NISTIR 6293

PAINTED STAIRWELL FIRES IN HIGHRISE BUILDINGS

Henri E. Mitler 

Building and Fire Research Laboratory

Gaithersburg, Maryland 20899

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology

Prepared for:

Federal Emergency

Management Agency

U. S. Fire Administration
NISTIR 6293

PAINTED STAIRWELL FIRES IN HIGHRISE BUILDINGS

Henri E. Mitler 

February 1999

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899 

U.S. Department of Commerce 

William M. Daley, Secretary

Technology Administration

Gary R. Bachula, Acting Under Secretary for

Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director

Federal Emergency Management Agency

James L. Witt, Director

U.S. Fire Administration

Carrye B. Brown, Administrator

Contents

1 Introduction ...............................................................1

1.1 Background .........................................................1

1.2 Previous New York City Fire Incidents ...................................2

1.3 Stairwell Fire Damage ................................................3

1.4 Overview of Flame Spread .............................................4

2 Experiments ...............................................................5

2.1 Bench-Scale Tests ....................................................5

2.1.1 Lateral Ignition and Flame Spread Test (LIFT) .....................5

2.1.2 Cone Calorimeter .............................................7

2.2 Intermediate-Scale Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.1 Test Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.2 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.1 Heat Flux and Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 Flame Spread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3 Examination of the Paint Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

iii
Acknowledgments

The author would like to thank Robert McCarthy and Andrew Giligo at the United States Fire

Administration for their support of this project. Additionally, the author wishes to thank Jack

Lee, Gary Roadarmel, and Lauran DeLauter from the Building and Fire Research Laboratory for

the preparation and execution of bench- and full-scale experiments. 

iv
Painted Stairwell Fires in High-Rise Buildings

Henri E. Mitler*

Building and Fire Research Laboratory

National Institute of Standards and Technology

1 Introduction

The United States Fire Administration (USFA) sponsored the National Institute of Standards and

Technology’s (NIST) Building and Fire Research Laboratory (BFRL) to conduct a preliminary

examination of fire spread in painted stairwells to provide a better understanding of reported

stairwell fires in high-rise buildings. This report provides an analysis of such fires and

documents the results of bench- and intermediate-scale tests involving painted surfaces in

stairwells.

1.1 Background

Starting in 1994, officials of the New York City Housing Authority (NYCHA) and the New

York City Fire Department (NYCFD) noted an increase in the frequency of stairwell fires in

high-rise buildings. The number of reported incidents increased from 5 in 1993 to 17 in 1995.

These fires were observed to spread with unusual speed and severity, traveling rapidly up

multiple floors of the stairwell. A number of such fires extended all the way to the stairwell’s

roof bulkhead. These were reportedly started by ignited furniture, rubbish, and/or old mattresses

that had been placed on a stairwell landing. Lasting only a few minutes; the fires were generally

out by the time the fire department arrived. The real danger of such stairwell fires lies in the fact

that the occupants rely upon the stairwells as a means of egress. In a fire on November 4, 1995,

set on the first floor, the fire raced up eighteen flights, killing two people in the stairwell. There

were at least three known fatalities as of December 1995, and the potential for further fatalities.

These fires are not unique to New York; they have occurred elsewhere in the U.S. and abroad.

The Fire Research Station, in England, reports similar occurrences in London. Murrell and

Rawlins1

 discuss the response of multi-layered paint to fire and concluded the following points:

*

 Current address: Department of Fire Protection Engineering, University of Maryland,

College Park, MD 20742

1
 • Each repainting, even with water-based paints, increases the hazard during a fire.

 • Any propensity of a coating to blister when heated increases the spread rate.

 • The flame spread rating obtained by a standard fire test method bears little relationship to

either the heat release or the fire propagation index of the paint as obtained with another

method.

 • The presence of a lacquer layer (sometimes used as an anti-graffiti measure) seems to

cause a delamination/adhesion problem, and increases the potential fire hazard.

The most famous example of paint contributing to a fire is an escalator fire in the London

Underground, known as the King’s Cross fire.2,3 While wood was found to be the principal fuel

load, the next most significant fuel source was paint: much of the surface area in the station was

painted with many coats. The layers were about 1 mm thick on average, and 2-3 mm in places. 

