When to Modify CMDA in a New System Design?

4/10/2026

I am doing CMDA overhead sprinklers for in-rack class 4 storage at 15' - a new system.

As I was navigating through NFPA 13 2022 Chapter 21, I came to 21.4, which seems to be where I should be.

My question is: 21.4.1.1.1 is where you start, which states that if it's a new sprinkler system, the demand shall be determined in accordance with 24.4.1.2.2.1. BUT, I skipped that and went on to 21.4.1.2.1.1, which seems to be for the EXACT SAME CRITERIA (unless I cannot read/comprehend, which is possible)!

So when would you use 24.4.1.2.2.1 over 21.4.1.2.1.1?

When we use 24.4.1.2.2.1 with the modification chart, we get a density of 0.297, and when we use 24.4.1.2.1.1, it calls for 0.30.

So they're pretty much the same... but I'm not seeing why you would ever use 24.4.1.2.1.1 since 24.4.1.1.1 directs you to 24.4.1.2.2.1 and bypasses the other. Hopefully, this makes sense.

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21.4.1.1 General

21.4.1.1.1 Ceiling sprinkler water demand for new systems shall be determined in accordance with 21.4.1.2.2.1 for single and double row racks or...

21.4.1.1.2 Ceiling sprinkler water demand for the modification of existing systems...

21.4.1.2 Protection Criteria for Single or Double-Row Rack Storage of Class I-IV Commodities over 12 ft Up to and Including 25 ft in Height.

21.4.1.2.1 New Systems Criteria for Single- or Double-Row Rack Storage of Class I-IV Commodities Stored Over 12 ft Up to and Including 25 ft in Height.

21.4.1.2.1.1 For single- or double-row racks for Class I, II, III, or IV commodities, encapsulated or nonencapsulated, the ceiling sprinkler demand.... shall be selected from the criteria in Table 21.4.1.2.1.1(a) through Table 21.4.1.2.1.1(c) that are appropriate for... and shall be modified as appropriate by 21.4.1.4.

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3 Comments

Pete H

4/10/2026 06:24:16 am

I think it's just a redundancy. It's changed in 2025 edition to 21.4.1.1 being general and only referring you to use 21.4.1.2 for single row and double row, 21.4.1.3 and 21.4.1.5 for multiple row.

Dan Wilder

4/10/2026 09:45:06 am

I think I am missing something....The sections referenced above from Chapter 21 have no reference to 24...

"So when would you use 24.4.1.2.2.1 over 21.4.1.2.1.1?"

24.4 is "Hose Stream Allowance and Water Supply Duration"?

Assuming you meant to stay within Chapter 21:

Otherwise, this is a single point design density (Section 21.4.1.2.1) change as all the referenced curves we have been using are now existing systems (21.4.1.2.2) now fall into the revised name of:

21.4.1.2.2 Evaluation or Modification of Existing Systems for Single- or Double-Row Rack Storage of Class I Through Class IV Commodities Stored Over 12 ft (3.7 m) Up to and Including 25 ft (7.6 m) in Height.

Jose R Figueroa

4/10/2026 01:09:18 pm

Right from the NFPA for a new overhead sprinkler system, such as your in-rack Class IV at 15 ft storage height, do not use the modification chart or figures — instead, rely on the New Systems Criteria tables.

This is the exact distinction that NFPA 13 (2022) makes in Chapter 21.4.1.1, which explains why the two sections look similar but are deliberately separated.

Quick Overview of the Sections (NFPA 13 2022)

- 21.4.1.1.1 (the initial paragraph) — Ceiling sprinkler water demand for new systems must be determined according to the New Systems Criteria (21.4.1.2.1 and its tables). (Note: In your original post, there was a typo — it’s 21.4, not 24.4. Everything stays within Chapter 21.)
- 21.4.1.1.2 — This paragraph applies exclusively when evaluating or modifying an existing system, which refers to the Evaluation/Modification Criteria (21.4.1.2.2 and its legacy curves/figures, plus the modification chart).
- 21.4.1.2.1 — “New Systems Criteria...' 21.4.1.2.1.1 instructs selecting the density per area from Tables 21.4.1.2.1.1(a)–(c) (the new tabulated values). For your specific case (Class IV, 15 ft storage, single- or double-row rack, CMDA overhead), the table shows 0.30 gpm/ft², generally over a 2000 ft² design area. No interpolation or modification chart is needed.
- 21.4.1.2.2 — “Evaluation/Modification of Existing Systems Criteria...” This section (and 21.4.1.2.2.1) allows using legacy density curves/figures from the 2019 edition, along with the modification chart in 21.4.1.4. The 0.297 gpm/ft² value results from interpolation on the old curve plus an adjustment using the chart. This path is explicitly prohibited for new systems.

Reason for the Two Values (0.30 vs. 0.297)

The 2022 edition intentionally “froze” the new-system tables at standardized, clear densities—often based on a 2000 ft² design area—to ensure consistency and simplify design for new installations. The legacy curves and modification chart were kept only for existing systems, so owners aren’t required to replace sprinklers due to minor code updates. That’s why the modification path might give a slightly different, sometimes lower, number.

Bottom Line for Your Job
- For a new system, only use Tables 21.4.1.2.1.1, which provide 0.30 gpm/ft².
- Never apply the modification chart (21.4.1.2.2.1) for a new system.
- The standard explicitly directs you away from it in 21.4.1.1.1.

If the AHJ asks why you didn’t use the modification chart, just cite 21.4.1.1.1 and 21.4.1.1.2 — the code clearly separates these requirements.

(There was also a minor erratum (13-22-3) that restored the 2019 figures for existing systems, but it does not affect the new-system table values at all.)

You’re all set with 0.30.

When to Modify CMDA in a New System Design?

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