PARIS (AP/WAND) — Investigators think an electrical short-circuit most likely caused Notre Dame Cathedral fire.

That's not uncommon, if that is the case. Officials said the 856-year-old church had fragile walls that were "shored up with wooden planks".

That's something home inspectors in central Illinois know all too well. There are many homes in Decatur that were built in the 1900s. Joe Spalding, a home inspector, said the West End homes were built up to the 1930s. 

In his experience, the electrical wiring of a house is a reason behind house fires. 

"It's over a hundred years old. The insulation starts to defray, it gets bare wiring. Plus, people have been in it and modified it," Spalding said. 

Compared to a house that's built in the 1990s and 2000s, an older house requires more maintenance. It is usually structured with wood. It's not similar to a modernized home where a person can take the walls out. 

"You don't want to go into the older wiring systems and splice into the new systems," Spalding advised.

Notre Dame's $6 million project

The peak of the church was undergoing a 6 million-euro ($6.8 million) renovation project.

The soaring spire of Notre Dame Cathedral in Paris collapsed in flames.

Much of the interior was salvaged.

A massive fire engulfed the roof of the cathedral in the heart of the French capital on Monday afternoon as Parisians watched in horror.

The cathedral is home to incalculable works of art and is one of the world's most famous tourist attractions.

Speaking to BFMTV, Deputy Mayor Emmanuel Gregoire said first responders worked to salvage that art and other priceless pieces stored in the cathedral.

Smoke could be seen billowing from the building. Witnesses said the smoke could be seen from miles away in the French capital. 

The cathedral dates back to 1160. It is considered one of the world's greatest examples of French Gothic architecture.