Depending on my project I would choose from one ore several of some possible solutions.
1-Roof based family. Make the rods variable length with instance parameters. Inside the project you can drag them to the right length.
2-Roof based family. Inside the family add an angle parameter, which you can make equal to the roofslope. Then use formulas inside the family to calculate the rod length for the rods on both sides.
3-Use a face based family and add an angle parameter so that it hangs straight, in combination with rod length calculation
4-Not modelling the rods.
Louis
![EESignature EESignature](http://autodesk.i.lithium.com/t5/image/serverpage/image-id/1272565i464DC3DC646B4F11/image-size/original)
Please mention Revit version, especially when uploading Revit files.