All About Roofs: Pitches, Trusses and Framing
Roofs may be pitched (angled), or "flat" (which, in reality, is very slightly angled). Most roofs are pitched. There are many types of pitched roofs to suit different situations. As a result, there are many variations on the basic design and numerous combinations of design elements with construction methods.
Types of Pitched Roofs
Four main designs of pitched roofs are shown here. There are many variations on these themes. Roofs are usually defined according to their shape. Each type can be built in different ways, and from different materials.
Gabled (image 1) The roof slopes around a triangular extension of the end wall. This piece of wall is the gable.
Hipped (image 2) A hip is the joint between two adjacent slopes of a roof. Some complex roofs have several hips.
Shed (image 3) This simple roof has only one slope. It is commonly used on lean-to structures, such as additions.
Mansard (image 4) A modified version of the pitched roof that creates a spacious living area in the roof space.
Ridges, hips and valleys are the corners or joints where a roof changes direction; they are the points at which pitched roofs meet. Verges, abutments and eaves are the "edges" of a roof. The eaves are horizontal joints between a roof and a wall, whereas the verges are angled joints between a roof and a gable wall. Not all roofs feature all of these details, and some of them can be constructed in a number of ways.
Pitched Roof Frames
A pitched roof has a network of frames to support the structure and its covering. There are two main types of wooden frames — a cut roof and a trussed roof — which are sometimes combined to achieve more complex roofs. Both types of construction will support any common roof coverings.
Cut Roof Frame
Traditionally, all roofs were "cut" — carpenters cut rafters on site during construction. To cover greater spans, some of the roof’s weight may be transferred onto internal loadbearing walls using purlins (beams that brace the rafters). This forms a "double" roof. Although they are labor-intensive, single- and double-cut roofs are still constructed.
Common Trussed Roof Frame
Often referred to as A-frames because of their shape, modern trusses (lumber frames) are manufactured off-site by specialist companies. The A-frame combines rafters, joists and jacks. A roof is made up of several A-frames. Because of technological advances in calculating the stresses and loading requirements of roof lumber, trusses can be made slimmer than the boards in a cut roof. Trusses are manufactured in a number of different shapes and sizes to suit the needs of various types of roofs. For example, some trusses are designed to leave a lot of open space in a roof, so that it can be used as a room. Lean-to, or shed, trusses are commonly used for additions.
Roof Truss Stress Tests 05:23
Other Trussed Roof Frame Designs
Trusses are manufactured in many different designs. Some are designed for a particular type of roof type or to fulfill a particular structural need. Others are made to match a span. The trusses for shorter spans usually have fewer web members than the trusses intended for larger buildings. The truss types illustrated below offer just a sample of some of the more common types.
Scissor (image 1) Gives a vaulted or cathedral ceiling.
Room-in-Attic (image 2) Provides a living space in the attic.
Clerestory (image 3) A clerestory truss, pronounced "clear story," allows for a high wall with a band of narrow windows along the very top.
Triple Howe (image 4) For very wide spans of 54 to 80 feet.
Copyright 2009 Dorling Kindersley Limited
Text copyright 2009 Julian Cassell and Peter Parham