The explosion happened in the 4400 block of Pearson Ave. at about 12:50 p.m.
"My whole house shook, and I just heard 'boom,'" said Angela Gallagher, who lives across the street from the home that exploded.
No one was hurt, much to the surprise of neighbors.
"Literally you can see where it looked like the front of the roof popped up," said neighbor David Rynkiewicz.
Chopper 6 flew over the scene and showed the roof was collapsed and a wall was blown out.
Crews battling fire after apparent explosion in Torresdale section of Philadelphia
The fire was out by 1:40 p.m.
Investigators think the explosion may have been caused by gas, though it's too early to say for certain.
"One of our concerns is not to extinguish the fire until you have the source of the gas because when you try to extinguish it, you put out the fire and you still have a gas leak," said Philadelphia Fire Assistant Chief Richard Davison.
Gas service was turned off on the block.
Neighbor David Rynkiewicz said he heard that no one lived in the apartment that was damaged, though there were other occupied units on the opposite side of the building.
"The house had been vacant, they said. But people were doing work there," he said.
Philadelphia Police Chief Inspector Carl Holmes says the house was sold a little over a week ago.
Investigators said they will be looking into whether construction played a role in the explosion and whether the construction was done under a valid permit.