"If you're not a criminal when you are brought in here, you definitely become one during your stay," he added in a letter sent to AFP through a local non-governmental organisation.
His case is by no means unique at the prison, where 3,700 inmates are crammed into a facility built to house some 1,000 men -- and where only 721 have gone to trial.
Some inmates have completed their sentences yet remain behind bars for months afterwards, waiting for a judge to sign their release form.
Within Roumieh's mouldering walls, laundry hangs from the small windows of overcrowded cells, rank with the smell of urine, and with juveniles and refugees living cheek by jowl with hardened criminals.
Noisy, narrow corridors link tiny cells that house up to six fully grown men each, with only small, barred windows to let in sunlight and barely enough room for them to stretch their legs.
Their dire living conditions recently grabbed headlines following a string of riots to demand speedier trials and better treatment by their warders.
Four inmates died when police stormed the facility to contain the mutiny.
One of them, a man in his late 20s, had been granted a presidential amnesty after serving time on charges related to homosexuality, which is a crime in Lebanon.
"They treat us and our children like animals," lamented Umm Ahmed, whose son is serving time for drug use.
"Every week when I go in, I make my son lift his shirt to make sure he has not been beaten," she added, standing outside the prison located northeast of Beirut.
"I can tell he has not showered and he grows thinner by the week."
Like hundreds of mothers, she regularly turns up at the jail with home-cooked food and basic provisions for her son, neatly wrapped in plastic bags, queuing outside for hours amid increasingly stringent security measures.
For years, experts have warned that the prison was a ticking time bomb, a call that, until this month, went largely unheeded by the authorities.
"The interior minister two years ago warned that the situation was going to explode and urged intervention before the worst. No one did anything about it as it was not the priority of the then government," said criminologist Omar Nashabe, author of the book "If Roumieh Could Speak."
"Now they cannot ignore it any longer."
Acting interior minister Ziad Baroud has acknowledged the state's shortcomings, and authorities have pledged a five-million-dollar (3.4 million euro) reform plan.
Prosecutor General Said Mirza has promised more efficient trials, and construction has begun for two new prisons in the country as well as a small courthouse outside Roumieh.
Lebanon's General Security, which is in charge of border security, has also announced it has begun to transfer illegal immigrants and refugees out of Roumieh and into cells near its headquarters in the capital.
Yet experts fear the plans for reform are simply window dressing aimed at temporarily quelling the unrest.
Father Hady Aya, founder of the non-governmental organisation Association Justice et Misericorde (AJEM), says prison management must be transferred from the hands of the police to the civilian-run justice ministry, as per a decree adopted in the 1960s.
"This humanitarian problem runs back decades and cannot be solved overnight," he told AFP at his makeshift office outside the jail.
"The prison is still run by security forces who have no training in prison management or how to deal with inmates, which only makes the situation worse," he said.
Nashabe said a key problem was the fact that the prison staff are police officers who are ill-trained and ill-equipped.
"They don't have surveillance cameras around the prison," he said. "They don't have proper separation doors between the different sections, nor an emergency plan or negotiation unit.
"Will these reforms suffice? The issue is not just about injecting cash."
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