Weekend Cabin Retreat Project
Roofs
Sacramento City College
EDT 300
Kenneth Fitzpatrick, P.E.
EDT 300 - Cabin Design Project - Roofs
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Types of Roofs
 The
appearance of a house is
affected greatly by the roof lines and
material.
 The roof should compliment the
structure.
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Gable Roof
 The
gable roof is very popular.
 Easy to build.
 Sheds water well.
 Provides for ventilation.
 Can be used on a variety of house
designs.
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Hip Roof
 The
hip roof is slightly more difficult
to build than a gable roof.
 The
hip roof is a very popular choice.
 Does not provide for ventilation as
well as gable roof.
 Increased chance of leaks because of
hips and valleys.
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Flat Roof
 The
flat roof is the most economical
roof to construct.
 Doesn’t add much to the appearance
of the house.
 Requires “built-up” or membrane roof
covering instead of conventional
shingles.
 A built-up
roof consists of layers of roofing
felt and tar.
 Usually
pitched about 1/8” to 1/2” to
provide for drainage.
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Flat Roof
 The
flat roof is popular in warmer
areas of the country where wide
overhangs are desirable for shade
and where little or no snow falls.
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Shed Roof
 The
shed roof is similar to flat roof
but has more pitch.
 Frequently
used for additions to
existing buildings.
A
built-up roof is often required.
 Pitch
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must be 3:12 to use shingles.
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Butterfly Roof
 The
butterfly roof has not been used
widely in the past.
 Provides plenty of light and
ventilation.
 Drainage is the problem.
 Flashings must extend far up the roof
to prevent leaks.
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Gambrel Roof
 The
gambrel roof is sometimes called
a “barn roof”.
 It
has been used extensively for
barns.
 Provides the additional headroom
required for the “Dutch colonial”
building style.
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Winged Gable
 The
winged gable is a variation on the
gable roof.
 Ridge
point.
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peaks are extended to form a
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Dutch Hip
 The
dutch hip is a combined gable and
hip configuration.
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Mansard Roof
 The
mansard roof has not been
popular for several years.
 Is
returning to popularity.
 Is a French design.
 Is more difficult to construct than
hip or gable.
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A-Frame Roof
 The
A-frame is a combined wall and
roof.
 Originally
used for cottages.
 Now is used for homes, churches, and
other structures.
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Curved Plate Roof
 Similar
to Folded Plate roof.
 Limited residential application.
 Some use in prefabricated modules.
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Folded Plate Roof
 The
Folded Plate Roof is a
contemporary design.
 It
is finding some limited application
in residential construction.
 Quite popular for motels and small
commercial buildings.
 Many modular, prefabricated units
have this type of roof.
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Traditional Frame Roof Construction
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Rafters
 Roof
covering material is supported
by roof framing.
 The
framing must be strong and rigid.
 Roof framing consists of several
distinct structural elements.
 The first and most basic of these
elements is the rafter.
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Rafters
 Common
rafters are perpendicular to
the top wall plate.
 They
extend from the plate or
beyond to the ridge of the roof.
 Note
that several types of rafters
other than common rafters are
identified.
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Rafters
 Rafters
are cut to the proper
dimensions by locating the ridge cut,
seat cut, plumb cuts, and tail cut.
 The
precise layout of these cuts is
determined by
 The
slope or pitch of the roof.
 The inside width or clear span of the
building.
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Rafters
 Terms
that must be understood
before calculating rafter dimensions
and roof pitch are:
 Rise.
 Run.
 Span.
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Terminology
 Rise
- the vertical distance measured
from the top of the wall plate to the
underside of the rafters.
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Terminology
 Run
- one-half of the distance of the
clear span.
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Terminology
 Clear
Span - is the horizontal
distance from the inside of one stud
wall to the inside of the opposite stud
wall.
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Roof Slope
 Roof
slope may be indicated on the
drawings by:
 A slope
ratio diagram
 A fractional pitch indicator
 An angular dimension.
 The
angular dimension method is seldom
used
 It is difficult to measure as accurately as
either of the other methods.
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Roof Slope
 When
designing a roof, use a
standard roof pitch.
3
in 12.
 4 in 12.
 5 in 12.
 6 in 12.
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Rafters
 Select
rafters based on
 The
distance to be spanned.
 The spacing of the rafters.
 The weight to be supported.
 Rafters
for low-slope roofs may also
serve as ceiling joists.
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Roof Weights Per Square Foot
A
“light roof” is one that weighs less
than 4 lbs per square foot.
 Examples:
 Cedar
shake roof.
 Composition roof.
 Metal roof.
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Roof Weights Per Square Foot
A
“heavy roof” is one that weighs
more than than 4 lbs per square foot.
 Examples
 Slate.
 Clay
tile.
 Concrete tile.
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Cornice
 The
cornice is the overhang of the
roof at the eaves line.
