You have to use the "shed hipped roof" (second in the roof options) drag a roof 1 tile in on all four sides, then use CFE false (boolprop constrainfloorelevation false) to bring the area where you would put the flat tiles of the roof (aka floor tiles) in line with the top of the roof....
you would need to build your cfe false adjusting area to the side of your house and then use the elevation down tool to bring that adjuster areas top level in line with the roof... then put a temporary wall on your actual house roof level and then click and drag to flatten that level down.... if you want it steeper, just adjust your roof angle first, then adjust the levels maybe needing to bring the adjuster location levels up instead of down...
If your house is 2 stories tall, build an area of three walls on top of each other... the top of the third floor is your "roof level" to be seasons safe you need to be sure to put walls and floors under the area... if that makes any sense.. sorta kinda maybe anyway....
If you don't know how to use CFE false yet, here are some vids I have of it being used - the first 2 show using cfe false erm actuall no only the garage attached to foundation shows it being used... also my square stair video also shows it you can find that if you click the "more from user" on the google video page...
http://www.modthesims2.com/showthread.php?t=213193... I'm sorry but my trackball is fucked and won't let me click and drag, or I would make you a vid of how to do this roof.... but the vids I gave will show you some basices of CFE false...
here are some pics of the process...
OR OH YEAH you can just use the stage option (split level option it's called now i think) and drag a stage across your roof area and it should match up pretty good.... doesn't need CFE false for that... but I'm not sure of the seasonal impact on the house occupants...