Multiple callers reported the blaze at 4 p.m. at 45 Londonderry Drive, according to a post at the Greenwich Professional Firefighters' Facebook page.
Upon arrival, firefighters found heavy structure fire burning through the roof of the large two-story house, the post said.
The occupants of the house were not home, and no injuries were reported, fire officials said.
The nearest fire hydrant was about 2,000 feet away, fire officials said.
Mutual aid from several other towns was called for tankers and station coverage, fire officials said.
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