In the early hours of December 2, Grindlay poured accelerant into the bedrooms, and opened all the doors and windows and set fire to the house.
When his ex-wife texted to tell him the fire alarm had been activated, he led her to believe it was under control.
He then changed his clothes and watched the house burn.
Nobody was injured and police arrested and charged Grindlay that morning.
In the Masterton District Court yesterday, Judge Barbara Morris said the arson had caused "huge emotional and financial grief to your ex-wife and her parents". Judge Morris said in deliberately setting fire to the house he had acted inexplicably and in a "crazed fashion".
The fire had caused considerable damage to the uninsured house, Judge Morris said.
"You had changed clothes and sat on the lawn and watched it burn while the fire service put it out."
She acknowledged Grindlay had been under a lot of stress at the time and had removed gas bottles from inside the house which could have caused more damage.
"You were under huge pressure for a long period of time and on that night you simply lost your senses. I don't consider that it was out of spite, for as the saying goes, it would have been cutting off your own nose. I consider that you simply lost your mind for a short time when the pressure got too much."
He was lucky the victims were not seeking retribution by seeing him imprisoned, she said.
In sentencing Grindlay, she took into account the fact he had continued to make payments towards what she called "a complete financial mess. You've done all you can since this occurred to make good this loss."
However, she also took into account the arson was premeditated and put lives in danger.
Judge Morris sentenced Grindlay to six months' community detention between 7pm and 7am and ordered him to pay reparations of $60,000.
He was also placed under supervision for a period of nine months and ordered to attend alcohol and drug programmes as directed by probation.
