Not join but addā¦
Select a fill.
Switch to the tool "fill"
Begin to edit your fill.
Select the "Add a fill" āwith a +ā¦ā in your contextual menu
Space bar clic on the contour of the fill that you want to add at your first one.
The first fill is increased by the second one
Don't forget to remove the second fill āwhich don't disappear in the operationā.
The two fill can have differents attributesā¦
You can do that with a fill and different elements : slab, roof, polyline, etc. āeverything which can space bar clickingā¦ā
You must recreate the hole(s) āif there isā¦āof the second fill to keep them in the result āby space bar clickingā¦ā
Hope this helps