Transcript Flat Roofs & Introduction to Gable Roofs

Flat Roofs & Introduction to
Gable Roofs
Ref:
M. S. Martin / March 2004 / Reviewed &
Combined March 2007
Review Ceiling Frame Calc’s
Spec for example following
copy on blank paper
• Joists 150 x 38 sawn Oregon @ 600mm
centres, max. span 3.6m
• Trimmers 100 x 38 sawn Oregon @ 600mm
centres
• Hanging beam 240 x 45 hardwood, max. span of
5.4m
• On sketch following, draw in joists and hanging
beam to scale and indicate trimmer positions
also.
Sketch picture below
allow 100mm for wall frames (not shown)
Internal room
sizes shown
4.100
3.300
5.800
8.350
Formula's
No. of joists = width of room - 1
spacing
Length of hanging beam = length of room +
2 x wall thickness
Trimmers = length of wall – 1
spacing
Joists
8.350 – 1
.600
= 13.916 – 1
= say 14 – 1
= 13
Order 13 @ 6.0m
Hanging beam
4.150 + .200 = 4.350
Order 1 @ 4.5m
Trimmers
5.800 – 1
= 9.666 – 1
.600
Say 10 – 1 = 9
Trimmers at two ends plus internal wall
9 x 3 = 27,
27 x .600 = 16.2 lin/m
Order 2 @ 6.0m, 1 @ 4.2m
Flat roof construction
• Section 8 – from P306
• Flat roofs combine the roof and
ceiling frame in one
• Generally described as one with a
pitch less than 10 deg.
• To allow for extra loads the rafters /
ceiling jiosts are increased in
sectional size
Ceiling types
• Ceilings can be either lined on the rake
or rafters laid flat and use graded
battens to give fall to roof
Variations to ceilings
Skillion roofs - P308
• Single sloping roofs
• Walls at either end of roof at different
heights
• Internal walls parralell to end walls will
be at different heights
• Walls at 90 deg. to end walls will taper
in height under the sloping rafters /
joists
Two ceiling designs
Simplest method of marking
• With skillion roofs the simplest thing is
to scribe the rafter over the supporting
plates
Lean – to – Roof construction
P309
• This type is constructed against an
existing wall or other roof
• Simple extensions
• Carports
• Verandahs
• Awnings
Rafter / Ceiling variations
• Common on carports, open exposed
rafters
Other types
Calculation of skillion rafters
(eg) calculate the quantity of 125 x 50
rafters required for the roof 4.2m
long and 2.7m wide, rafter spacings
600mm centres, the difference in
height of plates is 900mm
Length of rafter = √ (run of rafter)² + (rise)²
No. of rafters = (length of building) + 1
spacings
Answer – length of rafter
√ 2.7² + .900²
= √ 7.29 + .81
= √ 8.1
= 2.846 lin/m
900
2.700
Order 3.0m lengths
4.200
Number of rafters
(4.200) + 1
.600
=7+1
=8
Order 8 @ 3.0m rafters
Gable Roofs and
Components
Name the components (handout)
Answers
Ridge
Roof Tiles
Rafter
Tile Battens
Collar Tie
Hanging
Beam
Anti-ponding
Board
Purlin
Purlin
Strut
Top Plate
Strut
Top Plate
Block
Strutting
Beam
Soffit Bearer
Soffit Lining
Stud
Internal Wall
Ceiling
Joist
Ceiling Lining
Cornice
Fascia
Gable roof definitions
• Roof span: Is the horizontal distance
measured overall the wall plates
Span
Plate
Stud
Section A-A
Rise: Is the vertical height measured
from the x – y line to where the upper
edge of the rafters would intersect
Rise
Continue on roofs
Ridge
Rafter
Strut
Purlin
Ceiling joist
Hanging beam
Set Out for Common Rafter
• Plumb cut is at the
top of the triangle
• We reduce the centre
line length of rafter by
½ the thickness of the
ridge to get our true
length
• Birdsmouth has a
maximum cut out of
one third the rafter
depth
• Level bevel is shown
here at base of
triangle
• Back of birdsmouth, is
outside of frame and
if you take a vertical
line up, this is where it
meets the x – y line
X-Y
LINE
Set Out on Steel Square
• Calculate rise per m. run
• Transfer to steel square
Reduced
rise
Reduced run
Set out pattern rafter
• By either stepping out with square, or by
calculation