I think UL 1482 covers the stove testing requirements, not the hearth.
You might find this document on hearth design requirements from the tile institute helpful:
U.L.127 January 26, 1998 12.9 When the fireplace is fired as described in 12.5 – 12.7 , the maximum temperature rise above ambient zone temperature shall not exceed:
a) 117 degrees F (65 degreesC) on exposed surfaces of the test enclosure and
b) 90 degrees F (50 degrees C) on concealed surfaces of the test enclosure, such as beneath the hearth (fire chamber), beneath the hearth extension, behind the wall mounted shields, within the chimney enclosure and surrounding the fire chamber.[5]
The purpose of the Hearth Extension is to protect the combustible sub-floor from the radiant heat that the fireplace generates during normal operation, and to provide a non-combustible surface for sparks and flying embers to encounter. In the event of a log roll-out the Hearth Extension is subjected to higher heat values. The U.L. test procedure 1618 for wall protectors, floor protectors, and hearth extensions9 exposes the intended hearth extension design to extreme heat and physical abuse:
U.L. 1618 1991- 12.5 The floor protector is to be mounted such that the underside of the floor protector can be observed. The burning brand is to be placed on the surface of the floor protector at the location considered most vulnerable with respect to ignition of the floor protector.
The “Burning Brand” referred to in this section is 500 grams +or- 50 grams of kiln dried and conditioned Douglas Fir configured to maximize combustion.
U.L. 1618 1991- 12.6 The test is to continue until the brand is consumed and until all evidence of flame, glow, and smoke has disappeared from both the exposed surface of the material being tested and the underside of the floor protector, or until unacceptable results occur, but not more than 1-1/2 hours.
U.L 1618 1991-13.2 The floor protector is to be subjected to three successive impacts from a 5-inch (127mm) diameter hardwood log, 18 inches (457mm) long, weighing 12 pounds-mass (5.4kg), and dropped from a height of 48 inches (1.22m) with its major axis parallel, perpendicular, and at a 45 degree angle to the surface of the floor protector.
U.L.1618 1991-13.3 The floor protector is to be subjected to a 600-pound-mass (272-kg) static load applied to a 1-square-inch (6.45-cm2) area for 30 minutes.
http://www.ctioa.org/reports/fr77.html
