What recourse do I have when personal property I stored at work was damaged by rain and flooding due to the negligence of independent construction contractors doing renovations at my employer's office?
If the contractors who were responsible for the damage of your personal property were truly "contractors,” then your boss is not liable for your loss. A person, entity, business, etc. that hires independent contractors is not liable, or financially responsible, for damage they negligently, or carelessly, do to others' property, since there is no employer-employee relationship. You may be able to hold the contractor liable, since it was through its negligence, or unreasonable carelessness, that your property was damaged or destroyed. If the contractor will not voluntarily reimburse you for your loss, you could sue them for the money.
When someone damages or destroys your property “negligently,” or unreasonably carelessly, they become liable, or financially responsible, to pay to repair or replace that property. The law imposes a duty or obligation on all of us to not carelessly damage others’ property. So if this was negligent, someone may be liable for your losses.
The first issue then is: was this negligent? That depends on the facts. Leaving your property in an area exposed to the elements (e.g., a room with the windows open; a basement prone to flooding; etc.) for an extended period (so that rain sometime during that period was likely or when the weather reports called for rain) would very likely be negligent, and support holding someone liable. But if the property was left in the location briefly, on a day when rain was not expected, and there was a surprise storm or flooding, then there may have been no negligence (i.e., no carelessness in doing this), and, thus, no liability. There is a good chance that there was liability, but don’t assume there necessarily was—only if there was carelessness (negligence) would there be. That said, there would be some situations where there was no negligence and therefore no liability, since liability depends on fault—on having done something wrong.
Assuming that the independent contractors had been negligent, then they may be liable for the then-current economic value of the destroyed property. The actual workers or contractors who did this could be personally liable. Their employer–if they work for someone other than themselves (the company or business which was hired to do the work; i.e., the real “independent contractor” in this case)–could also be liable under the legal theory of “respondeat superior” (basically, “let the superior answer”—the employer must “answer” for the actions of its workers).
But your employer would not be liable if these were independent contractors. The doctrine of “respondeat superior” does not apply between independent contractors and the clients or customers who hire them. It only applies between employers and their non-independent-contractor employees. In this case, if the workers who did this were independent contractors, there was an insufficient legal connection between them and your employer, as to hold your employer liable.
The last factor to consider is what you could get in compensation—and whether it is worth taking legal action. You can only recover the actual, then-current fair market value of the destroyed property. What you paid for it is irrelevant; its sentimental value is irrelevant; all that matters is its economic value at the time. Some things hold their value well: e.g., art, collectibles, quality furniture, jewelry. These valuables, it may be worth suing over (since if they don’t voluntarily pay you compensation, your recourse—that is, the only way to make them pay—is to sue them). But other things do not: personal photographs; electronics or computers or phones, unless they were very new at the time; non-collectible books and magazines; inexpensive, utilitarian furniture. Depending on what was destroyed, it may or may not be worth taking legal action to seek compensation; for example, suing over, say, $300.00 of destroyed belongings is likely not worth your while.