Roof pitch is the steepness of a roof expressed as a ratio of inch(es) rise per horizontal foot (or their metric equivalent), or as the angle in degrees its surface deviates from the horizontal. A flat roof has a pitch of zero in either instance; all other roofs are pitched.
A roof that rises 3 inches per foot, for example, would be described as having a pitch of 3 (or “3 in 12”).
The pitch of a roof is its vertical 'rise' over its horizontal 'run’ (i.e. its span), also known as its 'slope'.
In the imperial measurement systems, "pitch" is usually expressed with the rise first and run second (in the US, run is held to number 12; [1] e.g., 3:12, 4:12, 5:12). In metric systems either the angle in degrees or rise per unit of run, expressed as a '1 in _' slope (where a '1 in 1' equals 45°) is used. Where convenient, a reduced ratio is used (e.g., a '3 in 4' slope, for a '9 in 12' or '1 in 1+1⁄3').
Considerations involved in selecting a roof pitch include availability and cost of materials, aesthetics, ease or difficulty of construction, climatic factors such as wind and potential snow load, [2] and local building codes.
The primary purpose of pitching a roof is to redirect wind and precipitation, whether in the form of rain or snow. Thus, pitch is typically greater in areas of high rain or snowfall, lower in areas of high wind. The steep roof of the tropical Papua New Guinea longhouse, for example, sweeps almost to the ground. The high, steeply-pitched gabled roofs of Northern Europe are typical in regions of heavy snowfall. In some areas building codes require a minimum slope. Buffalo, New York and Montreal, Quebec, Canada, specify 6 in 12, a pitch of approximately 26.6 degrees. [3]
A flat roof includes pitches as low as 1⁄2:12 to 2:12 (1 in 24 to 1 in 6), which are barely capable of properly shedding water. Such low-slope roofs (up to 4:12 (1 in 3)) require special materials and techniques to avoid leaks. [4] Conventional describes pitches from 4:12 (1 in 3) to 9:12 (3 in 4). [4] Steep is above 9:12 (3 in 4) [4] (21:12) (7 in 4) and may require extra fasteners.
US convention is to use whole numbers when even (e.g. "three in twelve") or the nearest single or two-digit fraction when not (e.g. either "five and a half in twelve" or "five point five in twelve", each expressed numerically as 5+1⁄2:12 and 5.5:12) respectively.
Definitions vary on when a roof is considered pitched. In degrees, 10° (2 in 12 or 1 in 6) is considered by at least one reference a minimum. [5]
In trigonometric expression, exact roof slope in degrees is given by the arctangent. For example: arctan(3⁄12) = 14.0°.
Framing carpenters cut rafters on an angle to "pitch" a roof. Lower pitched roof styles allow for lower structures with a corresponding reduction in framing and sheathing materials.
Historically, roof pitch was designated in two other ways: A ratio of the ridge height to the width of the building (span) [6] and as a ratio of the rafter length to the width of the building. [7]
Commonly used roof pitches were given names such as:
A roof is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temperature, and wind. A roof is part of the building envelope.
In mathematics, the slope or gradient of a line is a number that describes the direction of the line on a plane. Often denoted by the letter m, slope is calculated as the ratio of the vertical change to the horizontal change between two distinct points on the line, giving the same number for any choice of points.
The grade (US) or gradient (UK) of a physical feature, landform or constructed line refers to the tangent of the angle of that surface to the horizontal. It is a special case of the slope, where zero indicates horizontality. A larger number indicates higher or steeper degree of "tilt". Often slope is calculated as a ratio of "rise" to "run", or as a fraction in which run is the horizontal distance and rise is the vertical distance.
A mansard or mansard roof is a multi-sided gambrel-style hip roof characterised by two slopes on each of its sides, with the lower slope at a steeper angle than the upper, and often punctured by dormer windows. The steep roofline and windows allow for additional floors of habitable space, and reduce the overall height of the roof for a given number of habitable storeys. The upper slope of the roof may not be visible from street level when viewed from close proximity to the building.
In architecture, a cornice is generally any horizontal decorative moulding that crowns a building or furniture element—for example, the cornice over a door or window, around the top edge of a pedestal, or along the top of an interior wall. A simple cornice may be formed with a crown, as in crown moulding atop an interior wall or above kitchen cabinets or a bookcase.
A dormer is a roofed structure, often containing a window, that projects vertically beyond the plane of a pitched roof. A dormer window is a form of roof window.
The steel square is a tool used in carpentry. Carpenters use various tools to lay out structures that are square, many of which are made of steel, but the name steel square refers to a specific long-armed square that has additional uses for measurement, especially of various angles. It consists of a long, wider arm and a shorter, narrower arm, which meet at an angle of 90 degrees. Today the steel square is more commonly referred to as the framing square or carpenter's square, and such squares are no longer invariably made of steel ; they can also be made of aluminum or polymers, which are light and resistant to rust.
A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.
Framing, in construction, is the fitting together of pieces to give a structure, particularly a building, support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.
A gambrel or gambrel roof is a usually symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maximizing headroom inside the building's upper level and shortening what would otherwise be a tall roof, as well as reducing the span of each set of rafters. The name comes from the Medieval Latin word gamba, meaning horse's hock or leg. The term gambrel is of American origin, the older, European name being a curb roof.
A purlin is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
This page is a glossary of architecture.
Cavaedium or atrium are Latin names for the principal room of an ancient Roman house, which usually had a central opening in the roof (compluvium) and a rainwater pool (impluvium) beneath it. The cavaedium passively collected, filtered, stored, and cooled rainwater. It also daylit, passively cooled and passively ventilated the house.
A cricket or saddle is a ridge structure designed to divert water on a roof around the high side of a large penetration, typically a skylight, equipment curb, or chimney. In some cases, a cricket can be used to transition from one roof area to another. On low-slope and flat roofs with parapet walls, crickets are commonly used to divert water to the drainage, against or perpendicular to the main roof slope.
Domestic roof construction is the framing and roof covering which is found on most detached houses in cold and temperate climates. Such roofs are built with mostly timber, take a number of different shapes, and are covered with a variety of materials.
A gable roof is a roof consisting of two sections whose upper horizontal edges meet to form its ridge. The most common roof shape in cold or temperate climates, it is constructed of rafters, roof trusses or purlins. The pitch of a gable roof can vary greatly.
A speed square, also called a rafter square, rafter angle square, and triangle square, is a multi-purpose triangular carpenters' tool used for marking out. Its functions encompass many of those offered by combination squares, try squares, and framing squares. Carpenters use it to make basic measurements and mark lines on dimensional lumber, and as a saw guide for short 45 and 90 degree cuts.
A hip roof, hip-roof or hipped roof, is a type of roof where all sides slope downward to the walls, usually with a fairly gentle slope, with variants including tented roofs and others. Thus, a hipped roof has no gables or other vertical sides to the roof.