Sitting in the concrete skeleton of the Hilton being constructed in downtown Des Moines is a large box wrapped in white plastic.

Pull off some tape, poke your head inside and you'll see an entire bathroom: toilet, sink, shower, tile floors, and polished fixtures — everything but the mirror.

It’s one of about 300 prefabricated bathroom pods general contractor Weitz Co. plans to install at the eight-story convention hotel.

The bathrooms are being built on an assembly line in Ohio and will be trucked to Des Moines, where a crane will hoist them into the hotel and crews will wheel them into place.

For large-scale projects like hotels, hospitals and dorms around the country, contractors are increasingly turning to prefabrication, installing the modular bathrooms instead of building them from scratch on-site, industry experts say.

“It’s becoming more common because (it offers) a higher level of accuracy, better finishes and less variability than on the construction site,” said Larry Cormicle, a senior lecturer in construction engineering at Iowa State University.

The trend started with hospitals, which often feature hundreds of identical rooms conveniently assembled in a factory setting, Cormicle said. More recently, hotels, universities and senior housing developers have followed suit.

Cormicle expects prefabrication to become more popular in the coming years, thanks to a shortage of skilled construction tradesman and demands for efficiency.

Fast food companies, for example, might start to building their restaurants in modular pieces in a factory and assembling them on-site, he said.

It’s not new. Disney was stacking prefabricated resort rooms as early as the 1970s.

But the $100 million Hilton is the first project in Des Moines and likely the first in the state to use completely prefabricated bathrooms, said Ben Bunge, a project manager for Weitz Co.

“We’re not reinventing the wheel, we’re just bringing it to Iowa,” he said.

A few of the bathroom pods have arrived. The rest will be shipped by mid-July.

Construction workers buzzed around the Hilton site one recent morning. The 330-room hotel connected to the Iowa Events Center is scheduled to open in March 2018.

Weitz is gearing up to pour the concrete roof and has started to install windows and exterior panels.

White rectangles painted on the concrete floors mark will each bathroom pod will be installed. Once in place, hotel guests won’t be able to tell the bathroom was built three states away, Bunge said.

Only the hotel's handicapped accessible bathrooms will be built on site. 

For Weitz, there isn’t much cost savings, but the prefab bathrooms are expected to reduce construction time by about a month and improve safety and consistency, he said.

The 10-foot by 5-foot bathrooms are built on a rolling assembly line by PIVOTek, a suburban Cincinnati manufacturer.  

When built on-site, bathrooms are often a time-consuming part of the job. It can take 10 or more tradesman to complete a bathroom (think paint, tile, plumbing, fixtures and electrical systems) and because bathrooms are small, only one person can work at a time.  

And when bathrooms are built on-site, tradesman have to carry their tools and material from room-to-room, which creates more job sites hazards and waste that must be hauled away, Bunge said.

Prefabrication might not be a welcomed trend for all. Work that would have been done by local electricians, plumbers and drywall installers has been transferred to an out-of-state factory.

Bunge said Weitz hasn’t received pushback from trade unions about the prefabricated bathrooms. There are so many construction projects underway in Des Moines that contractors are having to bring in tradesmen from other areas, he said.

The downtown Hilton is the first project on which Weitz is using prefab bathrooms. The Des Moines-based contractor plans to use them again for a senior housing development in Nebraska.

They require more upfront planning. Weitz had to get buy-in from subcontractors and building inspectors. And there is some added risk to calculate: Will the on-site cost savings outweigh the added costs of trucking the bathroom pods?

But Bunge thinks Weitz and other contractors will increasingly considering using prefabricated rooms.