We're going to use the LidarRooftopAnalysis tool from the WhiteboxTools plugin to derive the properties of roofs directly from the point cloud data.
1. In the Processing Toolbox go to WhiteboxTools | LiDAR Tools | LidarRooftopAnalysis. Or search rooftop.
2. In the LidarRooftopAnalysis dialog, choose your LAZ file as an input. Note that you need to browse to the layer.
For now, we keep all the defaults. If the results are not good enough, you can play with the parameters. The Help button in the lower right of the dialog will point you to the documentation of this tool.
Note that it will give an error that it misses the input file. This is because it doesn't accept the extension in capitals. Rename the LAZ file to 37HN1_03.laz.
3. Save the result to a shapefile with the name roofs.shp and click Run.
This can take some time.
This video summarizes the steps:
Next, we'll look at the result.