An improved hot air welder which includes a blower communicating with a heater assembly mounted on a chassis wherein the chassis has coupled thereto a pressure wheel and tracking wheel, includes means for adjusting the angle of orientation of the pressure wheel and tracking wheel with respect to the chassis in order to obtain a proper pressure bias through the pressure wheel on the seam over which the pressure wheel rides, and for providing a proper orientation of the tracking wheel according to the slope of the roof upon which the welder is driven. The welder can accommodate inside and outside seams with equal facility. The air dam has an improved roller track to more evenly apply pressure against the underlying roofing material. A guide member slidably engages and accommodates roofing material from both the left and right sides to assist in guiding the hot air welder along the seam to be welded. A constant speed blower controlled by a gate valve controls the amount of air to the heater assembly. An alarm is included within the heater assembly for sensing insufficient temperature when the welder is operating and for sounding an audible alarm to indicate to the operator that insufficient heat is being delivered to the seam to manufacture a suitable weld.