Flames could be seen on the corner of the roof of The Range in Shrewsbury as alarms went off inside.
Fire crews were called out to the store on Sundorne Retail Park, which sells home, garden and leisure products, at 11.18am.
The fire service confirmed contractors were on site at the time the fire started.
Video and photographs taken by a passersby show a workman on the roof in a high-vis jacket trying to put out the fire - at one point appearing to be in danger of falling off the roof - while another watches on from a cherry picker.
Watch: Note - video has no sound
Staff and customers were evacuated from the building and gathered in front of the store.
Shropshire Fire and Rescue Service said the roof had caught fire and crews were using a 13.5 metre ladder, and one hose reel jet. A covering jet was also in use.
Reporter Keri Trigg was at the scene:
Crews were sent from Shrewsbury and Wellington. An operations officer was also in attendance.
The fire was quickly extinguished and a firefighter at the scene told staff fans would be used to clear smoke from inside the building through the roof vents.
People were allowed back into the building at 12.30pm and the fire service left the scene after checking over the affected area of the roof with a thermal imaging camera.