The young man didn't intentionally set the Feb. 18, 2018 blaze, Superior Court Justice Edward Gareau said, describing his actions, which resulted in a $150,000 loss to the property owner, as reckless.
Trudeau lit the fire to keep warm, but he and his hitchhiking companion didn't extinguish it when they left the building to continue their journey to Sudbury.
He sentenced Trudeau, who pleaded guilty last week to arson, to an 18-month jail term, followed by three years probation.
With the credit Gareau gave the 23-year-old for pre-sentence custody - the equivalent of 14 months - Trudeau must serve a further four months.
The Crown was seeking two years less a day, while the defence asked the judge to consider a sentence of time served.
Gareau imposed 18-month concurrent terms for three other offences - break and enter, mischief and breach of probation - stemming from the incident.
In an agreed statement of facts presented to the court on April 15, the judge heard Trudeau and the woman had been drinking when they stopped at the Highway 17 property.
The pair broke into the residence, where he started a fire on the living room floor because they were cold.
They then went to a shed, where Trudeau grabbed some spray paint and returned to the home.
He sprayed black paint on the walls.
After that, they went to the barn. Trudeau gathered some newspaper and hay and used a lighter to start a fire.
When he imposed sentence, Gareau said he had balanced the aggravating and mitigating factors after hearing from the lawyers and reviewing pre-sentence, Gladue and assessment reports.
He noted Trudeau suffers from fetal alcohol spectrum disorder, has cognitive and intellectual disabilities, and limited judgement and problem solving capabilities.
The judge referred to a report that indicated he doesn't tend to learn from past mistakes and will make them over and over again.
An assessment done in Sudbury found that Trudeau isn't able to recognize risky behaviour and its' possible outcomes.
He has been coping with drugs and alcohol and is "reckless in the consumption of both," Gareau said, adding that when he was out of control he would take everything he had.
Trudeau had a troubled home life that included substance abuse, domestic violence and foster care.
His maternal grandparents attended residential schools.
"It is difficult to think of someone with more Gladue factors," Gareau said.
The 1999 Supreme Court of Canada Gladue decision directs judges to consider an Indigenous offender's background during sentencing.
Trudeau has faced great disadvantages in life, and his "serious behavioural difficulties must be taken as mitigating factors," the judge said.
Gareau cited aggravating factors, such as Trudeau's criminal record, including a previous conviction for arson, and numerous offences for failing to comply with orders.
He also pointed to the nature of the offence - Trudeau "burnt a barn to the ground and caused extensive damage."
No people or animals were injured, but "arson always jeopardizes the lives of firefighters and first responders."
While on probation, Trudeau must take any recommended counselling and treatment.
As well, he can not possess any lighters, matches or incendiary devices that can start fires.
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