AP Photo/Rafiq Maqbool
Rescue workers look for trapped people after a residential building collapsed in Thane, Mumbai, India, on April 4.

An eight-story apartment building that collapsed and killed more than 70 people in Mumbai has exposed India's illegal housing sector, corruption among builders and a lack of policy in place to keep the public safe. Before it toppled, the building had been under construction for about five weeks, sources familiar with the accident say.

As developing cities expand, the demand for housing has attracted unreliable builders who exploit the imminent need for low-cost housing, forcing those with limited means to move into illegal buildings. In 2012, India’s urban housing shortage was estimated at nearly 19 million households, notes a report by the Ministry of Housing and Urban Poverty Alleviation.

The recent case, which the BBC says has led to nine arrests so far, is not one in isolation, as an estimated 2,000 illegal structures stand in the neighborhood of the tragic site. The tragedy is not endemic to Mumbai, either. In Delhi, an illegal building sector comprises roughly 70% of buildings, experts say.

“Rules are made for just … the 30%," says Arvind Nanda, chief executive officer of Interarch Infrastructure. "The push of population leads politicians wanting votes to ensure the illegal buildings are legalized."

Contractors working on illegal structures rarely follow traditional construction methods and use cheap materials, such as an incorrect mix of concrete that is cut with sand to reduce the cost of cement. This corner-cutting trick gives little time for concrete to cure, speeding up the building process before the defect gets noticed by authorities, explains Nanda. On average, each floor requires between 10 to 15 days for curing.

"A building that should have taken at least six to eight months to complete was instead constructed in four to six weeks,” says Nanda, who advocates a shift to steel buildings because steel doesn’t need curing. “Though it is more expensive and faster, there is no need to resort to taking shortcuts.”

The Mumbai building had four occupied floors. The eighth was under construction when it collapsed, Police Inspector Digamber Jangale says. A forensic report, which investigated samples taken from the site, is expected to be ready within a week.

There is likely more than one cause for the collapse, including soft soil, failure to compact the foundation's soil with rollers and a wrong mix of materials, which could cause air bubbles. “It was basically a mixture of speed and stealth with unskilled workers constructing at night,” says Nanda.

"Illegal construction is a major problem,” adds Sandeep Malvi, a local government spokesman. “Each time we demolish a building, someone builds again. We demolish it, and they build.”

The wake-up call has led to a law that requires officials to inspect the structural stability of new and old buildings in urban and semi-urban areas. While this is the second time the order has been issued since 2007, it has yet to be enforced. The requirement also stipulates deployment of "beat marshals" to ensure that new illegal construction does not come up.

Because about 400,000 people live in unauthorized structures in Delhi, Urban Development Minister Kamal Nath has announced that the Delhi Development Authority will build 100,000 new affordable houses a year. But the program has been slow to get off the ground as “a decision has been made to grow vertically because land use has to be judicious,” says Nath.

Others blame the Indian government for allowing the illegal housing to thrive.

“If the administration is really serious about waging a war against the illegal housing industry, it should act against the top bureaucracy that has ignored repeated pleas to pull down illegal structures," says Mumbai-based activist Mangal Patil.