Goto Level then Volumes,
Click “Navmesh Clip”
Hit ‘e’ to create the node
Make sure the top of the node is over the surface you need (in my case it is the roads)
Make it as big as you want
Place your nav and bake it
Slightly jenky with some flying zombies but that’s ok, they were fine on the streets.
I assume this works to make bunker navs too as you can limit the height. But as you can see if any buildings/objects are capable of entering that mesh they will.
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