We are a structural engineer currently work for an architect on a small assembly hall project; a wedding venue that will seat over 99, which will require sprinkler protection.
The roof structure will be wood scissors trusses, 5’-7” deep at the center – see below.
I mentioned to the architect that they will have to protect the “attic” space also with sprinklers.
In response, the architect said the owners want to finish the ceiling and insulate the attic space.
Where is it written about the maximum cavity size without sprinklers? Is it any cavity?
If so possibly we fill the entire attic space, which is small because of the unique scissors trusses. I don’t think this would be economical or practical.
But, what would the cavity depth have to be (underside of roof deck to top of batt or blown insulation) so that sprinkler protection of that concealed combustible space is not required?
I assume if this space is not sprinkler protected that “attic” compartments would be required. Any guidance on this matter would be appreciated. Thank you.
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2/10/2023 08:14:13 am
I thought NFPA 13 allowed for a 2" gap at the top of the cavity if desired? We have seen this and it allowed for the required ventilation of the roof deck...
2/10/2023 08:14:10 am
NFPA 13 provides the guidelines of when ceiling cavity spaces (See 9.2.1 in the 2019 edition).
2/10/2023 08:29:29 am
We have had the entire attic space filled before in this situation that would be hard. I believe you are referring to the 6" space allowed by NFPA 13, but I do not think you could keep almost 5' of insulation in that space. Depending on how much you want to spend for the fire sprinkler system, you could leave the concealed combustible space unsprinklered and increase your remote area also allowed by NFPA 13. Another option would be to put firewalls in the attic to limit the area of each compartment.
2/10/2023 10:43:52 am
You cannot voluntarily choose to not protect an attic and justify it by increasing your remote area to 3,000 sqft. That's a relatively common misnomer that NFSA (and AFSA I believe) have clarified (and it's not the way NFPA 13 reads either).
2/10/2023 08:38:36 am
184.108.40.206.7 Concealed spaces filled with noncombustible insulation
2/10/2023 09:16:27 am
I would recommend sprinklering the attic or you can compartmentalize with sheetrock draftstopping into 160 ft3 areas.
Todd E Wyatt
2/10/2023 09:50:41 am
The scoping Code (e.g. 2021 IBC) determines “Where Required” an automatic sprinkler system (ASPS) is required per the Occupancy Classifications (OC) of the building.
2/10/2023 11:28:27 am
Note that 903.3.1.2.3 are for sprinkler systems that fall under NFPA 13R.
2/10/2023 09:57:47 am
We run into this a lot, and usually end up protecting the space with automatic sprinklers. In one case where the arch wanted to insulate, the fire marshal required plywood sheathing installed to hold the insulation rolls in the joist space - which in effect created another combustible concealed space below that. And thuis required protection.
2/10/2023 03:10:10 pm
Even if they sprayed foam insulation, would that even be considered a "noncombustible insulation"? I believe the NFPA 13 requirement would require the insulation to be considered noncombustible.
2/10/2023 10:30:25 am
Maybe I am missing something, but if they already have to sprinkler the space below the ceiling,then there already has to be a sprinkler piping network installed and suspended from the structure. So I am not understanding the reluctance to sprinkler the attic space. If it is because wet-system piping sill be below, and the attic is not a conditioned space (such as if the insulation is at the bottom chord) I have often protected the attic space with intermediate-temperature dry upright sprinklers supplied by the wet piping below. Once I had to order them longer than their listing allows; that could be an issue.
2/11/2023 11:01:44 am
What the engineer is not telling us, is if they are spray foaming at the top chord or using batt insulation at bottom chord. As Dave is saying, there will be pipe in the attic truss area regarless. The cost is either spray foam to create a conditioned space or a dry pipe fire sprinkler system for the heads below the ceiling in the occupied space. Batt insulation of a wet system in theory sounds good but always ends up in a freeze up. Spray foam is used today on 99% of the jobs we see in NY. The cost savings is to spray foam top chord and end walls with a wet fire sprinkler system protecting the attic and the occupied space below. This is the least expensive fire sprinkler system. We all know dry systems fail at a greater rate than wet systems due to lack of proper maintance. A wet system with sloped roof has a design area of 1950 square feet, compared to as Wes noted a 3000 sq ft design with out sprinklers in attic. That 3000 sq ft is based on a flat ceiling and we would add the additional 30% to the design area. Thats double the water demand, increased pipe size, increased cost.
2/15/2023 03:58:39 pm
If you have a sprinkler system in the building already it will most likely be cheaper to install back to back attic heads rather than fill the void with insulation.
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