Use the Generate Roof plugin. Even better, install the Plugin Manager plugin. Using that you can easily install and uninstall every plugin available. In this thread you can read more about it.

Here is the roof generator plugin thread where you can find tips&tricks for using the plugin. The later pages show some excellent ways to use this plugin.
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it generates an extra floor or some element that I can’t remove in Sweet Home 3D.

This is caused by the room (floor) you include in the export. If you still want to use the walls for a roof then only export the selection, the selected walls. If you need the floor too then modify its properties and uncheck the ceiling. That is the element you can't remove.