Crews from Dee Road were called to a garage in Bourne Road shortly after 5.45pm as plumes of thick black smoke blew over Tilehurst.
Six appliances were at the scene, along with three police cars, after an explosion from the building behind the garage and neighbours were evacuated from their homes.
Paramedics treated one person but they did not need to be taken to hospital.
Jane Williams, who lives in a bungalow three doors down from the garage believes it is used to fix cars but does not serve petrol.
She said: "I'd already left my house out of curiosity and it seemed more sensible not to be in the house.
"I heard the 'kapang' and saw it was on the premises of the garage.
"I couldn't tell you if anyone was still working in there."
Anthony Skillin heard the blast in Kennedy Drive and added: "I was having an afternoon nap and I was woken by a boom. My bed shook.
"My mum told me to look out the window at what had happened."
The cause of the fire is still unknown, but officials are not treating it as suspicious.
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