The Structure and Materials of Roofing

Roof structures

Although, theorists tried, most roofs until the 20th century involved a sloping pitch.

40˚ or greater is a steeply pitched roof35˚ or less is a normal pitched roof20˚ or less is a normal pitched roof

The pitch, or slope of a roof is defined as the ratio of the rise or vertical increase as measured in inches to the inches of horizontal projection

Trusses

Buildings usually only involve sloping forces in the roof structure. These triangulated forces involve the creation of trusses. The other triangulated force is the brace in timber buildings.

Trusses. “A truss is a framed structure consisting of a group of triangles arranged in a single plane so that loads applied at the points of intersection of the members will cause only direct stress, (tension or compression) in members. Loads applied between these points cause flexural stresses. ”

structural components of roofs

1. Gable. Slope in two directions. The two slopes meet to form a ridge.2. Hip roof. Slopes in four directions and has the same slopes as a gable roof.3. Gambrel roof. Slopes in two directions, but has a double slope on each

side. 4. Mansard roof. Slopes in four directions, has a gable, or deck above.

5. Flat. Contrary to the name, a flat roof is sloped, but only enough for drainage, usually only on-eighth to two inches for every foot of horizontal

6. Shed. Is a roof that slopes in only one direction.

7. M Roof. Is a roof that combines multiple gables over a single massing.

Roof shapes

Intersections, joints

The cornice is the intersection of the roof and the wall. It involves tying the roof forces to the wall supports

The slope is called the rake when exposed along the gable walls. Hip roofs do not have a rake. By extension any joint of a sloping line with a vertical surface.The projection of the rake beyond the vertical surface of the walls is called the eave (usually plural).

Peyton Randolph “M” roof

Roof framing

1. rafters. Rafters maybe the upper chord of a roof truss. The roof truss might also be the support structure over which the common rafters transfer the load to the walls.

a. Principal rafters are heavy rafters that support purlin and then the weight of other smaller common rafters.

b. Common rafters are rafters of small size. In a common rafter roof all the rafters are of the same size.

2. purlins are horizontal supports for rafter trusses. Principal or common.

3. tie beams/ collars. Web members of a rafter truss that are in tension.

4. False plate. A board resting on top of the ends of joists or tie beams used to support common rafters.

5. Ridge board or ridge. Horizontal timber at the top of the roof. Rarely used with common rafters until the late 19th century. Without a ridge board the rafter joint was an open mortise.

6. Boxed Cornice. A hollow cornice with a fascia and soffit board nailed over the ends of the rafters and joists or beams.

Lower chord

Upper chord

Tie beamStrut

rafter

Queen Post Truss

Queen post

Scissor Trusstop chord

Bottom chord

Common rafter roof framing

Ridge of common rafter roof

Tie beams

Roof covering

Sheathing is the timber that provides the continuous structure upon which the covering is placed. The sheathing is mechanically attached by nailing to the rafters. decking can be open for thatch or slate, or close for shingle and metal.

Roof coverings are the surface materials that protect the roof structure and provide moisture proof protection.

thatch, sod, board, shingle, tile, metal roof, flat=tar or membrane)

Additional parts

PurlinsRafter foot

Dormers