The site, in Newport Road, is believed to have been deliberately set alight last Tuesday, police said.
Now beams can be seen through the roof, which was almost completely destroyed by the fire.
Stafford Rugby Club relocated from its ground near Newport Road to the new site at Blackberry Lane, near Doxey Marshes, after securing planning permission for the move in 2016.
Residents living near the new ground fought against the plans and even took their battle to the High Court.
Concerns raised included potential loss of open space on the Virginia Park estate for an access road and the siting of facilities on land affected by flooding.
Anyone with any information should message Staffordshire Police on Facebook or Twitter quoting incident number 379 of 8 September, call 101 or alternatively contact Crimestoppers anonymously on 0800 555 111.