One possibly significant observation was that in maintaining the station, “anti-graffiti” coatings

of clear polyurethane (PU) varnish were sometimes used on top of each new coat of paint; such

layers make it harder to apply the graffiti and easier to clean it off. Additionally, the angle of the

stairs may also affect the spread of the fire. In the King’s Cross fire, the escalator rose at a 30

degree angle. The resulting buoyant flow followed the contour of the channel.

1.2 Previous New York City Fire Incidents

Between 3/18/90 and 7/17/96, there were 175 stairwell fires reported by the NYCFD. Of these,

92 were in buildings presumably protected against such fires either by having the walls scraped

clean or repainted with a fire-retardant or an intumescent paint. The remaining 83 were in

stairwells with many layers of paint. In 1995, the statistics were refined to indicate which fires

demonstrated significant extension.

 

Starting at the end of 1995, the NYCHA decided to repaint the stairwell walls with water-based

coatings. Approximately half of these were covered with an intumescent paint, half with a fireretardant paint. The use of these fire-protective coatings has been partially successful. As of

January 1996, 6 out of 8 fires that occurred did not extend beyond the floor of origin. In the two

cases where the fires continued to the roof, the wall surface may not have been correctly

prepared before the paint was applied. In June 1996, a new strategy was implemented by the

NYCHA: they repainted only buildings with ten or fewer floors. In higher buildings, they

proceeded to strip the paint off the walls entirely.

Table 1 shows an analysis of painted stairwell fires from 1990 to 1996 in New York City.

Stairwells which had been repainted with a fire-protective paint are included as well as those

which had not been repainted. The fires are categorized as either small (no) extension or large

extension. Small or no extension refers to fires that were limited to the floor of origin or the

flight above. Large extension refers to fires that involved more than two flights of stairs. They

are further subdivided into small and large igniters. Small igniters are objects, such as rubbish

bags, likely to yield 150 kW and less; and large igniters, such as mattresses, upholstered chairs,

sofas, couches, etc., are likely to yield 250-1000 kW or more.

2
Figure 1. Stairwell fire on floor of fire origin

Stairwells Without FireProtective CoatingsStairwells with FireProtective Coatings

Large Extension Large Igniter 17 4

Small Igniter 21 0

Small Extension Large Igniter 2 16

Small Igniter 43 72

Total 83 92

Table 1: Stairwell Fires in NYC 1990 - 1996

While the authorities decided to repaint the stairwells with fire-protective coatings, they also

stipulated that the bulkheads (the doors to the roof) be closed when not in use. This makes a

very large difference in the ventilation available to the fire. 

For the stairwells with protective coatings, 4 out of 20 large ignitions produced extended

burning. For small ignitions (72 cases), there were no failures, but the bulkheads were closed in

all but one case. 

For the stairwells which had not been repainted with protective coatings, 17 out of 19 large

ignitions produced extended burning. For small ignitions, 21 out of 64 cases led to extended

burning.

1.3 Stairwell Fire Damage

Three sites where stairwell fires

took place in NYC during 1994

and 1995 were examined. In all

three cases, fire damage in the

stairwells was severe. The

typical appearance of a stairwell

post-fire is shown in figure 1. In

the three fires, the damage was

most severe on the floor of fire

origin with lesser damage on

higher floors. Not all of the paint

was consumed in any of the fires

and in some of the fires the

spread was limited to several

floors. 

3
1.4 Overview of Flame Spread

Flame spread up a painted stairwell involves classical vertical flame spread. A large ignition

source ignites the wall through direct flame impingement and radiant heat flux to the walls. As

the paint layers on the wall ignite, a flame front develops. The preferred direction of flame

propagation is upwards, due to buoyancy forces.4 Buoyant forces then carry heated gases up the

stairwell. The heated gases convectively preheat the walls as they travel up the stairwell. The

flame front propagates up the stairwell until either the fuel runs out, conditions become oxygen

limited, the burnout region at the end of the flame region reaches the pyrolysis front, or the fire

department manually suppresses the fire. The critical variables in the propagation of the flame

include heat flux at the flame front, flammability of the surface, thickness of the fuel (thermal

inertia, kρc), the geometry of the surface, and composition of the atmosphere.5 Painted stairwells

present a favorable flame spread environment, provided the paint surface is sufficiently

flammable. All combustion gases are contained within the stairwells, all heat fluxes produced by

the flames are absorbed by the walls and ceiling, the surface area is large, and the orientation of

the wall surfaces is vertical, which leads to favorable flame spread conditions. 

