Ask a Pennsylvania Attorney what to do...Simply stated, Pennsylvania property law allows a landowner to cut off over-hanging branches of a tree belonging to an adjacent landowner, without regard to the degree of physical harm done to his property. The courts feel that cutting the protruding limbs is an adequate remedy when the branches are "intruding into the landowner's air space" and, therefore, the landowner who cuts the branches will not be subject to liability in the absence of special damages. The caveat here is that this self-help approach applies only to the protruding (offending) branches. In essence, the landowner cannot trespass on a neighbor's property and cut down the tree or cut branches that do not protrude on the landowner's airspace. The landowner may also seek reimbursement of reasonable expense incurred in exercising the self-help remedy. Irrespective of the above law, I highly suggest that you consult a lawyer before resorting to any self-help remedy.
When the landowner whose airspace is being offended has sustained actual substantial damages, he may (1) resort to cutting the branches or (2) bring suit for damages or (3) maintain an action for abatement of the nuisance (i.e. the hanging branches) and file for injunctive relief which would require removal of the branches.