Sprinkler Locations for Tight Wood Trusses?

6/24/2025

I have a building where the 3rd floor is open to the structure. It is open-web joists (wood) with 2x6 members on top and bottom, and 1x6 supports. They are spaced 16" apart.

The bottom of the joist elevation is parallel with the floor throughout, but the top varies, as the roof slopes slightly. I cannot come up with a passable solution to sprinkler this area, since the joists are too close together to pass the 3X rule, and the bottom of the joist is more than 22" below deck for the majority of the space.

How would you approach this from a sprinkler design perspective, considering the obstruction rules as well as unobstructed vs obstructed construction?

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7 Comments

Dan Wilder

6/24/2025 08:17:23 am

Well, applying 10.2.7.2.1.6 (19' edition) is out due to the spacing being less than 24" on center.

You'll be left applying 10.2.7.2.1.7 (19' edition) and placing sprinklers centered on a truss and a minimum of 6" clear away from the bottom chords and using the 3x rule for the diagonal.

The enhanced content within the book has a good example of this location and spacing applications.

Dan Wilder

6/24/2025 08:24:45 am

And as for spacing, 10.2.4.2.1 (19' Edition) for Combustible Obstructed with members less than 3' on center puts you at 130 ft² & max 15' on center, unless this turns into a combustible concealed space, then you may need to look at a combustible concealed space application. I am taking "slightly slopped" as less than 2:12 which doesn't trigger the 120 ft² and more restrictive spacing requirements along the slope.

Jesse

6/24/2025 08:22:17 am

I love questions like this because they require some outside the box thought. I agree with Dan's assessment. I've posted the text from 10.2.7.2.1.7 here

10.2.7.2.1.7

Sprinklers shall be permitted to be installed on the centerline of a truss or bar joist or directly above a beam, provided that the truss chord or beam dimension is not more than 8 in. (200 mm) and the sprinkler deflector is located at least 6 in. (150 mm) above the structural member and where the sprinkler is positioned at a distance three times greater than the maximum dimension of the web members away from the web members.

Anthony

6/24/2025 08:24:37 am

I wouldn't, call this obstructed construction. These joists are more than 70% open. I think you might have a shot for sprinklers in the A's and V's at their most open areas.

Note you'll be at 130 sqft spacing combustible construction with members less than 3'-0 on center.

I think the best option is put in a ceiling do above and below protection with cpvc and combustible concealed space sprinklers.

Casey Milhorn

6/24/2025 08:57:40 am

Agree with Dan and Jesse. I think you have a shot at placing upright in the "A" centered on the truss, however I don't think you will defeat the 3X rule in every direction relative to the adjacent trusses. BUT, depending on the spacing of the As and Vs, you might be able to install extra branch lines, while still keeping uprights 6' apart, and achieve adequate coverage (at least from a common sense perspective). It might meet the letter of the law here. If you do overload the space to provide better coverage, I would consider 4.2k sprinklers since your spacing will be so dense and the 7psi will get you in your calcs if you go 5.6. The other alternative is a ceiling attached directly to the bottom chord and fill the space with non-combustible insulation.

Jack G

6/24/2025 04:51:56 pm

I m with Jesse / Anthony on locating in the the A s and V s.
I think “ pipe lengths “ would be a challenge locating sprinklers thru the closely spaced joists.
Branches below the truss would knock out a future ceiling unless they did a dropped ceiling beneath the sprinkler piping.
I ve run branches parallel to the joist bottom, do an arm to get the head where it should be, and can tilt the sprinkler on the sprig to match roof pitch and / or adjust sprig length, ( if more than 2 inch slope) .
The supply can be below at an end ( one small box out) or can be thru the joist so that is the only pipe needing short lengths.

Jay (OP)

6/27/2025 08:04:04 am

Thanks, all, for the replies. The roof is only sloped at .25 / 12, but due to the length of the building the truss depth varies from about 14" on the north end to 38" on the south end, with the average in the middle being around 26" - 30". The angle of the diagonal truss members will be slightly different on every truss, making it virtually impossible to strictly adhere to the 3x rule. I think the best I can do is space the heads close enough to allow for some field adjustments, and hope the AHJ takes the common sense perspective mentioned by Casey.

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Sprinkler Locations for Tight Wood Trusses?

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