Drag&Drop one of the Roof Presets from the Asset Library to any space in the viewport. A piece of roof will appear in the scene (you can edit it to shape your own roof or copy its Modifier to the roof mesh you already have).
Select your roof mesh and holding Shift select the preset object you just added from the Asset Library.
There are two ways here:
Press Ctrl-J to join the mesh of your roof to the preset mesh (then delete the preset mesh in Edit Mode).
Go to Modifier Tab and copy the Geometry Node Modifier to selected. Then go to the Material tab and copy material to selected.