I am wondering if it is a good idea to chisel out the edge of rafters for wires? We want to floor our garage attic, and there are wires criss-crossing on top of the rafters.
Update: These are regular 2x4 rafters. Or isn't that regular
The top or bottom edges of a beam are the worst places to cut. It is where the most stress occurs. This is one of the reasons why it is best to run wiring/plumbing through holes in the center of your beams. Will it cause a problem? That is impossible to know, unless we also knew how much of a load you would be adding up there, the dimensions of these beams, etc.
While it is very possible there would be no problem, remember that those rafters were put there with no plans for a floor above. The structure may not have been designed to take a load. So if these are 2x4 rafters, on which you will then install a waterbed or a hot tub, expect problems. If you will be just using that floor to store a few light boxes, it probably won't be an issue.
If the floor that you add consists of 4x8 sheets of plywood, they will add considerable stiffness to the floor, but they will also add weight. As an alternative, you might consider adding sleepers on top of the rafters, which would allow you to avoid cutting into these members at all.
Finally, remember to be very careful in nailing/screwing down the new floor. A nail through an electrical line tends to be bad news.
Look at the picture above. See how the beam curves? This means the upper edge of the beam is (under load) shorter than the bottom edge. In other words, the upper portion of the beam is being compressed, and the lower edge is under tension. This means, that somewhere in the middle of the beam, there is a portion that is not under compression or tension. This is known as the "Neutral Axis" - X on the image - It is not being stressed at all.
Most of the forces on the beam occur at the outer edges, and are zero in the middle. This is why we make I-beams -- We can remove some of the material that isn't doing anything for us and make the beam lighter.
For a symmetrical beam, the neutral axis is dead center, and that is where you should drill. In theory, a small hole through the neutral axis will have no effect on the load bearing capacity. In practice it will, but it will be minimal. However, chipping out wood on either of the edges, is the worst place to remove material.
Image from: http://en.wikipedia.org/wiki/Neutral_axis