
Two fire fighters were injured in the blaze, which broke out at 9:30 am at the shop located in Chiang Mai's Sarapee district.
Police said the fire was started by a worker who used a welding machine to fix an air-conditioner. The welding machine sparked a fire to the plastic wrapper of the ceiling thermal insulator.
The fire spread along the plastic sheet, and dropped to cans of thinner and paints, causing the blaze to spread quickly through the building.
The fire prompted about 250 workers of the shop and some 50 shoppers to run for their life.
Some 20 fire engines spent over 4 hours but failed to control the blaze, which engulfed the entire building, causing it to partially collapse.
Somkiart Wiangchansua, the manager of the Global House Wiang Kumkam branch, said the shop was housing some Bt3 billion worth of construction materials and the building itself was cost about Bt300 million.
He said some 250 employees at the branch would not be laid off.
The Chiang Mai provincial administration declared the shop as a danger zone.
Chiang Mai deputy governor Phairoj Saenphuwong said engineers would later inspect the structure of the building, which might be demolished if it was considered not stable.
He said unauthorised persons would not be allowed to enter the building before the inspection.
At the time of the fire, the owner, Withoon Suriyawanakul, was in Khon Kaen. He left on a plane to Chiang Mai immediately.