4
Figure 2. Lateral Ignition and Flame spread Test

2 Experiments

Two kinds of experiments were conducted to study flame spread in stairwell configurations:

bench-scale tests to establish material and flammability properties of multi-layer paint on a given

substrate and intermediate-scale tests to determine if the phenomenon can be reproduced in a

laboratory and to compare to the results to the bench-scale tests.

2.1 Bench-Scale Tests

The bench-scale test methods used in this study are the LIFT6 (ASTM E 1321) and the Cone

Calorimeter7

 (ASTM E 1354). The LIFT test determines ignition temperature, the parameter Φ

for lateral (counter-current) flame spread rate, and the mean thermal inertia, (kρc), for a material. 

The Cone Calorimeter yields the heat release rate, and the time to ignition under varying external

fluxes, from which the critical flux (and hence the ignition temperature) can be determined. 

2.1.1 Lateral Ignition and

Flame Spread Test (LIFT) 

The Lateral Ignition and

Flame Spread Test (LIFT)

was proposed in 1983 by

Harkleroad and Quintiere.8

The purpose of the test

procedure is to determine

flame spread parameters for

materials related to downward

or lateral flame spread on a

vertical surface. These

parameters include the

effective ignition temperature,

the parameter Φ for lateral

(counter-current) spread rate,

and the mean thermal inertia,

(kρc). The rate of flame spread for a particular material can be calculated as a function of

external heat flux. The geometry of the experiment consists of an angled radiant panel, a

vertically oriented sample, and an acetylene-air pilot flame located in the out gassing flow. 

Figure 2 shows the geometry. 

5
A series of LIFT tests was performed using two substrates, gypsum board and masonry-block to

determine effect on flame spread with multiple layers of paint. Six and twelve layers were used

to represent initial estimates of the bounds of the numbers of paint layers observed in post-fire

examinations.

Table 2 summarizes the LIFT experiments. Figure 3 shows the results of the LIFT tests. Tests 1

and 2 (figure 3a) were replicate tests consisting of 12 coats of paint on gypsum board at a peak

incident flux of 30 kW/m2

. The maximum extent of spread was 0.37 m, along the upper edge. 

Figure 3b shows tests of 12 coats of paint on masonry block at a peak incident flux of 30 and 50

kW/m2

. At a peak incident flux of 30 kW/m2, the painted masonry block shows a lower value of

0.14 m compared to 0.38 at the higher incident flux of 50 kW/m2. Tests at 50 kW/m2 on masonry

block (figure 3c) show similar results to the gypsum tests.

From table 2, the amount of paint on the surface also has an effect on the flame spread. With six

coats of paint, flame spread to 0.20 m. With 12 coats of paint, the spread is nearly double at 0.37

m.

Substrate # of Paint Layers Heat Flux (kW/m2

) Flame Spread (m)

Gypsum Board 0 30 0.42

Gypsum Board 6 30 0.20

Gypsum Board 12 30 0.37

Gypsum Board 12 30 0.37

Gypsum Board 12 50 0.40

Masonry Block 12 30 0.14

Masonry Block 12 50 0.38

Table 2: Summary of Results from LIFT Tests

6
(a) Two replicates with 12 coats of paint on

gypsum board

(b)12 coats of paint on masonry block at 30

and 50 kW/m2

(c) Two replicates with 12 coats of paint on gypsum board and masonry block

Figure 3. LIFT test samples for paint on gypsum board and masonry block

2.1.2 Cone Calorimeter

The Cone Calorimeter, ISO 56609

 or ASTM E 13547 is used worldwide to measure heat release rate

and other material flammability parameters for a 100 x 100 mm material sample. The apparatus is a

general-purpose one, which may be used to test products for various applications. Thus, the heater

had to be capable of a wide variety of heating fluxes; the actual capability spans 0 to 100 kW/m2.

