Why a 'birthday' may have sparked a rooftop siege and bedroom fire

A birthday may have sparked a siege in Western Sydney on Tuesday night after a man allegedly set fire to a home and held police at bay in a rooftop stand-off.

The agitated and armed man is alleged to have held police at bay on a neighbour's rooftop after setting his bedroom alight.

Police were called to the Oakhurst street just before midnight after the 26-year-old allegedly doused his bedroom with petrol and set it alight.

"He was waving around a massive knife... it was huge," resident Brittany Muller said.

The concreter is alleged to have barricaded himself in the room but soon relocated to a neighbour's roof and began pulling it apart.

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"He was on my fence with a knife, when the cops came he got on the roof and started throwing tiles," Ms Muller said. "When he ripped the chimney off, we thought he was going to come through he roof because we heard tiles slide everywhere."

Ms Muller and her family were told to get out of their house.

"They waited until he was at the back of the house for us to run to the car so we could drive away," Ms Muller said.

The siege only came to an end when the 26-year-old slipped off the roof. There was a brief scuffle as police attempted to arrest him and officers were forced to use their tasers.

The man was taken to Nepean Hospital for assessment.

Friends said it was the man's birthday.