The percentage of Americans who belong to a church, mosque or synagogue has declined in the past 20 years, forcing some religious leaders to make a difficult decision: sell their houses of worship and downsize.

In the U.S., many religious buildings were built during periods of religious growth, said Cleveland State University professor of urban planning Robert Simons.

“The buildings we have that were built in the ’40s, ’50s and ’60s are not really functional for today’s perspective,” said Simons, author of Retired, Rehabbed, Reborn: The Adaptive Reuse of America’s Derelict Religious Buildings and Schools. “Too many classrooms, a little bit too big.”

These large religious buildings can fall into disrepair, placing a financial burden on shrinking congregations. The process is a “vicious circle,” said Simons, because congregations in deteriorating buildings may have trouble attracting new members, which in turn reduces donations.