[header] How to keep birds from nesting [title] Use porcupine wire to deter birds from landing on rafters. [step] Porcupine wire creates an uneven landing place, which makes any birds nesting there unlikely. Lay out a length of porcupine wire over any rafters where you don't want birds to nest to keep them away. 
[substeps] Porcupine wires have heavy prongs that stick out in every direction, and they can be purchased online or at most hardware stores. [title] Lay out bird netting over large areas where you don't want birds to nest.