26 BULLETIN 801, U. S. DEPARTMENT OF AGRICULTURE. 
The size of the door should be no greater than necessary and in no 
case should it exceed 120 square feet in area of opening or 12 feet in 
width. It is preferable that the door opening be not wider than 10 
feet. Doors of large size are not nearly as strong in their resistance 
to fire as smaller doors. Generally a door should be used on both 
sides of the wall so as to protect the opening against a long-sustained 
fire; this also gives greater certainty that at least one of the doors 
will close. 
It is extremely important that a door which has closed from any 
cause be easy to open from either side, as this is a matter of protec- 
tion to both life and property. If a door closes during a fire a person 
may be confined in the fire area, or, on the other hand, persons desir- 
ing access to the fire in order to extinguish it may be delayed by 
their effort to open the door. 2 
always the more satisfactory provided the purchase is confined to doors ap- 
proved by the Underwriters' Laboratories. Insurance associations will furnish 
detailed specifications and explanatory drawings showing the construction of 
tin-clad doors, but, ail too often, these are not followed by the mechanics. 
The istallation of all types of fire doors should be such as to insure the hold- 
ing of the door very securiy against the wall and so the hardware (including 
the track for sliding door) should be extremely heavy and securely applied, 
since it may be exposed to a long-continued fire. Pieces of hardware attached 
to doors should be fastened with bolts or rivets extending entirely through the 
door, while parts applied to the wall should be bolted entirely through it. 
The door should overlap the wall 4 inches at both sides and top of the opening 
unless it closes properly into a recessed steel frame securely anchored to the 
wall. The joint formed at the floor should be close, the door closing upon a 
steel or iron or concrete threshold extending under it 6 inches. 
2 A variety of fire-door arrangements is shown by Plate XIV. Rolling steel 
doors with various methods of installation are shown in figure 2. Drawing I. 
"Detail A" shows an economical method which is satisfactory in most cases. 
The sliding fire door is usually to be preferred, as it? operation is easy and 
positive. The usual installation, shown by figure 5, provides an inclined track 
(having a slope of three-fourths or 1 inch per foot), which causes the door to 
roll shut by gravity when automatically freed from a restraining weight. 
This weight (shown to the right of the door) is suspended by a cord which 
passes over a pulley attached to the track or wall and thence to a bracket se- 
cured to the side of the door and projected beyond it and into the opening. 
The connection between this bracket and the cord is made by a " fusible link " 
which consists of two thin pieces of metal soldered together with solder, which 
melts at a temperature of 165° F. The action in case of fire is that this 
fusible link melts apart, thus disconnecting the restraining weight from the 
door, which rolls shut by its own weight acting upon the inclined track. The 
fusible link should be projected into the opening in order that the natural 
draft of heat through the door may melt the fusible solder as soon as possible. 
In situations where the story height does not allow the necessary incline 
for the track, it may be installed level and the closing action of the door 
secured by an additional weight, as shown to the left of the opening in figure 
3, Plate XIV. This second weight should be connected securely to the door 
by a substantial sash chain in order that it may hold the door shut during a 