The technical features of the Cone Calorimeter are documented in several references.10, 11, 12, 13 Some

of the most salient features include:

7
Figure 4. General view of the Cone Calorimeter

(ASTM E 1354, ISO 5660)

 • horizontal or vertical specimen orientation

 • composite and laminated specimens can be tested

 • continuous mass loss load cell readings

 • feedback-loop controlled heater operation

 • HRR calibration using methane metered with mass flow controller

 • smoke measured with laser-beam photometer and also gravimetrically

 • provision for analyzing CO, CO2, H2O, HCl, and other combustion gases

Applications of the Cone

Calorimeter are discussed in detail

in a textbook which

comprehensively examines heat

release in fires.14 Figure 4 shows

the Cone Calorimeter.

Tables 3 – 5 summarize the Cone

Calorimeter tests for this study. 

Each table provides the imposed

external flux, the time to ignition

for the sample, the peak HRR, peak

mass loss rate, average HRR, and

the total heat released by the

sample. An important result is the

determination of the critical

external heat flux necessary to

ignite the paint. The critical value

lies between 27.4 and 37.5 kW/m2,

depending on the paint/substrate

combination. The second crucial observation is the relationship between the number of layers of

paint and HRR. For 6 layers of paint on masonry block, the average HRR ranges from 20 to

49 kW/m2

, depending on the external heat flux. With twelve layers of paint on masonry block,

however, a significantly higher average HRR was observed, ranging from 41 to 61 kW/m2

,

depending upon the external heat flux. The six layer paint/gypsum board combination exhibited

higher HRRs, leading to the conclusion that pyrolysis of the gypsum board contributed to the HRR. 

The average HRR ranged from 120 to 240 kW/m2

. As a basis for comparison, Babrauskas, et al.15

reported Cone Calorimeter data for unpainted 13 mm gypsum board having a peak HRR of

97 kW/m2

.

8
External Flux

(kW/m2)

Ignition

Time

(s)

Peak Heat

Release Rate

(kW/m2)

Peak Mass

Loss Rate

(g/s-m2)

Average Heat

Release Rate

(kW/m2)

Total Heat

Released

(MJ/m2)

50 22 340.1 18.8 211.4 3.1

50 23 369.9 20.3 240.1 3.5

40 37 289.1 17.4 167.5 3.3

40 52 289.9 17.2 140.6 2.8

40 39 332.0 19.4 193.0 2.8

30 83 227.4 14.5 177.9 2.6

30 87 216.5 14.7 120.0 2.4

27.4 4 -- -- -- --

25 4 -- -- -- --

Table 3: Cone Calorimeter Data for Gypsum Board Substrate with 6 Layers of Paint

External

Flux

(kW/m2)

Ignition

Time

(s)

Peak Heat

Release Rate

(kW/m2)

Peak Mass

Loss Rate

(g/s-m2)

Average Heat

Release Rate

(kW/m2)

Total Heat

Released

(MJ/m2)

50 37 95.6 3.9 48.7 10.0

50 33 78.7 3.5 49.4 8.4

40 92 56.8 2.9 25.2 5.7

40 631 39.4 5.6 19.5 2.1

40 633 64.2 5.1 30.8 3.1

37.5 631 62.3 4.9 32.3 3.9

35 4 –– – –

30 4 –– – –

Table 4: Cone Calorimeter Data for Masonry Block Substrate with 6 Layers of Paint

External

Flux

(kW/m2

)

Ignition

Time

(s)

Peak Heat

Release Rate

(kW/m2)

Peak Mass

Loss Rate

(g/s-m2)

Average Heat

Release Rate

(kW/m2)

Total Heat

Released

(MJ/m2)

50 22 187.7 6.8 61.0 21.0

50 18 193.5 7.6 58.2 18.6

40 42 160.7 7.5 49.8 7.7

40 284 186.3 4.7 63.2 9.7

40 36 168.4 6.7 49.7 8.0

30 98 100.6 4.7 40.7 6.5

30 88 124.4 5.4 47.0 6.3

27.4 4 –– – –

24.7 4 –– – –

Table 5: Cone Calorimeter Data for Masonry Block Substrate with 12 Layers of Paint

9
Figure 5. Experimental arrangement for test 1.

Location A to D are locations of total flux meters

embedded into the side and back walls.

