Buckley Rumford Fireplaces
Is Drywall Combustible?
An interesting question. Can you attach drywall directly in contact with a masonry chimney?

Two Views

Drywall in not Combustible
According to USG in an article pointed out by Jeff Lockhart

Date: Wed, 23 Oct 2002
From: Jeff Lockhart
Subject: Fire code & sheetrock
To: Jim Buckley


I just came across something which I've never seen mentioned in any article dealing with fireplace & chimney clearances. I've wanted to wrap my (interior) chimney & the sides of the firebox in sheetrock, as I've seen in many pictures, but both the old & new codes wouldn't permit it. But this data sheet from USG lists sheetrock as a limited-combustible, and states that it would be allowed. Here's the article:


USG Data Sheet
Case Studies

In the National Fire Protection Association's NFPA 101 Code for Safety to Life from Fire in Buildings and Structures, a noncombustible material is defined as a material that, "in the form in which it is used and under the conditions anticipated, will not aid combustion or add appreciable heat to an ambient fire." Materials are tested for noncombustibility in ASTM E 136 Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 deg. C. The test exposes small samples of the material to a stream of air heated to 750 deg. C, (1382 deg. F). The material is deemed noncombustible if:

    1) Sample temperatures at no time exceed 780 deg. C, (1436 deg. F).

    2) There is no flaming after 30 seconds. 3) Once the sample loses 50% of its weight, there is no flaming and sample temperatures never exceed 750 deg. C, (1382 deg. F). ASTM E 136 is an extremely strict test and under its criterion, few building materials qualify as noncombustible. Two USG Interiors products which do are CERAMIC HERITAGE and most THERMAFIBER insulation products.

In regard to gypsum wallboard, the product's paper facing prevents it from passing ASTM E 136. However, because it does have a demonstrated ability to perform in fire rated assemblies, the NFPA has placed it in a special classification called limited-combustible This category distinguishes gypsum wallboard from other, more highly combustible products. To qualify as limited-combustible, a material must have a noncombustible structural base or core, a surface less than 1/8 in. (0.3 cm) thickness and a flame-spread rating of 50 or less.

This last requirement can be confusing because the three national model building codes, (ICBO's Uniform Building Code, SBCCI's Standard Building Code and BOCA's National Building Code), all allow composite materials that meet the NFPA's definition of limited-combustible, to be classified as noncombustible.

Drywall is Combustible
According to the State of New Jersey as pointed out by Michael Thomas 2/26/07

Thought you might find this Bulletin (below) of interest:

Michael Thomas
Paragon Home Inspection, LLC
PO Box 1455 Evanston,IL 60204-1455

State of New Jersey
Department of Community Affairs
Division of Codes and Standards
PO Box 802
Trenton, New Jersey 08625-0802
Richard J. Codey
Acting Governor
Charles A. Richman
Acting Commissioner
Date: December 1990
Revised: December 2005 Subject: Gypsum Wallboard Classification
Reference: N.J.A.C. 5:23-3.14, Building Subcode, Section 703.4.2
N.J.A.C. 5:23-3.20, Mechanical Subcode, Chapter 2
Page 1 of 1

It has recently come to the attention of the Department of Community Affairs that gypsum wallboard is something being mistakenly considered a noncombustible material when applying the clearance-to-combustibles requirements of the Mechanical Subcode.

The definition of "noncombustible" in the Mechanical Subcode differs from the definition in the Building Subcode. The Building Subcode's Section 703.4.2, Composite Materials, states that a material having a structural base of noncombustible material (meeting ASTM E136), with a combustible surface not more than 0.125 inch thick, and having a flame-spread rating not greater than 50 (when tested in accordance with ASTM E84) is acceptable as a noncombustible material. Because the Building Subcode allows such "composite" materials, gypsum wallboard can be considered noncombustible as defined by the Building Subcode.

The Mechanical Subcode does not contain such a provision for composite materials. Noncombustible materials in the context of the Mechanical Subcode are those materials which pass ASTM E136. Gypsum board, because of its combustible facing, will not pass ASTM E136. (The only exception to this applies to Type I commercial kitchen hoods at Section 507.9 of the Mechanical Subcode.)

The difference in standards is due to the different purposes of the two subcodes. The Building Subcode is concerned with the performance of a material under the conditions during a fire, while the Mechanical Subcode is concerned with performance during exposure to a constant high-heat source. Therefore remember, under the Mechanical Subcode, gypsum wallboard is classified as a combustible material.


That is interesting. Thank you. In my opinion the State of New Jersey Department of Community Affairs Division of Codes and Standards got it right and their view is at odds with USG. How has USG reacted? I notice the data sheet on their website that Lockhart pointed out for me is no longer on line.

I think I will just post both the USG and New Jersey views. This is a small issue as it relates to masonry fireplaces. I'm disappointed that drywall can't just be glued to masonry chimneys but there are other materials and larger clearance to combustibles code issues.

Thanks again.

Jim Buckley


I've not heard any resolution to this issue. Frankly I think the State of New Jersey got it right but, wow, that ASTM E 136 Standard tests the material at 750 deg. C, (1382 deg. F). That's really hot. The general standard built into the code is that combustibles such as wood framing cannot exceed 90 deg. F above ambient. That's about 160 deg. F or about 1,200 deg. F less that the ASTM noncombustibility test.

While the New Jersey is correct about the Mechanical Subcode being concerned with performance during exposure to a constant high-heat source, I wonder at what temperature the "limited-combustible" draywall beginis to catch fire or sacrefice itself. Is it around 1,382 deg. F? What happens to drywal at 200 or 300 deg. F? The outside of a masnry chimney will never get that hot.

I don't like framed out chases. Exposing the masonry walls and eliminating the air spaces helps dissapate the heat and is safer.

Very conservatively, if you can't glue drywall straight on the maosnry at least you could use masonry backer-board whcih, without the paper backing, does seem to be "Non-Combustible" per ASTM E 136.

Jim Buckley

Back to Technical Discussion
Buckley Rumford Fireplaces
Copyright 1995 - 2015 Jim Buckley
All rights reserved.