What is "Exposed" Combustible Construction?

10/16/2023

NFPA 13 in talking about concealed spaces uses the language "Concealed spaces of exposed combustible construction shall be protected by sprinklers."

What does the term "exposed" construction mean?

We have a wood deck structure with a steel frame and a un-heated attic; so trying to figure out how to solve this without a dry system (historic building so spray foam on the interior is economical).

It would be nice to put an ignition barrier/thermal barrier on the wood deck and then not provide a sprinkler system. (concrete deck of the attic).

Thanks in advance for your take.

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12 Comments

Dan Wilder

10/16/2023 08:58:30 am

So, when there is no direct explanation of a term within NFPA, Websters Dictionary is supposed to provide the meaning of a term, in this case Exposed = "Open to view" and/or "Not shielded or protected"

As I dive into the IBC/IFC more (always learning)..in order to consider the space non-combustible, there would need to be some protection of the combustible materials via covering it (i.e. Type X gypsum board covering the entire exposed combustible face of x" thickness). Spray foam would not qualify and may make things worse. This would also need to be defined by an architect and approved by the AHJ as to a compliant building design.

You may try and use something from NFPA 914, an FPE, and a performance-based design approach. The building owners, local AHJ, and the Historic Preservation overseeing the building are all stakeholders that may prefer spending the money to avoid an invasive installation process.

Chad

10/16/2023 12:36:13 pm

As Dan said, you have to wrap in in Type X or something similar, if allowed/accepted. This is the only way to not sprinkle that attic.

Casey Milhorn

10/16/2023 09:14:01 am

This is from NFPA 13, 2016 ed. Not sure which edition is applicable for you, but I don't believe this has changed much lately. (verify that of course).

8.15.1.2.1* Concealed spaces of noncombustible and limited combustible construction with minimal combustible loading
having no access shall not require sprinkler protection.

A.8.15.1.2.1 Minor quantities of combustible materials such
as but not limited to cabling, nonmetallic plumbing piping,
nonstructural wood, and so forth can be present in concealed
spaces constructed of limited or noncombustible materials
but should not typically be viewed as requiring sprinklers (see
8.15.1.1). For example, it is not the intent of this section to
require sprinklers, which would not otherwise be required, in
the interstitial space of a typical office building solely due to
the presence of the usual amount of cabling within the space.
The threshold value at which sprinklers become necessary in
the concealed space is not defined.

I think it's going to be a call by the AHJ and whether or not the wood deck is deemed as structural.

Anthony

10/17/2023 07:05:55 am

The phrase "The threshold value at which sprinklers become necessary in the concealed space is not defined." has always lead to a judgement call. IMO if your whole roof deck is wood then you have a fire spreading hazard and possible structural problems in the event of a fire. As compared to a framed out wall with studs extending to a concrete deck where the structure wont collapse in a fire.

I think case is right this is an AHJ call to determine structure qualifications on the deck.

DAVID T WILLIAMS

10/16/2023 09:38:31 am

The AHJ is NAVFAC, Since we were trying to avoid a dry system we had gotten some preliminary approval of installing Tyvek with cap nails on the deck (to allow removal), then spray foam, then DC315 intumescent coating.. but our historic steel framing system pushed that cost well over an additional $500,000 do to labor of installing the Tyvek around all the framing.

I did discover some paint on products that conform to E814 that provide fire proofing of wood, and our historical architecture consultant says that could be okay. But then we are in an uninsulated attic so the temps would drop below 40 degrees and the dry system add is $250,000.. so if we can coat the wood so it is not "exposed" we could eliminate the sprinkler.

(BTW the attic was retrofitted with sprinklers in the '50s, but with steam heat in the building the attic was historically toasty... not expected after we are done)

Streeter

10/16/2023 10:26:29 am

Fireproofing wood with paint does not typically make the material non-combustible, it just creates a non-combustible surface. Per NFPA 13, A.4.10.1.1(1) commentary, intumescent coatings can be applied to trusses and plywood to create a noncombustible or limited combustible coating, they typically will not meet the NFPA ASTM standards for a noncombustible material, and need AHJ approval. I don't think painting the wood creates a code-compliant non-combustible space, and think the only options are dry system, fill with non-combustible insulation, which is not reasonable, gyp the entire attic, or what might be the most reasonable option, seal the attic and heat it properly to just have a wet system.

Jon Nisja

10/16/2023 12:14:14 pm

Extreme caution urged here; there is no way to make wood (a combustible material) into a noncombustible material by applying some after-market product. Most of these products - as Streeter points out - help with flame spread reduction by making surface ignition or burning slower.

You can provide an intumescent coating that swells up when exposed to heat and protects the wood. There are often some restrictions on those products such as strict application thicknesses, weathering considerations, reapplication requirements after a certain age of the product, environmental factors (temperature, humidity, etc.), and so on.