2.2 Intermediate-Scale Tests

The primary objective of the intermediate-scale tests was to replicate fire spread through a painted

stairwell in the laboratory. Space restrictions in the test facility precluded the possibility of erecting

a full-story mock-up of a stairwell. Therefore, several half-scale models of one flight of stairs were

built. 

2.2.1 Test Configurations

Test 1: The first test was conducted to evaluate the effect of small ignition sources upon the test

configuration. From Table 1, small ignition sources tend not to lead to full-burnout of an painted

stairwell. 

The test was conducted using 9.5

mm (3/8”) gypsum board as the

substrate (the “wall” and “ceiling”

material). Three 4’ x 8’ sheets were

each covered on one side with six

coats of an alkyd/oil-based paint

supplied by the NYC Housing

Authority. Aging was simulated by

covering the surfaces with dust,

soot, and graffiti, then rinsing the

surface with water to simulate

cleaning prior to repainting,

between coats. While no attempt

was made to age each layer, each

layer of paint was allowed to dry

thoroughly before the next coat was

applied. An average of 0.48 kg/m2

of paint was applied to the wall

surfaces. The three panels were joined so as to simulate the ceiling and two side walls in a stairwell

and inclined at approximately a 10 degree angle from the vertical (see figure 5). 

Flux gauge A was located on the left-hand wall, 0.3 m (1 ft) from the top surface and 0.6 m (2 ft)

from the back wall. Flux gauge B was located on the left-hand wall 0.6 m (2 ft) from both the back

wall and bottom surface. Flux gauge C was located on the back wall 0.3 m (1 ft) from the top

surface and 0.6 m (2 ft) from the right-hand wall. Finally flux gauge D was located on the back wall

0.6 m (2 ft) from both the right-hand wall and the bottom surface. A natural gas burner measuring

approximately 0.1 m2

 (1 ft2) was placed on the floor, in the bottom right-hand corner of the unit.

Upon ignition, the heat output of the burner was approximately 40 kW. 

10
(1)

Figure 6. Experimental arrangement for test 2 and 3

Test 2: For the second test, the experiment was modified by 

 • Using a longer simulated stairwell section, 3.6 m (12 ft.) in length.

 • Inclining the unit approximately 36 degrees from the horizontal to more accurately simulate

a stairwell configuration, and

 • Increasing the ignition source to a heat release rate scaled to reflect those typically involved

in stairwell fires. 

Figure 6 shows the experimental

setup. Four total heat flux gauges

were used. Two were located along

a side wall centerline and two along

the ceiling centerline, 1.2 m (4 ft)

and 2.4 m (8 ft) from the leading

edge, respectively. The heat flux

gauges provide an indication of the

spread rate of the fire, and the

magnitude of the flux incident on

each surface as a function of

position. 

Typical scaling shows heat release

rate is proportional to the relative

size to the 5/2 power of the size ratio16, that is,

where l and lo are the characteristic dimensions of the intermediate-scale and full-scale units,

respectively. Assuming that the ignition source for actual fires in the stairwells is of the order of

800 to 1000 kW (typical of a fully-involved stuffed chair), the heat release rate for the

intermediate-scale experiment should be approximately 150-200 kW. For test 2, a 150 kW

ignition source was used.

For test 2, six layers of an alkyd/oil-based paint were applied. Each coat was allowed to dry for

24 hours before application of the next coat of paint. For this test, an average of 0.40 kg/m2

 of

paint was applied to the wall surfaces.

Test 3: Test 3 was conducted as a continuation of test 2. The experimental setup of test 3 was

identical to that of test 2, with the exception that the heat release rate of the burner was increased

to 200 kW. 

