General manager Andrew Hanson said it was lucky there were still staff in the building to fight the fire.
He said: "If it had happened an hour later when we had all gone home I think it would have all been ablaze."
About 10 members of staff were on site when the arsonists set fire to ply-board that was covering a window and threw burning rags inside.
Mr Hanson said: "I was here with a couple of the lads when one of them jumped off the forklift and ran down shouting 'fire'.
"We got the extinguishers out. It had just caught hold when he saw it. There was smoke filling the roof space. If it had spread to the roof space there's no way we would have got it under control."
They used at least eight fire extinguishers in their bid to get the flames under control.
The company, a bathroom manufacturer, uses the building for storage and distribution of hundreds of thousands of pounds worth of stock and equipment.
And he praised the fire service's quick response.
"The fire brigade were here in about three minutes, which was incredible," he said.
He added: "If the fire had caught hold it could have killed somebody, never mind risking the lives of the firefighters," said Mr Hanson.
Four youths were arrested and questioned and have since been bailed.
Only two hours later another factory building – Clough Works, Brow Bridge, Sowerby – was hit by an arson attack.