Three thicknesses of seven-eighths inch dressed, tongued and grooved white pine or other non-resinous wood board not more than eight inches wide, securely clinched with wrought-iron nails and covered with 14 inch by 20 inch sheets of terne plate of at least 1-C quality, joints locked full one-half inch and nailed under seams with at least 12 and one-half gauge flat head, full barbed-wire nails, two inches long.

Wrought-iron or steel plates, at least three-sixteenths inch thick, having two inch by two inch by three-eighths inch angle iron stiles and rails, center rails and center stiles on one face, or three inch by one-quarter inch flat bar stiles and rails on both faces, no unstiffened panel shall be more than nine square feet in area; all parts shall be securely riveted with three-eighths inch iron rivets.

A skeleton framework of at least one and one-half inch by one-quarter inch channels, angles or T's, covered on both sides with at least one-eighth inch wrought-iron or steel plates, filled solid with mineral fibre, asbestos, gypsum or other incombustible material.

(d) [Other.] Or, any form of door construction that shall have successfully passed a standard one-hour fire and water test and approved by the commissioner.

Door counterweights shall be guarded on all exposed sides and shall run in guides, from which they cannot be accidentally dislodged. The bottom of the guides shall be of such construction that the counterweights will be securely held if the counterweight ropes should break.