Working on a new green house. Many greenhouses have spikes or points along the top ridge. I presume one of the main reasons is to discourage birds from landing there. Question ; How close together do they need to be, how high? How about putting in large nails? Would this help?

They are usually Victorian styled greenhouses and the spikes are for decorative purposes, although I guess they might keep birds off. IMO, you really don't need them unless you have a bird problem.