This happens when the Spline and the Surface are exactly the same size. Unfortunately it's a know issue and at the moment the only solution is to make the surface larger than the spline. Alternatively you can not use a surface and just rotation the RC object so that it's aligned to the pitch of the roof. I think you've seen this script
, but it can really speed up this process.
I hope that helps, please let me know if you have any further questions.