In-Rack Demand with Overhead Sprinkler Calc?

10/3/2025

When calculating overhead systems with in-rack sprinklers, how much in-rack demand is added to the calculations?

I assume all in-racks within the most remote area, but want to ask to be sure I'm getting this right.

Thanks in advance.

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7 Comments

Pete H

10/3/2025 06:05:20 am

All the in-rack sprinklers in the design area typically unless you're using some exception due to circumstances of storage in the code.

The trade off is typically with in-rack heads you get lower design densities than a full ceiling-only system. But given the difference in cost of labor, if you can design ceiling-only for your storage, I'd advise to pursue that option.

Brett

10/3/2025 06:42:14 am

The answer is it depends. It depends on the sprinkler type, commodity being protected, storage height, open shelves or solid shelves, and how many rows of in-racks are being provided. If this is an NFPA 13 question and not an FM or NFPA 30 question, then the specific section depends on which edition of NFPA 13 is applicable. If you're in the 2019 edition then look at section 25.12.

Anthony

10/3/2025 07:33:36 am

As other have said: "it depends" which I would totally understand is not as helpful as you'd like.

Hope this is helpful:

If you're a designer please look at the design criteria for each scenario outlined in NFAP 13 or the relevant FM document. It wont be in the most convenient spot but it should be in the same chapter. (NFAP 13-2019 and later)

If you're a reviewer demand the code section pulled for the design criteria. It's well within an AHJ's right to ask for the applicable code path for any design.

For everyone: Clearly define your commodity and storage arrangement before you dive into the in rack section of code. IF you're trying to min/mx for cost clearly define each storage arrangement, ceiling mixed or independent scheme, open racks or solid shelves; FOR EACH estimate. there will be operational implications to each different type of storage arrangements.

Dan Wilder

10/3/2025 08:04:27 am

Careful with this...hydraulically remote vs physically remote are not always the same location, especially with different piping configurations for overhead vs in-rack layouts.

For example, a grid system with a riser in the corner of the building will typically have the overhead RA on the opposite corner last couple lines but ~1/4 to ~1/3 between the two mains, closer to the far/dead/secondary main, in the case of a center ridge building with slopes down each half, that RA may shift to more the middle of the building (due to elevation loss exceeding and shorter piping run gains) and there are times the RA will be locked adjacent to the far main. An in-rack system RA may be located against the far wall, well outside of the overhead RA or possibly in the middle of the building where the racks are tallest or a higher commodity classification and design criteria is required.

As to how much of the in-rack demand....all of it as required by the design criteria, balanced at the point of connection to the overhead.

Adam

10/3/2025 08:23:04 am

I remember doing an in-rack design where the most demanding racking was not located under the most demanding ceiling area.

I ended up running multiple calculations, shifting the ceiling area one sprinkler at a time throughout the space (I did not have fully-automatic software). The most demanding total area ended up shifting slightly away from the most demanding ceiling area when the ceiling area was calculated independently.

It's interesting how the two can affect each other.

Glenn Berger

10/3/2025 08:21:14 am

You got some really good responses to your question.

The basic answer is that all sprinklers that are located in the hydraulically most demanding area are to be included in the calculations.

Franck

10/3/2025 09:40:00 am

You have to add the number of IRAS based on the commodity classification and number of level of IRAS.
But in no case it will be all the IRAS located below the ceiling demand area.
If your ceiling design is over 2000 sq ft, you will have several racks in this area and the IRAS demand is calculated only in one rack, for example with the following number of operating IRAS:
• 6 sprinklers for one level of in-rack for Class I, II and III commodities
• 8 sprinklers for one level of in-rack for Class IV and Group A Plastic commodities
• 10 sprinklers for two or more levels of in-rack for Class I, II and III commodities (5 sprinklers on the top 2 levels)
• 14 sprinklers for two or more levels of in-rack for Class IV and Group A Plastic commodities (7 sprinklers on the top 2 levels)

In-rack sprinklers should discharge at 15 psi minimum for storage up to 25 ft and at a minimum flow of 30 gpm for storage above 25 ft.

As Dan indicated, the final most demanding area is not always the physically farther remote areas as the supply to IRAS and ceiling may follow different paths and you have to balance your all system (at the same pressure) at the point of connection.

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In-Rack Demand with Overhead Sprinkler Calc?

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