Randy,

you can purchase the drawings at the Pullman Library, Illionois Railway Museum.
The original drawings state: 3/32" roof plates and rivets, see drawing.
The wooden construction must have been applied afterwards because WP MW2711 was converted from a caboose (that was converted from a 15001-series boxcar early 1940's).
Cabooses did NOT have steel roofs anymore, they were rebuilt with wooden roofs, covered with muleskin.
(© Picture: John Ryczkowski)

regards
Fred Jansz