What's the proper way to make perfect slanted roofs?

Not sure if its proper, but an easy way is make a cube the size of the roof (don't add or subtract yet)
Click or double click the upper corner of the cube on the side you want to move downward, a cross will appear in that corner
Hold ctrl (or it might be alt, I'm not sure anymore lol) and left click while dragging that corner downward

Don't try this with very small cubes or your unrealed will probably crash, works fine for roofs though.

http://wiki.beyondunreal.com/Legacy:Vertex_Editing

Thats Vertex editing Masta. It can be done 2 ways even 3 ways or 4 maybe 5 ways here is 2. Do what Masta said, Make a cube say 128 wide and 64 height and drag 2 corner points down down by holding Alt key to leave say an 8 unit gap from the bottom of the roof to the point you just moved which then would make the slope of the roof. (Tip if it gets to close to another point more than one pivot point will move at the same time) 2nd use 2d shape editor and make a triangle and extrude it to 128 and hold alt to get the pivot points to where you need them. Also textured after re size will be off set to size 1 then realign the texture to match.
No expert here but that's all you get from me after I have had a beer

Edit. Just follow the link posted by Masta, I failed to notice it before I posted!

well 2D editor would make sense, make a brush.

For WW2 it was best just to make a "wooden frame" and have panels of tiles, if players are coming through the attic or there's ton's of chimneys, pipes, windows and stuff that might be easier to not have one big brush, unless it is like out of bounds roof that no-one is going to travel through.