In the 17th century, the timbers used for the construction of the dome of the Invalides were barely edged so as not to reduce their solidity. The joists were made of oak, the only wood used in the timber roof trusses at the time. Oak is a regular wood, from the surface to the heart, hard, and heavy. The pieces of the Invalides dome are curved and their assembly is very complex. In total, nearly 6,500 joists make up the dome’s “forest”. In 1988, bituminous cardboard sheets had to be inserted under the lead sheets of the roof to protect them from the corrosive action of the oak tannins.