Shelter is the task of a house’s exterior—the roofing, siding, windows, doors, and related components that provide a barrier between indoors and outdoors.

Today’s houses are designed to provide somewhat semi-transparent shelter—the comfort and safety of enclosure without the feeling of being “closed in.” To this end, houses incorporate plenty of windows, doors, and skylights to bring in natural light, views, and a sense of connection with the outdoors.

Roofing, a key part of the house’s shell, consists of several components. Finish roofing, roofing felt, and sheathing—in concert with flashing and gutters—create the waterproof surface.

Exterior walls are faced on the outside with some type of durable siding. Beneath the siding, there is often building paper or house wrap and, depending on the kind of siding, sheathing. Insulation provides an additional barrier to the movement of heat through a wall.

Exterior materials are assembled to take advantage of the law of gravity: the simple fact that water doesn’t flow uphill. From a wall’s lowest point to the peak of the roof, each material overlaps the one below it, providing a continuous surface that sheds water downward. This fundamental principle of overlap is used with all exterior materials, including roofing felt, flashings, sidings, and window and door frames.