This is a very elementary question, but rather than walk around Lowe's looking at tools, I figured I'd come here: What's the easiest way to determine the slope of the ceiling/roof joists in my attic? I'm building a knee wall and the top plate will be attached to the joists, so the studs will need to be cut at the angle of the ceiling. I assume there's a simple tool for this?

If Rise over Run is 3 over 10, the angle is the arctan of 0.3; 16.7 degrees. The angles have to add to 180 so one angle is 90 and the other must be 90-16.7.

Its called a level and a block of wood....trace the angle on the wood when it is perpendicular to the angle you need.

I use one of these: http://www.sears.com/shc/s/p_10153_...ngle+indicator&prdNo=6&blockNo=6&blockType=G6