My home insurer denied my claim calling it an "act of God". Is that a valid reason?

"Act of God" is not a rule in your policy. It is a phrase. What counts is the policy itself. Texas explains that an all-risk policy covers any event the policy does not specifically exclude, and a named perils policy covers only the events listed in the policy [1]. So the first question is which type you have, and which words in your policy the company relied on.

If the company rejects a claim, Texas says it must say why in writing [2]. California goes further. When a denial rests on a policy provision, condition or exclusion, the written denial must point to it and explain how it applies to your claim [3]. Rules differ by state, so check your own state's rules.

If you still disagree, you have options. Texas says appraisal is for disputes about the amount of a claim, not for whether the policy covers a loss [4]. Texas also says a complaint to the state department must be in writing [5]. Before suing over a disaster loss in Texas, you must tell the company in writing at least 61 days ahead [6]. The sources give no deadline for filing a lawsuit.

Open your policy and find the list of covered perils or the list of exclusions. Look for the exact words in your denial letter. Your state insurance department's consumer pages say what your state requires. Texas and California are used here as examples. Your state's rules and your policy's suit deadline: "Don't know yet".