11
Figure 7. Experimental arrangement for test 4

Figure 8. Schematic side view for test 4

Test 4: For test 4, the walls and ceiling were

covered by fifteen coats of paint – five of

primer, to ensure that no water diffused into

the paint from the gypsum-board walls, and

ten layers of the alkyd/oil-based paint. The

right-hand wall received a coat of anti-graffiti

coating before the last coat of paint was

applied, to simulate the technique that had

been used to control graffiti. A total of

0.92 kg/m2

 of primer/paint/anti-graffiti

coating was applied to the wall surfaces.

The upper two thirds of the fourth (bottom)

side of the duct was covered with a sheet of

clear polycarbonate plastic to prevent the

(mostly horizontal) entrainment of cooling air

into the duct while still permitting

observation and videotaping of the test

(figure 7). Four flux meters were placed on

the left-hand side wall, 50 mm below the

ceiling (figure 8). A 200 kW propane gas

burner in the corner served as the ignition

source.

2.2.2 Test Results

The result of test 1, which utilized a 40 kW

burner, was limited upward flame spread

along the corner, and almost no lateral flame

spread. This result is consistent with

observations from actual stairwell fires with

small ignition sources (Table 1).

Test 2 was run with a burner heat release rate of

150 kW and a modified setup from test 1. The

major modifications were the angle of the

stairwell, the size of the setup, and the burner

HRR. The peak measured HRR was 270 kW

(at t = 45 s). Of this, about 150 kW was

directly attributable to the burner, so that the

paint contributed approximately 120 kW. At

105 s, the burner was turned off, and the flames

self-extinguished. 

12
Test 3 was conducted as a follow-on to test 2. The test used an approximately 200 kW ignition

source. The flames spread upward rapidly so that flames extended approximately 0.30 m (1.0 ft)

beyond the end of the mock-up, for several seconds. The peak measured HRR of 395 kW

included a 200 kW contribution from the burner, 38 s into test 3. The peak HRR as a result of

the paint was approximately 200 kW. The flame then gradually retreated and extinguished. 

Examination of the resulting damage showed that in the region where the ignitor flames

impinged, all six layers of paint had been consumed. The fire extended about 45 cm downward

from the ceiling of the stairwell. Near the exit, on the other hand, the fire narrowed to about a

0.15 m depth, and only the top one or two layers of paint were consumed. Although there was

rapid upward spread, the comparably rapid exhaustion of the flame suggests that the spread

might have been limited, rather than continuing up the stairs. 

Test 4 had a significantly different geometry, as the experimental setup was covered with

polycarbonate. The wall and ceiling paint layers ignited very quickly and the flame lengthened

continuously. Within 30 seconds, the flame tip had extended 20-50 cm beyond the test

apparatus. Less than three minutes into the test, the right-hand wall was significantly involved. 

Peak HRR for the test was 1170 kW.

13
Time (s)

0 100 200 300

Heat Release Rate (kW)

0

200

400

600

800

1000

1200

1400

Test 2

Test 3

Test 4

Figure 9. Heat release rate during several

intermediate-scale stairwell fire tests

3 Discussion

3.1 Heat Flux and Heat Release Rate

Like any vertical flame spread problem, spread of flame up the incline of the stairwell is driven

by the heat release of the fire and its resultant heat flux incident on the region in front of the

flame front. Both the Cone Calorimeter and intermediate-scale tests showed that the heat release

rate of multiple layers of paint can be significant. Peak HRR from the Cone Calorimeter tests

ranged from 220 to 370 kW/m2, including some contribution from a gypsum board backing..

Lower values were observed for tests on masonry block. It is presumed that these lower values

resulted from both the high heat capacity of the masonry block and from the lack of the paper

coating present in the gypsum board.

For the intermediate-scale tests,

peak heat release rate is shown in

figure 9. The qualitative

observations from each test are

confirmed by the heat release rate.

Tests 2 and 3 show similar growth

and decay, with the higher ignition

source contributing to a somewhat

higher peak HRR. For test 4, two

distinct peaks are apparent, one

for the initial spread along the left

wall followed by a second larger

peak corresponding to the righthand wall igniting and burning

much more vigorously. Peak

HRR for the tests ranged from 270

kW for test 2 to 1170 kW for test

4.

