Arches and lintels transfer the loads of walls over openings to the wall systems on either side. They must be strong enough and stiff enough to carry the vertical loads without deflecting and must be durable enough to withstand weathering, since they are exposed to the elements.
Arches are most often the same masonry as is used in the wall face (e.g., stone, brick or concrete), although architectural detailing sometimes dictates the use of different materials. For example, stone arches in brick walls are common. Lintels are typically steel, wood, or a large single piece of masonry.
Openings in interior and exterior bearing walls require lintels or arches. Arches and lintels are both common on exterior masonry walls. Lintels are almost exclusively used in interior wood walls and exterior wood frame walls with siding. Lintels are more common with masonry veneer although arches are sometimes used. Separate lintels are used for the exterior masonry and for the interior wood framing on a masonry veneer wall.
Arches and lintels in masonry walls have a different set of loads than in wood frame walls. In a masonry wall, the arch or lintel carries the weight of the masonry immediately above the opening. However, if you draw two lines up toward each other from the top corners of the opening at 45 angles, you'll end up with a triangle. This triangle of material is all that is carried by the arch or lintel above an opening in a masonry wall, no matter how much brick is above it. Therefore, if you have a ten story masonry building, the size of the lintel over a first floor window would be same as the size of the lintel over a tenth story window.
A lintel in a wood frame wall sees all of the vertical loads above that opening. You have to draw two
lines up from the corners of the opening and pick up all the loads that fall in that area to figure out how much load there is on that lintel. A lintel on the first floor of a ten story wood frame building may have to be significantly larger than the lintel on the top floor (although you can't build ten story wood frame buildings, for other reasons).
Lintels in masonry walls should typically have six inches of full and level endbearing. Lintels in wood frame walls typically have 1 1/2 inches of endbearing (on the jack studs below).
Various types of arches have different shapes and endbearing. Arches impose large lateral thrusts on walls beside openings. Lintels do not. With arches look for vertical movement (sag) and horizontal movement (outwards) of walls beside openings.