Each truss is made up of two opposing rafters as well as a ceiling joist. The rafters are compressed and pushing outward on the eaves walls, while the ceiling joists are tensioned and drawing inward on the eaves walls. This creates a counteracting force that keeps the roof from collapsing. Trusses were first used by French architects in the late 18th century as a more efficient way to build large roofs than using single timber beams. Before trusses were invented, builders often used heavy cranes to lift huge timbers into place or they would have to use smaller trees instead which would be expensive and difficult to find on site. Trusses allow builders to use much thinner timber than would otherwise be possible because it doesn't have to support so much weight. Also, since there are no longer any major supports under the middle of the roof, it can be made out of very thin material which reduces the amount of material needed for its construction. Today, trusses are still used in many large-scale building projects such as stadiums, but they also appear in small buildings such as churches when roofing material needs to be replaced or repaired without having to replace the entire roof. The type of truss used in a structure depends on how much load it has to carry as well as what kind of material it's made out of.