Materials in Blender can be reused. This keeps files smaller and lets you easily modify materials throughout a scene. Perform the following steps:
Select the Materials button on the Properties window header.
In the Material panel, notice that Hull Material is already there. One clue to why that is can be found next to the box where you input the material's name. Notice that it has the number 2, as seen in the following screenshot. The number represents how many objects use the material. Both Sloop and Cabin were made from the default cube, and the Hull Material started out as the default material. So, Blender has already been reusing the materials.
With the cursor over the 3D View window, press A to deselect all the faces.
Select the Object Data button on the Properties window header.
In the Vertex Groups subpanel, highlight the Cabin Roof group in the menu and press the Select button with the LMB.
Select the Materials button on the Properties...