Figure 10 shows the measured heat flux to the wall surfaces for tests 2 to 4. For each test, peak

heat flux is naturally lower at the locations further from the ignition source. Peak heat flux is

significantly different for the three tests ranging from less than 6 kW/m2

 for test 2 to 98 kW/m2

for test 4. Both the smaller ignition source (in test 2) and the test configuration (in tests 3 and 4)

contributed to these differences.

14
Time (s)

0 50 100 150 200 250 300

Heat Flux (kW/m2

)

0

2

4

6

8

10

Test #2, Gauge 1

Test #2, Gauge 2

Test #2, Gauge 3

Time (s)

0 20 40 60 80 100 120 140 160 180

Heat Flux (kW/m2

)

0

5

10

15

20

25

Test #3, Gauge 1

Test #3, Gauge 2

Test #3, Gauge 3

Time (s)

0 100 200 300

Heat Flux (kW/m2

)

0

20

40

60

80

100

Test #4, Gauge 1

Test #4, Gauge 2

Test #4, Gauge 3

Test #4, Gauge 4

Figure 10. Total heat flux on wall and ceiling surfaces during several stairwell fire tests

Tests 3 and 4 were both performed with a 200 kW ignitor. There was a significant difference

between the peak heat fluxes in the two tests. Test 3 (6-layers of paint, no polycarbonate floor)

had a peak flux of 24 kW/m2

, whereas for test 4 (15-layers of paint, polycarbonate floor), peak

flux was 98 kW/m2

. Beyond the obvious conclusion that test 4 was a significantly more severe

fire than test 3, conclusions are difficult to draw, as the increase in heat flux may be attributable

to:

 • number of paint layers (6 in test 3 and 15 in test 4)

 • area of involvement (one wall in test 3 and two walls and ceiling in test 4)

 • anti-graffiti coating (only in test 4), or

 • the polycarbonate floor lining, which may trap unburned fuel and reduce IR radiation

losses.

15
Still, two observations are apparent comparing tests 3 and 4:

 • Flame spread in test 4 continued beyond the wall adjacent to the ignition source with 

sufficient intensity to involve both the ceiling surface and opposite wall surface.

 • The clear anti-graffiti coating on the wall opposite the ignition source contributed to a

rapid rise in the HRR of the stairwell fire.

3.2 Flame Spread

The critical question regarding fires in painted stairwells centers around whether the paint can

sustain flame spread. Three tests were performed to test the flame spread capabilities of painted

substrates. The LIFT tests demonstrated that, for the masonry tests, the number of layers of paint

present on the substrate has a direct relationship upon the flame spread properties of the paint. 

The flame spread properties of the paint/gypsum board combination were confounded with the

effects from the substrate pyrolysis. The cone calorimeter tests also indicate that the number of

layers of paint on the masonry block substrate directly affect the HRR and mass loss rate of the

sample. More paint results in higher HRR and mass loss rates. Additionally, higher external

heat fluxes result in generally higher average heat release rates. The same relationship exists

between the average HRR of the gypsum board/ paint combination and the external heat flux. 

Finally, intermediate-scale tests were performed to determine the effect of the ignition source

size upon the spread of flames. Ignition sources less than about 200 kW did not produce flame

spread beyond the test apparatus. Tests 3 and 4 both produced flame spread beyond the test

apparatus. Test 3 however, exhibited flame spread beyond the test apparatus with only 6 layers

of paint, no enclosure (polycarbonate), and no anti-graffiti coating. This represents the minimum

configuration which may produce flame spread. Adding polycarbonate, more paint, anti-graffiti

coating, and changing the burner all enhanced the rate of flame spread. The intermediate-scale

tests support the evidence of the bench-scale tests that paint can sustain and propagate flame

spread through a stairwell enclosure. 