 It forms a connection between the
roof and the side walls.
 In a gable roof, the cornice is formed
around two sides of the building
 In a hip or flat roof, the cornice is
formed around all four sides of the
building.
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Cornice
 Three
types in residential
construction:
 Open
cornice
 Box cornice
 Close cornice
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Open Cornice
 The
Open Cornice may be used with:
 Exposed
beam construction.
 Contemporary designs
 Rustic designs
 Rafter
ends are exposed and are
usually tapered or curved to prevent
a bulky appearance.
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Three Types of Box Cornice
 Narrow
Box.
 Wide Box with Lookouts.
 Wide Box without Lookouts.
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Narrow Box Cornice
 Usually
between 6” and 12” wide.
 The soffit board is nailed directly to
the bottom side of the rafters.
 Usually used for Cape Cod or colonial
homes.
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Wide Box Cornice with Lookouts
 Is
usually between 6” and 12” wide.
 Requires additional support members
called “lookouts” for fastening the
soffit
 The soffit board is nailed directly to
the bottom side of the rafters.
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Wide Box Cornice without Lookouts
 Has
a sloped soffit
 The soffit material is nailed tot he
underside of the rafters.
 Frequently used where overhangs are
very wide.
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Close Cornice
 Is
one in which the rafter does not
project beyond the wall.
 The
roof is terminated by a frieze
board and molding.
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Rake or Gable End
 Is
the extension of a gable roof
beyond the end wall of the house.
 The
overhang should be about the
same size as the cornice.
 If
a close cornice is used, a close
rake should be used.
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Rake or Gable End
 Wide
overhangs provide side wall
protection and less frequent painting.
 In
California, wide overhangs provide
shading from hot summer sun.
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Roof Trusses
 Roof
trusses are an assembly of
members that form a rigid framework
of triangular shapes.
 They
permit wide unsupported spans
with a minimum amount of material.
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Roof Trusses
 Lightweight
wood roof trusses are
designed to span distances of 20 to
32 feet.
 Many
times trusses are less
expensive than conventional framing.
 Time
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and expense is saved.
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Roof Trusses
 Prefab
trusses for standard widths
are readily available.
 Custom
trusses for non-standard
dimensions are factory produced.
 Most trusses are made from 2 x 4
lumber.
 2 x 6 may be used for wider spans
and heavier loads.
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Roof Trusses
 W-type
truss
 King-post truss
 Scissors truss
 are commonly used in residential
construction.
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Roof Trusses
 Most
are designed to be installed at
24” on center.
 Ceiling materials are nailed directly to
the bottom of the truss.
 Information required to make
 Span
 Roof
pitch
 Spacing of the trusses
 Anticipated roof load.
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Ventilation
 Adequate
attic ventilation is critical.
 Moisture will form on the underside
of the roof sheathing and damage will
result.
 A well-ventilated attic will help to
cool the house in the summer.
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Ventilation
 Ventilation
is usually achieved by
louvered openings on the gable end.
 Ridge ventilators are also available.
 The total are of the ventilation
openings should be 1/300 of the
ceiling area.
 1200 sq ft. Ventilation openings at
least 4 sq ft.
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Flashing
 Flashing
should be used where
 the
roof comes in contact with a wood or
masonry wall.
 At a chimney.
 At a roof valley.
 Common
flashing materials:
 Aluminum
 Copper
 Galvanized
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sheet steel.
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Flashing
 The
flashing is placed under the
roofing material a sufficient distance
to prevent the penetration of water.
 Roof valleys
 flash
with metal or two thicknesses or roll
type roofing.
 Asphalt
roofing mastic is used under
the shingles adjacent to the valley to
aid in waterproofing this area.
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Flashing
A
small metal edging is normally used
at the gable and eave line to act as a
drip edge
 Its purpose is to prevent water from
entering behind the shingles and to
protect the fascia boards.
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Gutters and Downspouts
 Gutters
collect the water and prevent
it from running directly off the eaves
and splattering the house.
 Downspouts are the pipe to channel
the roof water to the ground.
 Copper, vinyl, aluminum and galvanized
sheet steel are common materials.
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Roof Sheathing and Roofing
 Sheathing
is placed over rafters
 Supports the roofing material
 May be
 planks
 individual
boards
 plywood
 Plywood
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is most common.
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Roof Sheathing and Roofing
 Sheathing
thickness varies with the
spacing of the rafters and supporting
beams.
 Sheets are laid with grain
perpendicular to the rafter
 Joints are staggered
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Roofing
 Common
materials:
 asphalt
singles
 wood shingles
 tile
 slate
 roll roofing
 copper, aluminum and galvanized sheet
steel
 layers of felt and tar
 rubber membrane roofing
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Roofing
 Factors
affecting choice of material:
 cost
 local
codes
 roof pitch
 design
 individual preference
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