Building codes on Puerto Rico unable to withstand Category 5 storms: Expert Many residences in Puerto Rico are built to withstand 125 mph winds, if that.

 -- With Hurricane Maria bearing down, residents in Puerto Rico are hunkering down, preparing for 175 mph winds, 6- to 9-foot storm surge and up to 25 inches of rain. Unfortunately, most of the homes in Puerto Rico are built to withstand just 125 mph winds, characteristic of a Category 2 hurricane, according to one expert on building codes on the island.

With current gusts reaching 175 mph or more, the Category 5 storm, which slammed into the eastern Caribbean islands of Dominica and Guadeloupe Monday night, is expected to wreak havoc on the island, with the governor calling it the "potentially most catastrophic hurricane to hit Puerto Rico in a century.”

According to University of Puerto Rico at Mayagüez engineering professor Dr. Luis Aponte-Bermúdez, Puerto Rico adopted the "International Building Code" in 2011, which requires residences withstand 140 mph winds, characteristic of a Category 3 storm.

When it comes to wind worthiness, these building codes are similar to the ones that govern mainland U.S. cities like Miami, an engineer at the Insurance Institute of Business & Home Safety tells ABC News.

However, the majority of the homes on the island were built prior to 2011, to a weaker code, and were "grandfathered" in.

Before 2011, Puerto Rico was using the "Uniform Building Code," which only required residences to withstand 125 mph winds, adopted after Hurricane Georges pummeled Puerto Rico in 1998.

Most legally built homes on the islands use that UBC 125-mph standard.

Worse still, many homes dotting the island fall into what's called "informal construction" -- built to no standard whatsoever.

These are homes built illegally, without proper regulation, by people who lack the economic resources to hire a constructor and instead just buy wood and other materials from the local hardware store.

These structures are "extremely vulnerable ... most of these homes are going to get destroyed," Aponte-Bermúdez says, noting that many similar homes on the nearby island of Culebra were recently wiped out by Hurricane Irma.

"With the passage of Hurricane Irma, the people of Puerto Rico not only demonstrated our resilience, but we banded together to show our kindness and hospitality to thousands of our fellow Americans in the U.S. Virgin Islands, BVI [British Virgin Islands], St. Marteen and beyond," Puerto Rico Gov. Ricardo Rosselló said Tuesday afternoon.

"Now we’re looking down the barrel of Maria, a historic Category 5 hurricane. Although it looks like a direct hit with major damage to Puerto Rico is inevitable, I ask for America’s prayers," he continued. "No matter what happens here in the next 36 hours, Puerto Rico will survive, we will rebuild, we will recover and with your support, we will come out stronger than ever."

ABC News' Melissa Griffin and Josh Hoyos contributed to this report.