The Spotswood home became engulfed in flames about 3.30am on Thursday when a fire broke out in the air-conditioning unit in the roof of the property.

The blaze started in the Hudsons Road home's second storey, where two children were sleeping.

According to MFB North Laverton station officer Rod O'Sullivan, the mother was woken by sounds of the fire. When she investigated the second floor, she found it engulfed in flames.

One child woke up and made their way downstairs but when the second child couldn't be stirred, the father went upstairs to rescue him.

The man suffered burns to his feet while saving his son, which Mr O'Sullivan hailed as a "pretty good effort".

"Our priority would be to rescue lives and he's got his son out, so it allows us to focus on the fire. It means we were able to contain the fire to the second floor."

The man suffered burns to his feet, and was treated by Ambulance Victoria paramedics at the site before being taken to the Alfred Hospital in a stable condition.

Four trucks and 20 firefighters were called to the scene, which was reported by the police air wing and a neighbour.

Mr O'Sullivan said most roof fires were caused by air-conditioning units.

"It's a lot of factors but being up in the roof, they tend to be a major cause of house fires," he said.

"The bearings seize up in them and no one notices because it's out of sight and out of mind, so it's in the middle of the night that these things can happen."

Reports that the family's smoke detectors weren't working were unconfirmed. Mr O'Sullivan said the fire was so intense, the property's roof may have collapsed before a smoke alarm would have even detected the blaze.

Families are being urged to have a fire plan, especially in two-storey properties where escape routes are limited.

Mr O'Sullivan also urged Victorians to ensure their smoke detectors were working.

"They're no good being on the ceiling if they don't work. It's extremely important."

The fire comes as the city experiences a heatwave, sending overnight temperatures soaring well above average.

Wednesday's overnight low was a stifling 24.8 degrees - nearly 11 degrees above the March average.

Damage to the weatherboard home is estimated to be in the hundreds of thousands, with Mr O'Sullivan saying the second storey would have to be destroyed.

He praised fire crews for their "extremely good" work in containing the blaze within four to five minutes of arriving.