I'm going to be installing a series of new smoke detectors. When running the cable in the attic, do I run it on top of the ceiling joists or drill holes in the joists and run it through them.
I'd probably use running boards.
If it is four feet from the access hole and this is not a walk up attic then on top of the joists is fine. It still needs staples every four feet.
Unless it will be a total PITA, you may as well run the wires through the joists. That way, if you ever want to use the area for storage, you won't have to worry about the wires.
The access to the attic is in the garage ceiling. No stairs (fold down or otherwise). I use a ladder to access it. Attic height is hands and knees only and will never be used for storage. If I do run the wire through the joists, the holes will be near the center of the joists.
As for the junction box, I'll use a metal one, but can I mount them on top of the joists or do they go on the sides?
It can be mounted wherever it's convenient as long as it's accessible.
No need to drill the joists in this instance, and if it is a truss system, they should not be drilled at all.
if it is a truss system, they should not be drilled at all.
Here is a shot of the attic space. It's a couple of years old (the image) and the junk was removed and the ducts repaired.
Here is a link that might be useful: Attic