8'-0" max spacing perpendicular to deck slope

[kolby15]

I have an attic space that is currently protected by BB attic sprinklers and AP sprinklers, it was an unoccupied attic area. Roof pitch is around 12/12.. They are going to occupy this space with offices and install a wood plank ceiling directly onto the roof structure so no roof joist will be exposed. It will not really be a "concealed space" at this point. There will be exposed wood truss's beams going across the space spaced 8'-0" on center +-.. They are proposing to remove the ATTIC sprinklers since it will be an occupied space, and want to install standard uprights for the new occupancy.

Per 2010 Nfpa 13, 8.6.2.2.1 (A) it says: For combustible concealed space under a pitched roof having combustible wood joist or combustible truss construction with members less than 3'-0" on center with slopes greater than 4 in 12 pitch. To the right under the Max Spacing column it says: 15'-0" parallel to the slope 10'-0" perpendicular to the slope, with an asterisk * that refers to 8.6.2.2.1.1 which says when 8 foot max spacing perpendicular to the roof slope is exceeded the minimum pressure shall be 20 psi...


Since this space will no longer be considered a concealed space, does the 8'-0" max spacing perpendicular to the roof slope rule still apply because of the roof pitch?

Does that rule apply to an occupied space in addition to combustible concealed spaces ?

I know I need to increase the remote area for the roof pitch, but 8'-0" max spacing will really kill the job as I will end up calculating the entire space, and it will not work most likely.

Thanks in advance.

 

[TravisMack]

A couple thoughts:

1 - If the space is occupied, it should be conditioned. At that point, you can use 4.2k sprinklers and a wet system. That will drastically cut down on the over-discharge.
2 - I would say that you no longer have a concealed space and the concealed spaces requirements are not applicable.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[Jondoeiowa]

Following off Travis, it would appear to be 8.6.2.2.1 (a) "Combustible Unobstructed with exposed members greater than 3' O.C." and also with a sloped ceiling so don't forget your 30% design are increase.

 

[MGXFP]

If the peak is less than 20' and the system is wet with QR sprinklers you could reduce the remote area as well.

 

[TravisMack]

Yeah, I assume you will still be limited to 130 sq ft per sprinkler. A 12:12 slope would lead me to believe that the 20' height will be surpassed pretty quickly. But, you will have the 30% increase for sure.

Since you "space on the slope and calc on the flat," you may even want to look at 2.8k sprinklers as that area per sprinkler on the flat projection is going to be pretty small. That is provided you have a wet pipe system.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[kolby15]

Travis,
Are you sure about the 130 sq. ft spacing? If it falls under Combustible unobstructed with exposed members more than 3'-0" O.C (The exposed beams/ trusses) 8.6.2.2.1 says I can go to 225 sq.ft spacing with a calculated system.

It's 18'-4" to the peak.. Would I still be allowed a slight reduction in my remote area for qr sprinklers if I have to initially increase it by 30% for the steep pitch deck?

Thanks guys!

 

[TravisMack]

I was assuming your trusses would be the standard 24" on center. In that case, you would be limited to the 130 sq ft for combustible members less than 36" on center. I missed your 8' on center in the original post. Is your construction considered obstructed? That would put you at 168 sq ft max.

Yes, you can apply multiple reductions. At 18.333', you would be 1438 sq ft design area.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[kolby15]

It's very old construction. The roof joist are 2'-0" o.c, but they will all be covered up by the new wood plank ceiling, so I am thinking the roof joist are no longer a factor in considering the spacing of the sprinklers, am I correct in that thinking? It's really just a steep pitch ceiling at this point,with exposed cedar columns and beams. No, it's not obstructed construction.

 

[kolby15]

I just got onsite for the first time today and the horizontal "beams" that I said were 8'-0" on center are actually wood collar ties spaced anywhere from 24" to 34" on center. Most are around the 24" mark, tied into the roof joist. (The cad files and pdf plans leave a lot to be desired on this job.)
The collar ties do not make this space "obstructed construction" but would these collar ties put the sprinkler spacing under the category of Combustible unobstructed with exposed members less than 3'-0" on center? for a max spacing of 130 sq.ft per sprinkler? This seems like it would, but I am just trying to be sure before I move ahead with my layout.

Sorry for any confusion.

Thanks for your help

 

[TravisMack]

I would take the conservative approach and say you are going to be limited to 130. Again, if this is a wet system, you will find that the 4.2k sprinklers may save you a pipe size or 2.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[kolby15]

I spaced @ 130, and I am close to getting it to work with 2.8 k sprinklers.. I have lousy pressure and it's 55'0" in elevation to boot!

Thanks for all your help.

 

[kolby15]

Is the minimum psi for a 2.8k head 7psi?

 

[TravisMack]

Yeah, 7 psi for 2.8k sprinklers, as long as they are not covering more than 74 sq ft.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692

 

[kolby15]

What is the formula for determining the min psi if it is over 74 sq.ft spacing?

Thanks

 

[TravisMack]

The formula is as old as hydraulic calculations in NFPA 13.

Q=k*P^.5

Once you know your Q, you can then divide by your density to get the spacing that corresponds to the Q.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

"Follow" us at

https://www.facebook.com/pages/MFP-Design-LLC/92218417692