As she stares forlornly at the teetering wreck she still calls home, Sobha Shakya knows it will soon be reduced to rubble by bulldozers poised to obliterate thousands more buildings in Nepal's devastated capital.

The mammoth April 25 earthquake that killed upwards of 7,600 people reduced large areas of Kathmandu to ruins, flattening hundreds of houses as well as several centuries-old monuments.

But nearly two weeks on from the disaster, surveyors warn as many as a fifth of all homes are no longer habitable and will have to be razed to the ground by bulldozers or wrecking balls in coming weeks.

As Shakya's poorly constructed and top-heavy house started to crumble in downtown Kathmandu on April 25, she and her neighbours grabbed what they could and set up a makeshift camp in a nearby courtyard.

"These houses could collapse in a second if there is another earthquake. It's scary," she told AFP as she stared at a row of empty houses on her street propped up by wooden planks and metal pipes.

The neighbourhood consists of buildings dating back around a century, all of which have expanded vertically to accommodate growing families.

- Shoddy construction -

Large parts of Kathmandu were also left in ruins by an earthquake in 1934 which killed more than 10,000 and seismologists have long voiced warnings that another disaster could be around the corner.

But builders who have added the extra floors in the decades since have routinely turned a blind eye to planning regulations and invariably used cheap cement and other sub-standard materials.

"Our grandfathers didn't think much while building these," said Shakya, who had been living on the third of a four-storey house along with her shopkeeper husband, two sons and a daughter.

An initial survey this week of more than 15,000 buildings conducted by 2,400 volunteer engineers, sporting yellow hard hats and fluorescent orange safety vests, concluded that a fifth were damaged "beyond repair".

"About 20 percent of homes and other buildings were totally damaged. Not collapsed completely but beyond repair due to weakened structure and foundations," Dhruba Thapa, president of the Nepal Engineers' Association, which is heading the surveys, told AFP.

Thapa said that it made no sense to try and shore up buildings which were at high risk of collapse.

"It will cost billions and billions of rupees because you see, houses have walls and roofs caving in and whole structures have twisted and turned," he said.

"In these cases, one will have to start afresh. Knock them (down) and build all over again," he said. "It's going to be a lot of work. A lot."

Thapa added that 30 percent of the buildings surveyed needed repairs before they could be deemed "safe and inhabitable" and half of those inspected were "completely safe".

As well as the thousands of homes, other buildings in need of repairs include hospitals, offices, hotels and schools.

- Web of cracks -

At Kathmandu's Campion School, the 7.8-magnitude quake knocked books off the shelves, leaving them strewn across the library floor in scattered piles with shattered glass. A web of cracks also runs down walls.

"Luckily the earthquake happened when no one was here. But I have to get everything fixed before they come back next week, else these cracks can have a psychological effect on the kids," said school principal Roshan Bhandari as volunteers chipped away at walls.

"If they see cracks, broken walls everyday, it will slowly become like a scar in their minds."

While Thapa said the destruction was less than he had expected, he warned the government must rebuild the city "100 percent perfectly" now or else another quake could leave even more devastation.

"The government cannot afford to take any chances from now onwards," Thapa said.

Impoverished Nepal's government has set aside 20 billion rupees (around $196 million) for a reconstruction and rehabilitation fund, and has asked for large-scale financial help from the international community.

Since the tragedy, Shakya and her family have been living under a tarpaulin tent, occasionally venturing gingerly back inside their broken home to retrieve essential possessions.

They did try and spend one night inside but had a rethink after being spooked by aftershocks and are now resigned to a long spell under canvas.

"We don't have the money to get these buildings fixed and we can't afford to stay in hotels, so we stay here together," said Shakya.