plumbing Code section that requires roof drains, overflow drains, and storm leaders to be sized for the local rainfall rate using the code’s published drainage tables. Every square foot of roof area must route to a drain sized to handle the peak design storm for your specific location. Single-family additions, ADUs, and commercial re-roofs all trigger this requirement whenever drainage work is part of the permit scope.
How to fix it
Confirm your jurisdiction’s design rainfall rate
Look up the 60-minute, 100-year design rainfall intensity (in inches per hour) for your project city in CPC Table D 101.1 (Appendix D). Its California entries range from 1.3 in/hr (Lake Tahoe) to 3.0 in/hr (Palmdale), with Los Angeles at 2.0. The zone for your project determines every downstream pipe size. If you’re not sure which zone applies, the plan-checker’s correction letter will often name it — or your civil engineer can pull it from the National Weather Service TP-40/NOAA Atlas 14 data for the site coordinates.
Calculate the total roof area draining to each outlet
Measure the horizontal projected area (not the slope area) of every roof surface draining to each drain or scupper. For sloped roofs, use the plan-view footprint, not the rafter length. Count any area where water will concentrate: skylights, equipment pads, and parapet pockets all count. If multiple roof levels drain to a common leader, add those areas together. Document each sub-area on the plans so the reviewer can follow your math without calling you.
Size roof drains and vertical leaders using CPC Table 1103.1
CPC Table 1103.1 sizes vertical conductors and leaders by projected roof area at your design rainfall rate, and Table 1103.2 does the same for horizontal storm drains by pipe diameter and slope. Work through it in sequence: area (sq ft) x rainfall rate (in/hr) / 96.23 gives you the required GPM per outlet, then match that GPM to a pipe size in the vertical-leader table. Most residential leaders end up at 3 or 4 inches, but in high-rainfall zones a single large roof may need 5 or 6 inches. Show the calculation on the plans — a table format with area, GPM, pipe size, and table reference is what reviewers expect.
Size horizontal storm conductors for slope and flow
Once you have the vertical leader size, continue through the CPC horizontal-conductor sizing table. Pipe size depends on both GPM and slope — a 4-inch pipe at 1/8 inch per foot carries significantly less than at 1/4 inch per foot. Label the slope on every horizontal run in the plans. If you can’t achieve the minimum slope shown in the table due to structural constraints, increase the pipe size to the next diameter that covers your GPM at the flatter slope.
Add overflow drains or scuppers sized for the same flow
CPC 1101.12.2 requires secondary (emergency) roof drainage by scuppers, an open side, or secondary roof drains set at least 2 inches above the roof surface. A separate secondary piping system is sized for the same rainfall rate as the primary system, but if secondary drains tie into the primary conductor, the combined system must be sized for double the local rainfall rate. Scuppers must be at least 4 inches high and as wide as the circumference of the required roof drain. Show both the primary and overflow systems on the roof plan with callouts for size and type.
Label drain types and pipe materials on the plans
CPC 1101.12 corrections sometimes come back a second time because the first resubmit fixed the sizing but didn’t identify drain types. Call out each drain as “roof drain,” “overflow drain,” or “area drain” on the plan. Note the material (cast iron, PVC, or other approved material per CPC Chapter 11) and the connection method. Roof drain bodies need to match the pipe material downstream, and some cities have local amendments requiring cast iron for the first 4 feet of leader below a roof drain.
Include a drainage calculations note block on the plans
Put a simple table directly on the plumbing or civil sheet: list each drain by ID, contributing area in square feet, design rainfall intensity in in/hr, required GPM, selected pipe size, and the CPC table used. This format gives the reviewer everything they need in one place and avoids the back-and-forth of a comment asking where the calculation is. If a civil engineer stamped the drainage design, include their stamp and license number on the same sheet.
Resubmit with a written response addressing each correction item
When you resubmit the revised plans, attach a written response to each correction item that cited CPC 1101.12. Reference the sheet and detail number where each fix appears. Plan-checkers reviewing a resubmit spend less time on sets that map each correction to a specific plan location — it’s not required, but it consistently shortens the review cycle.
Take the CPC 1101.12 fix steps with you
All 8 steps in one email, so you have them on site instead of on this page.
Why you got flagged
Plan-checkers flag CPC 1101.12 when storm drainage calculations are missing entirely from a set. Roof drainage is often treated as an afterthought on residential additions — the architect draws in a generic 4-inch leader without referencing the rainfall tables, and the plumbing reviewer sends it back. No calculation, no approval.
A second common scenario is using a flat default pipe size when the local design rainfall rate makes that size undersized. California’s hourly rainfall intensity varies from under one inch in the dry interior valleys to over four inches along parts of the North Coast. A 4-inch vertical leader that handles 3,450 square feet in a low-rainfall zone might only handle 1,500 square feet in a high-intensity zone. Getting the zone wrong means the plans show a system that will overflow during a design storm.
Overflow drain omission is another frequent flag. When a primary roof drain clogs, water needs somewhere to go before it pondes enough to exceed the structural load limit. If the plans show primary drains but no overflow drains or scuppers, the plan-checker will cite 1101.12 even if the primary drain sizing is correct.
A fourth trigger is re-roofing permits that add or relocate drains without updating the drainage calculations. The reviewer will ask for revised sizing to confirm the new drain layout matches the code tables for your jurisdiction’s rainfall zone.
The official code
California Plumbing Code — Part 5 of Title 24 (Building Standards Commission)
Read the source →