A thermal welding marking system for attachment to a thermal welding machine including: a hammer arm rotatably connected to a mounting bracket attached to the thermal welding machine, wherein a distal end of the hammer arm is shaped to receive a marker or pen therein; a cable or spring connected to the distal end of the hammer arm; a cable connecting the hammer arm to a hand lever mounted adjacent to hand controls on the thermal welding machine, wherein movement of the hand lever raises or lowers the distal end of the hammer arm and thereby raises or lowers the marker or pen therein. In operation, the thermal welding machine is advanced across the edges of overlapping building roofing material and the marker or pen is intermittently lowered/raised to mark locations along a seam of the edges of overlapping building roofing material where the seam is either open or closed.