Start by building walls in the shape you want for your base...I chose an octagon...
Selecting a small fence such as the monorail fencing and create a pergola lattice ontop of the walls....
remove the walls
place columns at the corners as markers... you will see why later
Open cheat box by pressing Ctrl Shift and C... type constrainfloorelevation false
go into terrain tools and select raise terrain. for this I used the medium square on the hardest setting...
Position square in the middle of the ground under your "roof"
I used 2 clicks it may take more or less but raise the ground under your roof until it reaches the height you want.
then you can use a bigger tool size to raise the whole roof up slightly...
Now we need to select soften terrain and soften the edges of the terrain under our roof....
Since we don't want gaps between the column and the roof select lower terrain and the smallest circle.
center the circle around a column and lower terrain until the fence hits the top of column...you may have to undo and try again a couple times since it isn't very consistent on how far down it goes.
do this to all columns
now delete all columns and type constrainfloorelevation true
level the ground under the gazebo roof
depending on how far you lowered the fence you may need to use MOO....but now put your columns back...
And there you go a domed gazebo roof.
Jaded