Download Timber Framing Code. INTRO TO ROOFING and PRELIMINARY

Timber Framing
Code.
INTRO TO ROOFING and
PRELIMINARY
CALCULATIONS
1
Previously.
We looked at the subject in general
 Discussed assessment criteria
 Section 1. Scope & General
 Section 2. Terminology & definitions.
 Section 7. Roof Framing

2
Flowchart
It is recommended that design starts
at the roof and works down to the
foundation.
Although the flowchart on page 17 tells us
to1. Determine wind classification.
2. Consider the bracing and tie-down
details.
 We will leave wind classification to the
structures teachers
 Consider bracing details after roof and
3
wall design.

Revision Quiz
1.
AS 1684 specifies the
requirements for building practices
for what classes of building?
4
2. List 5 limitations on building design
using AS 1684
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4.
Why is it necessary to determine
the wind classification of a site prior
to using AS 1684 to select section
sizes of members?
6
5. A site may be classified as N1, N2,
N3, N4, C1, C2, C3 or C4.
a. What do the letters N and C
indicate?
b. True or false : The higher the
number the greater the wind risk
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6.
What are racking forces and how
are they resisted?
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7.
How are overturning forces
resisted?
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10. The amount of ‘bearing’ of a
member is…….?
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11. What is stress grading and
how is it achieved?
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Let’s start.
Remember- throughout this module
we will consider Coupled roofs
 With single row of underpurlin.
 Without ridge struts

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Roof (and ceiling) Members
Ceiling joists
 Hanging beams
 Counter beams
 Strutting beams
 Combined strutting/hanging beams
 Combined counter/hanging beams
 Underpurlin

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Roof Members cont’d
 Rafters
 Hips
 Ridges
 Valleys
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Calculations
If you look at the supplement tables you
see that you need to determine
 Spacing of members
 Spans- single or continuous
 Ceiling load widths CLW
 Roof area supported
 Roof load widths RLW
 Rafter span
 Rafter overhang
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Calculations

Spacing of members such as ceiling
joists are measured centre to centre or
“in to over"
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Member sizes
Remember: The flow chart dictates that we
first Determine the wind classification
 Consider position and extent of wind
bracing and tie downs
Let us assume1. That wind classification for all our
exercises is N3
2. There is sufficient room for bracing and
tie-downs
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Preliminary calculations

Some calculations are required before
we have sufficient data to use the span
tables
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Preliminary calculations
You MUST have a scientific calculator.
 You only need to be able to do very
basic trigonometry.
 You must be able to use Pythagoras
theorem.
 You need to be able to perform basic
algebra

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Preliminary calculations






What do we mean by the term ‘true length of
rafter’?
We need to be able to calculate the true
length of the rafter so that we can determine
such things asThe span of the common rafter
RLW
Rafter overhangs
Areas supported
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Trigonometry
Comes from the Greek words ‘Trigon’
meaning triangle and ‘metre’ meaning to
measure.
 Trigonometry is based on right angled (900 )
triangles.
 It involves finding an unknown length or
angle, given that we know a length or an
angle or various combination of known and
unknown data.
 We will also use the “Pythagoras” theorem

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Trig ratios
The 3 basic trig ratios are
 Sine
(sin)
 Cosine
(cos)
 Tangent
(tan)

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Trigonometry
The ratios are related to parts of the
right angled triangle
 The ‘Hypotenuse’ is always the longest
side and is opposite the right angle.
 The other two sides are either the
‘opposite’ or the ‘adjacent’ depending
on which angle is being considered.

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Trigonometry
Sin =opposite ÷ hypotenuse
 Cos = adjacent ÷ hypotenuse
 Tan = opposite ÷ adjacent
OR
 S= O÷H
 C= A÷H
 T= O÷A

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Trigonometry
Some students remember this by
forming the words SOH CAH TOA
 Or by remembering



Some Old Hounds
Can’t Always Hide
Their Old Age
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The Pythagoras theorem
The square on the hypotenuse
equals the sum of the squares on the
other two sides.
 Or A2 = b2 + c2

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Roofing calculations.
If we know the roof pitch.
 And the run of the rafter.
 We will use the term RUN of rafter
rather than half span.
 We can use trigonometry and
Pythagoras to find the true length of the
rafter
 And it’s overhang.

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True length of the common Rafter
Centre
of ridge
Outside
edge of
top plate
Rise of
roof
Run of
rafter
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NOTE!
We are not calculating an ordering
length.
 We require the length from ridge to
birdsmouth.
 You may know this as the set out length
 We need to find the Eaves overhang
separately

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Problem
Calculate the true length of the rafter
 Pitch is 270
 Run of rafter is 4000

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Example:
Method 1. (using Tan)

Trade students may be more comfortable
with this method
 Find the Rise per metre of C.Raft
 Find the True length per metre of C.Raft
 Multiply TLPM x Run= True length of rafter.
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Method 1. (using Tan)
= Tan 270
= 0.5095
= 0.510
 T.L. per metre CR
= √ R2 + 12
= √ 0.5102 + 12
= √ 1.260
= 1.122m
 T.L.C.R.
= T.L per metre X run
= 1.122x 4.0
= 4.489m

Rise per metre run
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Method 2. Using Cosine (Cos)
Pitch is 270
 The run is 4.0m


Cos 270

Rafter length
= adjacent ÷ hypotenuse
= run ÷ rafter length
= run ÷ cos 270
= 4.0 ÷ 0.8910
= 4.489m
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Exercises
Calculate the following rafter lengths.
(choose either method)
 Pitch 370 , Run 3.750m
 4.696m
 Pitch 230 , Run 4.670m
 5.073m
 Pitch 19.50 , Run 2.550m
 2.705m

