Technical Bulletin #160613
Understanding Flame Spread Index (FSI) and
Smoke Developed Index (SDI) Ratings
Considering installing a plastic system in a plenum? No matter what type of piping system is
being installed, the subject of flame spread and smoke developed ratings continues to be a
confusing topic.
How is Suitability Determined?
ABS, PVC and CPVC pipe, like other plastic piping materials, are classified as combustible materials per ASTM E 136 (test method used
to determine the combustibility of building materials). Model mechanical codes in the United States require combustible materials in
un-ducted return air plenum spaces to meet the 25/50 flame spread (FSI) and smoke developed (SDI) ratings as defined by ASTM E 84.
In addition, some model and local mechanical codes require that combustible materials be listed by a third-party listing organization and
labeled to indicate compliance.
Piping materials such as cast iron, which are classified as non-combustible per ASTM E 136, can be installed within un-ducted return
air plenum spaces without restriction.
2015 Uniform Mechanical Code
602.2 Combustibles Within Ducts or Plenums. Materials exposed within ducts or plenums shall be non-combustible
or shall have a flame spread index not to exceed 25 and smoke developed index not to exceed 50, when tested as a
composite product in accordance with ASTM E 84 or UL 723.
2015 International Mechanical Code
602.2.1 Materials within plenums. Except as required by Sections 602.2.1.1, materials within plenums shall be
noncombustible or shall be listed and labeled as having a flame spread index of not more than 25 and a smokedeveloped index of not more than 50 when tested in accordance with ASTM E 84 or UL 723.
The Test Method
ASTM E 84, the standard test method for surface burning characteristics of building materials was originally developed in the 1950s to
test sheet goods like floor coverings. This test is conducted in a Steiner Tunnel which is a measurement device that enables objective
comparison of the surface burning characteristics of materials.
This protocol provides data, enabling judgments about the suitability of materials for an application. ASTM E 84 is referenced in the
major mechanical codes to measure the surface burning characteristics of combustible materials.
As this protocol was developed for sheet goods, it must be modified to test pipe. In fact, pipe tested to ASTM E 84 is said to be tested
“in general accordance” with the standard rather than in strict conformity with the standard.
"You can't beat the system" is a registered trademark of Charlotte Pipe and Foundry Company.
© 2016 Charlotte Pipe and Foundry Company
Charlotte Pipe and Foundry Company
TB-FSI (6-14-16)
P.O. Box 35430, Charlotte, NC 28235
www.charlottepipe.com
1-800-438-6091
704-348-6450
How Much is Too Much?
A growing issue in the plumbing industry is the extent of modifications performed to manipulate the ASTM E 84 test protocol in order to
achieve the required 25/50 flame spread (FSI) and smoke developed (SDI) rating. It has become common practice to manipulate the
test to generate favorable test results, so a valid comparative evaluation of the safety and suitability of a material is often impossible.
For example:
Test Specimen Size
Specimen Test Position
The standard specifically calls for a specimen to be 20
inches by 24 feet in length, completely filling the burn
chamber. Cement fiber board tested in a full chamber will
yield values of 0 (zero) flame spread and 0 (zero) smoke
developed. A sheet specimen of red oak will yield values
of 100 flame spread and 100 smoke developed. These two
materials create benchmarks allowing an “apples to apples”
comparison for a range of materials.
If an engineer wished to compare the surface burning
characteristics of white pine to red oak, and tested a single
white pine 2x4 rather than filling the burn chamber with a 20
inch by 24 foot specimen, would that be a valid comparison?
Obviously the resulting test data would not provide a fair
comparison of the two materials.
The Scope section at the beginning of the ASTM E 84
standard indicates:
The test is conducted with the specimen in the ceiling
position with the surface to be evaluated exposed face
down to the ignition source. The material, product, or
assembly shall be capable of being mounted in the test
position during the test. Thus, the specimen shall either
be self-supporting by its own structural quality, held
in place by added supports along the test surface, or
secured from the back side.
The intent of this language is clear and logical: the specimen
is to be held over the flame for the duration of the test.
Unfortunately, it has become common practice to suspend
the specimen above the flame in a manner that permits the
specimen to drop from the support during the test, thereby
discontinuing exposure to the flame.
Tests conducted in this manner do not yield an accurate
measurement of a material’s surface burning characteristics
and the resulting data is invalid.
Flame Spread & Smoke Developed Ratings
Hubless, Service and Extra Heavy Cast Iron
ABS, PVC and CPVC
Per ASTM E 136, cast iron DWV piping systems are
non-combustible. ASTM E 84 does not apply to noncombustible materials. Hubless, Service and Extra
Heavy Cast iron systems can be installed within
plenums without restriction.
When evaluating materials for suitability for inclusion within
un-ducted return air plenum spaces, the surface burning
characteristics for construction materials is an important
life safety issue. Accurate testing in conformance with the
intent of ASTM E 84 is critical.
Charlotte Pipe and Foundry has not seen reliable test data demonstrating that PVC, ABS or CPVC
pipe passes an ASTM E 84 test with flame spread/smoke developed values of 25/50 or less. These
materials have, therefore, not been shown to comply with ASTM E 84.
"You can't beat the system" is a registered trademark of Charlotte Pipe and Foundry
Company. © 2016 Charlotte Pipe and Foundry Company
Charlotte Pipe and Foundry Company
P.O. Box 35430, Charlotte, NC 28235
www.charlottepipe.com
1-800-438-6091
704-348-6450