Vandals set light to a wooden summer house display at a B&Q branch in Bolton town centre, in what the store manager called a "very dangerous" act.
It is thought the arsonists used petrol to start the blaze and police have appealed for anyone with information about the attack to contact them.
The wooden summer house was in a car park display at the entrance to B&Q.
Store manager Stephen O'Toole said: "If the summer house had been any closer to the building, the whole place could have gone up, especially if the people who did it were using an accelerant. It was very dangerous."
The incident took place days before a B&Q trading update reported sales for the DIY chain have fallen by 6.7% in the 11 weeks to 16 July.
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