How to select/extrude all the upper faces of a lot of buildings in one only step Hello to everyone (specially to Mzidare :D),

I found this blender tutorial How to Create a City Covered with Fog and I tried to do it.

The problem I have is that I can extrude a single building vertices/faces correctly but not all vertices/faces of all the building at one time by pressing A.

If I do it,the extruded part is shown above the old one. I switched the mode selection to face and also to vertex. But it didn't work.

The author of the tutorial,Gleb Alexandrov,did not explain it very well,because he talks very fast. At the beginning of the stage 2,he says :


"hit Ctrl + A" ; instead, I imagine that's "press A to select everything" and then he says
"press E to extrude". And finally he says
"press A and recalculate normal".


I'm stuck on point 1. If I select only the upper vertices or faces of one building only it works for me. But this takes a lot of time to select every upper vertex or face and Gleb didn't do it. Instead, he selected everything. 

Is there a solution to make faster ? Check the attached short video to understand. Many thanks.

What I did
Your problem is that your buildings seem to have already been extruded once, instead of being flat 2D shapes as in the tutorial.

Either delete all but the bottom faces and star over. Select one of the top faces then use Shift+G > Normal option to select all faces pointing up. Then use Ctrl++ once to grow the selection and add include side faces too. Then delete them with X and extrude as per tutorial instructions.



Or select only the top faces for extrusion, don't select side faces like you did in your video or you will end up with a very bad dirty topology and a model full of errors.

Select one of the top faces then use Shift+G > Normal option to select all faces pointing up, then extrude