IIRC, you need to enter IndividualRoofSlopeAngle 25 and the last roof you placed will magically change to 25 degrees. (Of course, you can replace the 25 with any other number you like.) If it's not that, then it's that you enter IndividualRoofSlopeAngle 25 and then place a roof on a empty spot (not somewhere there's a roof already). Pretty sure the answer is one or the other of those, but I never remember which one.
If you want all the roofs the same, you can place them and then enter RoofSlopeAngle 25.
And remember that you can replace 25 with whatever number you want in all the examples.