Legal Question in Construction Law in California

This isn't exactly what "joint and several liability" means, although if the manufacturer of the skylight were somehow also liable for the installation defect along with the contractor who did the installation, a court could hold them jointly and severally liable to the homeowner.

However, it is rather unlikely that the manufacturer would be held liable at all. That could conceivably happen if it (negligently) provided incorrect or insufficient installation instructions, even though there were no manufacturing defects per se. Most manufacturers of such products are far too cautious to fall into that trap these days.

For a good discussion of when and how two or more parties can be held jointly and severally liable, see http://www.wilsonelser.com/writable/files/Legal_Analysis/50_state-survey-joint-and-several-liability_mm4.pdf

As Mr. Whipple points out, joint and several liability is unlikely under the facts as you've presented them and its unlikely the manufacturer would have any liability.

Just to clarify - Joint and Several Liability ONLY applies where each party is at least SOMEWHAT liable for breach of contract, negligence or some other legal reason for liability and ALL parties malfeasance contributed in SOME way to the damages. As you laid out your question, the manufacturer did nothing wrong, so they cannot be liable individually or under joint and several liability. What joint and several liability does is allows the damaged party to collect ALL the damages from ANY party that is jointly responsible for the loss. Then if they are only partially responsible, they have to collect the other parties share from the other parties.