It happened around 2:30 Saturday afternoon on the 200 block of Daniels Way in Florence Township.
The warehouse is a distribution point for Christmas Tree Shops, a home goods store.
A Good Samaritan alerted authorities when the flames sparked.
"They saw the smoke coming from the turnpike as they were driving by," said Captain Brian Boldizar, Florence Township Police Department.
Two issues presented problems for firefighters.
"Very difficult to gain access to the roof itself. It's a very tall building, 60 feet in height, so we have to get aerial devices to get access to the roof," said Battalion Chief Kevin Mullen, Florence Township Fire Department.
Once fire crews did get access, they found themselves trying to navigate around solar panels, many of which were on fire.
Solar panels hampered responders in the blaze that gutted a Dietz & Watson plant in September.
Crews brought the Florence Township blaze under control around 5:20 p.m. No injuries were reported and there was no damage to the merchandise inside.
"The bulk of the fire is knocked down. They're now checking for extension into the roof," said Battalion Chief Mullen.
Meanwhile no word yet on a possible cause.