3.3 Examination of the Paint Layers

Microphotographs of the cross-sections of the remains from Test 3 are shown in figure 11. The

sample in figure 11a was taken from an unburned section of the left-hand wall, and shows a

cross-section of the unburned surface. The various layers can be distinguished. The paper layer

on the gypsum board lies at the bottom, followed by the five layers of primer, with the ten layers

of paint on top. The ten paint layers are about 300 μm thick, while the five layers of primer are

about 240 μm thick.

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(a) Painted surface from unburned surface of

test 4

(b) Burned surface taken from a position

intermediate between (a) and (c)

(c) Burned surface near ignition source

Figure 11. Microphotographs of cross sections of paint surface after test 4

Figure 11b is from a heavily burned region near the exit. It can be seen that large gas bubbles or

blisters have formed between the layers, probably from the anti-graffiti coating. When the heat

causes a gas bubble to form, it forces lower layers, which have been plasticized, to flow, making

the layer thinner. The lowest layer is about 110 μm thick; since the paper layer is only 60 ± 10

microns thick, this must be primer. This is consistent with having the lowest layers not

pyrolyzing. The upper set of paint layers has begun to delaminate, and is ~80 - 120 μm thick. It

shows that ~200 out of the 300 μm of paint have been pyrolyzed away; since there may be have

been charring and expansion or shrinkage of the layers, it is difficult to determine exactly how

many layers that corresponds to, but it is most likely between 6 and 7 layers.

17
Figure 11c is one of the most pyrolyzed of the samples which have not been totally burned away. 

It was taken from the right-hand wall, approximately 2 m from the exit. From the top of the

photograph, there is a layer of paint approximately 160 – 170 μm thick, with a division within it

~80 - 90 μm below the surface. Delamination separates the paint from the primer. The primer is

approximately 150 -- 160 μm thick. The dark band is probably the paper facing on the gypsum

board and shadows. Finally, the cottony-looking region below the paper facing is the gypsum

board. It is reasonable to conclude that 220 or more microns of paint have been pyrolyzed, and

that delamination occurred within the primer layers.

Samples could not be taken from the rear wall: all that was left was extremely friable ash,

loosely attached to the gypsum. Not only were the paint and primer all burned away, but so was

the paper facing of the gypsum board. The microphotographs above confirm what is apparent to

the naked eye; that is, the amount of paint removed is inversely proportional to the distance from

the origin of the fire. This is partly a result of having burned longer in the lower reaches, and

partly due to the higher heating fluxes there. 

18
4 Summary

This report provides the results from a preliminary examination of fire spread in painted

stairwells to better understand reported stairwell fires in high-rise buildings. This report provides

a description of the background and theory of such fires and documents the results of bench- and

intermediate-scale tests of fires involving painted surfaces in stairwells.

Cone Calorimeter tests were conducted to determine heat release rate and critical ignition flux

for painted surfaces. The results indicated that the critical ignition flux necessary to ignite

painted substrates was between 27.4 and 37.5 kW/m2

. Additionally, the HRR was a function of

the substrate and the number of layers of paint. 

Lateral Ignition and Flame spread Test experiments revealed that the rate and extent of flame

spread was also a function of the number of layers of paint and the substrate. 

Finally, four intermediate-scale experiments were performed to demonstrate that paint may

sustain vertical flame spread beyond the floor of fire origin. The first experiment was conducted

with a small ignition source (40 kW) and no significant flame spread was observed. The second

intermediate-scale test was performed with an ignition source of 150 kW and 6 layers of paint. 

Although there was modest flame spread, it was contained within the test apparatus. 

The third test was conducted as a continuation of test 2, but with a 200 kW gas burner. Flames

spread beyond the test apparatus for a significant period of time. 

Finally, the fourth intermediate-scale experiment was performed. This test was perhaps the most

realistic of the intermediate-scale experiments, as the horizontal entrainment was reduced with

polycarbonate, anti-graffiti coating was applied to one wall, the HRR of the ignition source was

maintained at 200 kW, and 15 layers of paint were present. The observed and measured flame

spread in the final experiment confirmed that paint, particularly when covered with anti-graffiti

coating, can sustain vertical flame spread in a stairwell configuration. The extent of the flame

spread beyond the parameters of the intermediate-scale experiments cannot be determined or

predicted from these experiments.

19
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