It's generally avoided due to the difficulty of it. The only things you can do to help are keep to easily altered/chopped angle ratios like 1:1 or 1:2. Also... don't use the rotate function, that will make a mess, use Vertex Manipulation to do it by hand.
I usually make the structure first on a level axis. Once completed, I group them together and use Ctr+M to angle them. If I ever need to make edits, I could reverse the Ctr+M and make my edits.
That isn't reliable though. Rotating an object is bound to put verticies off the grid, which will be snapped upon reload of the VMF, there is then no way to rotate it back to normal.
I use a nice 1:2 ratio. Lowering the grid helps massively to get brushes the right size, I use the vertex editing tool and sometimes the clipping tool to get my brushes at the 1:2 ratio. This method doesn't need func_detailing but if you're doing it for just a small hut then its probably best to func_detail it anyway you make it.
I build the entire structure on grid, at normal angles, then select the whole lot and func_detail it and rotate it in its place and don't move it. This way only works for small buildings really.
In line with this, how bad is it to have overlapping brushes? For example, If you have a 20º angled slope, and you build a house standing on 4 legs, but you adjust the legs so that there is minimal brush-overlapping, and even use the Vertex Tool to reduce it even more, but still have a slight overlap?
Well, the easy way to fix the legs would be to select them all and use the clip tool to cut them at an angle all at once.
But if they sink into sidplacments I would just leave the ends flat and get them to have as little overlap as possible.
If they sink into a brush you don't even have to clip them, the engine will do the work for you apon compile. However make sure they don't extend past the bottom of the brush or you will get extra polys down there.
3. I build the entire structure on grid, at normal angles, then select the whole lot and func_detail it and rotate it in its place and don't move it. This way only works for small buildings really.
I've found that this creates waterindices like a mother-f***er. Just something to keep in mind. If you want to rotate something like that on a big/complicated map, you best be using models.
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