Transcript The Structure and Materials of Roofing

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 roof
35˚ or less is a normal pitched roof
20˚ 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.
Queen Post Truss
rafter
Upper chord
Strut
Tie beam
Lower chord
Queen post
Scissor Truss
top chord
Bottom chord
Common rafter roof framing
Tie beams
Ridge of common rafter
roof
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
Purlins
Rafter foot
Dormers