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True length of eaves overhang.
Firstly you must be aware of the
difference between eave width and
eaves overhang.
 For a brick veneer building with an
eaves width of 450mm; the actual width
to the timber frame is 450 + 150mm for
brick and cavity = 600mm

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True length of eaves overhang.
The true length of the eaves overhang
is the measurement ‘on the rake’ from
the ‘x y’ line to the back of the fascia
along the top edge of the rafter.
 It can be calculated the same way you
calculate you calculated the rafter
length

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True length of the common Rafter
Centre
of ridge
Outside
edge of
top plate
Rise of
roof
Run of
rafter
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Student exercises 2.
Calculate the true length of eaves
overhang for each of the following (all
brick veneer)
 Pitch 270, eaves width 450mm
 .673m
 Pitch 370 , eaves width 500mm
 .814m
 Pitch 230 , eaves width 480mm
 .684m

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Span of the common rafter.

The ‘Span of the rafter’ is the actual
distance on the rake between points of
support.
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Span of the common Rafter
Centre of
underpurlin
Outside
edge of
top plate
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Span of the common rafter.
Span of rafter is the true length of the
rafter divided by 2
 That is:- from our previous example
= 4.489 ÷ 2
= 2.245m

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Student exercises 3.

Calculate the span of the common rafter
for the 3 roof pitches from previous
exercise.
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Exercises (calculate span)
Pitch 370 , Run 3.750m
 4.696m / 2 = 2.348m
 Pitch 230 , Run 4.670m
 5.073m / 2 = 2.537m
 Pitch 19.50 , Run 2.550m
 2.705m / 2 = 1.353m

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Fan struts
We can make more economical use of
underpurlin by using fan struts.
 The fan strut does not increase the
allowable span of the underpurlin.
 It enables the points of support (walls or
strutting beams) to be further apart

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Fan struts

1.
2.
3.
To estimate the spread of the fan strut
we make 3 assumptions
The underpurlin is in the centre of the
rafter length.
The plane of the fan strut is fixed
perpendicular to the rafter.
The fans are at 450
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Underpurlin
90 x 35 mm spreader
cleats either side of
struts fixed with M12
through belt
Strut nailed to
underpurlin with
4/75 mm nails
Struts (see Table 7.5)
Equal angles not
less than 45 o
Min. angle 60 o
to horizontal
Each strut 30 mm min.
bearing to top plate
Chock nailed
to plate
Stiffener
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Fan Struts
60O
max.
45O max.
45O max.
EQUAL
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Fan struts (use Tan)
The formula is ½ spread of the fan struts=
 Span of rafter x tan angle of pitch
 For our previous example
 = 2.245 x 0.510 (tan 27 deg.)
 = 1.142m

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Fan struts
Therefore distance apart of strutting
points for a given u/purlin can be
increased by 1.142m using a fan strut at
one end of the underpurlin span.
 Distance apart of strutting points can be
increased by 2.284m using a fan strut at
both ends of the underpurlin span.

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Supplement Tables
 Once
the preliminary
calculations have been done we
can start to use the span tables
in the supplement
 But which supplement????
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Using supplement tables
Firstly you must choose the correct
supplement (see page 3 of the
standard)
 Depends on wind classification, stress
grade and species of timber
 Then choose the applicable table within
the supplement (see list of tables page
3 of the supplement)

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Class Exercise N3 wind classification
 Using MGP 15 seasoned softwood
 Single storey building
 Tile roof
 Roof load width 3.000m
 Rafter spacing 600mm
 Select a lintel size to span 2100m

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Class ExerciseWhich supplement?
6
 Which table?
 18

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Class Exercise





Choose one of these
2/120x45 ?
2/140x35
170x35 ?
Which one is the smallest cross section?
But is this section commercially available?
60
Class ExerciseNotice that in the last exercise the RLW,
Rafter spacing and required span of
lintel were all values included in the
table.
 What if the RLW is 3300 or the rafters
are spaced at 500mm or the lintel needs
to span 2.250m?
 WE need to INTERPOLATE

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Interpolation
Simply put, to interpolate is ‘To
estimate a value between known
values’.
 It is not possible to show every span or
spacing related to member sizes.
 Convenient regular increments are
used.
 Linear interpolation is allowed for
calculation of intermediate values.

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Cross multiplication
Before we start doing interpolation
calcs.
 You need to be conversant with a
mathematical procedure called cross
multiplication.

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Cross multiplication
In an “equation” such as
A = C
B
D
 we can cross multiply so that
AxD=BxC
 Same as the ration A:B::C:D

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Class Exercise
If RLW is now 3.300
 Rafters still at 600
 Required span 2.100


We need to interpolate between two
columns to find the most economical
section size.
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Class Exercise
Run your fingers down the 3000m
column and the 4500m column for 600
spacing.
 We can tell that 2/140x35 will probably
span.

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67
Class Exercise
RLW
3000
RLW
3300
RLW
4500
2300
?
2000
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Class Exercise
RLW
3000
RLW
3300
RLW
4500
2300
2240
2000
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Class exercise
Using Table 18 of supplement 6
 Interpolate to find the maximum
allowable span for a 290x45 lintel
 RLW 5300
 Spacing of rafters 1200mm

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Class Exercise
RLW
4500
RLW
5300
RLW
6000
3400
?
3100
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Class Exercise
RLW
4500
RLW
5300
RLW
6000
3400
3240
3100
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Next week
We will start work on selecting
suitable roofing members from
a given plan and specification
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