A group of cloistered nuns in rural Pennsylvania who trace their history back to 16th-century Spain are building a sprawling monastery meant to last 1,000 years — and they’re doing it almost entirely by hand.

Expected to be completed in 2041 at a cost of $100 million, the 48-acre Carmel of Jesus, Mary and Joseph monastery in Fairfield, Penn., is entirely the vision of one woman — Mother Thérèse of Merciful Love, the 41-year-old daughter of a firefighter from Cape Cod, Mass.

Built entirely with traditional methods and materials, the 48-acre Carmel of Jesus, Mary and Joseph monastery in Fairfield, Penn., is expected to be completed in 2041. @HaleyMonicaPhotography

Architect Riccardo S. Vicenzino looks up at the brick vaulted ceiling of a building under construction at the monastery.…