Here is a good reference on some of these similar (yet different and confusing) terms and their applicable tests.
https://dps.mn.gov/divisions/sfm/fire-code/Documents/Fire-Code-Information-Sheets/Fire-tests-and-terminology.pdf

Jack G

10/16/2023 02:02:25 pm

Not knowing how big your attic is, would it be possible to remove the wood plank and reinstall lightweight concrete planks—— no combustibles. ( ? ) bucks material, ( ? ) bucks labor demo as you install ?

Glenn Berger

10/16/2023 05:28:13 pm

"Exposed Combustible" is one of the easier definitions in NFPA 13. If non-fire-rated materials is visible, then exposed combustible construction is occurring. Encapsulated the combustible construction with an approved fire retardant material is typical accepted to eliminate the sprinkler requirement. Check with the authorities before proceeding down this path and get their approval.

David Williams

10/16/2023 08:08:07 pm

Well... it doesn't say you can eliminate sprinklers in a combustable SPACE... but it DOES say if the space has no EXPOSED combustable construction, sprinklers are not required... thus my question. What makes the wood roof deck boards not EXPOSED or the surface non-combustable ? Only 5/8 Gypboard? Intumescent spray? Rockwool. The UL 814 product I found?

No sources of ignition... Concrete floor of the attic, steel frame structure.

Todd Wyatt

10/17/2023 04:02:49 pm

NFPA 13 – 2019 references “exposed” (107) times (e.g. 9.3.17.1.2 Localized Protection of EXPOSED Combustible Construction or Exposed Combustibles) references it (10) times in Chapter 2 Definition but does not specifically define what “EXPOSED combustible construction”.

Determine if the Attic meets the requirements of a “concealed space” and if so, determine if it meets the “Allowable Sprinkler Omission Locations” per 9.2.

“Concealed Space” is not defined in “Chapter 2 Definition” but it is included in the definition of “Small Openings”. “Concealed Space” is referenced (73) times in NFPA 13 – 2019.

REFERENCE
NFPA 13 – 2019
Chapter 2 Definition
3.3.195* Small Openings.
Openings in the ceiling or construction features of a concealed space that allow limited amounts of heat to enter the concealed space.
A.3.3.195 Small Openings.
A return air diffuser can be 4 ft by 2 ft (1.2 m by 600 mm) and meet the definition of a small opening. A linear diffuser can be longer than 4 ft (1.2 m) but is then limited to 8 in. (200 mm) in width (or least dimension). Spaces between ceiling panels of architectural features that create a concealed space must meet the same criteria.

Chapter 9 Sprinkler Location Requirements
9.2 Allowable Sprinkler Omission Locations.
9.2.1* Concealed Spaces Not Requiring Sprinkler Protection.
9.2.1.1*
Concealed spaces of noncombustible and limited-combustible construction with minimal combustible loading having no access shall not require sprinkler protection.
A.9.2.1.1
Minor quantities of combustible materials such as, but not limited to, cabling, nonmetallic plumbing piping, nonstructural wood, and so forth can be present in concealed spaces constructed of limited or noncombustible materials but should not typically be viewed as requiring sprinklers (see 9.3.17.1). For example, it is not the intent of this section to require sprinklers, which would not otherwise be required, in the interstitial space of a typical office building solely due to the presence of the usual amount of cabling within the space. The use of acoustical tile ceilings does not negate that the space above the tile is a concealed space because a tile could be removed. The threshold value at which sprinklers become necessary in the concealed space is not defined.

Enhanced Content
Paragraph 9.2.1.1 is intentionally nondescript in its reference to the combustible loading or opening sizes used to determine the need for sprinklers in the noncombustible concealed space. At the time of design and installation, it is possible to consider the construction of the space and determine what mechanical systems (duct, piping, electrical cabling) might be present in the space. Some authorities have attempted to provide specific requirements for the type and amount of cabling. However, it is almost impossible to determine how much cabling will result in an increase in hazard and what type of cabling will eventually be present in the space, since this material typically is installed long after the sprinkler system. The following exhibit shows a noncombustible concealed space with typical building service systems present. Note that some communications cabling is in the space, but sprinklers have not been required.

James Art, FPE

10/18/2023 05:02:17 pm

2 questions:
1. If a building (made up of modulars) has a metal roof on exposed wood joists (structural) above a hangng tile suspended ceiling,
is the concealed space "combustible"?

2. Same building, but if the underside of the roof is exposed plywood, on all metal joists, and the actual roof above is metal sheeting.

My gut feeling is both of these are exposed combustible,
so sprinklers Above and Below? What spacings?

Thanks

Jim

What is "Exposed" Combustible Construction?

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