Roof Framing – Design Criteria and Process
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HOUSE 1 – RAFTERS AND CEILING JOISTS
Box 1
Building Analysis- Assessing roof type and design criteria. Section 10 -3604
Review building noting elements in structure such as rafters, purlins, ridge beams, ridge boards and record the
following design factors by highlighting those which apply.
ROOF TYPE - :
Couple Close Roof
ROOF WEIGHT based on cladding type - :
WIND ZONE - :
TRUSS CENTRES - :
Low
Medium
480
600
Skillion Roof
Light Roof
High
Heavy Roof
Very High
900
Extra High
1200
Box 2
Determine Rafter Spans: Table 10.1(a) NZS 3604 2011
Analyse the roof structure from the section drawing to verify the RAFTER SPAN = __________m
Important to note that Table 10.1 gives rafter spans for EXTRA HIGH wind zones which then have to be factored
to suit the wind zone of the building under consideration
Select a rafter thickness (options are 45,70,or 90mm – normally 45mm if covered by ceiling) = _______mm
Check the columns related to rafter spacings selecting that which matches the spacings determined in Box 1.
RAFTER SPACINGS = _____________mm
Adjust the span value above to suit the building wind zone from Box 1.
FACTORED SPAN = Actual RAFTER SPAN x Wind zone factor in Table 10.1 = ________ m
Go down the column under relevant rafter spacing until a value exceeding FACTORED SPAN is reached and
record the RAFTER SIZE = ________ x ________ SG___ rafters @ ______ crs.
Fixing Type = _______________________________________________
Alt. Fix Capacity = ________ kN
Design Ridge Beam: Table 10.2 NZS 3604
Box 3
Check the drawings or criteria to verify the span of the proposed ridge beam.
RIDGE BEAM SPAN =_____________m
To use Table 10.2 we must also determine the LOADED DIMENSION of the ridge beam.
Refer to NZS 3604 Figures 1.3 E and 1.3 I for guidance and also the BRANZ guide to loaded dimensions which
is available on the Construction Systems 1 Moodle website.
Ridge Beam LOADED DIMENSION = _____m.
ROOF TYPE = ________________ RIDGE BEAM WIDTH (normally 90mm used) = _______ mm
Apply the roof type, beam width, loaded dimension and span to Table 10.2 find a ridge beam size.
RIDGE BEAM = ___________________ SG ____ ridge beam
Note fixing type also. FIXING TYPE = ____________________________________ or Alt. Fixing _______ kN
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Roof Framing – Design Criteria and Process
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Box 4
Design Roof Purlins: Table 10.10
Are you installing sarking (plywood)? No / Yes (skip this section – no purlins used)
Are you installing roof tiles (concrete or pressed metal) No / Yes (skip this section – tile battens sized)
Verify the span of the purlins which equals the RAFTER SPACING (Box 2)
PURLIN SPAN = ____________mm max.
Use this figure and check the column second from left in Table 10.10 to find a max span value which
corresponds to the PURLIN SPAN. There are 3 rows that will comply.
Read across this row to the right until it intersects the WIND ZONE (Box 1). Select the purlin spacing (this
spacing has to be checked against the recommendations of the manufacturers of the Roof Cladding material. eg.
For Corrugated Steel max spacings are normally up to 900mm).
PURLIN SPACING = ________mm max.
PURLIN SIZE = _________ x _________SG___ purlins on edge / flat (normally flat) @_________ crs.
Note also the fixing type and description and record a summary of your findings.
FIXING TYPE = __________________________________________
Alt. Fix. Capacity= ______________
Box 5
Design Tile battens: Table 10.12
Are you installing Roof Tiles (concrete / asphalt / pressed metal)? Yes / No (skip this section – use purlins)
Roof claddings are classed as LIGHT or HEAVY so confirm which category applies to your roof
and the proposed roof cladding type. ( Eg Concrete tiles would be classed as Heavy.)
ROOF CLADDING = ______________
WIND ZONE = ______________
Using Table 10.12, go to the relevant section on the left for either LIGHT or HEAVY roof, and select the max.
batten span (Tile batten span equal the rafter or truss spacings – see roof criteria).
BATTEN SPAN =_________ mm
Table 10.12 NZS 3604 allows us to check compliance for common batten sizes.
Read along the row to the right to match the WIND ZONE recorded in Box 1. This gives the maximum spacing of
the Tile batten = _________ mm
This figure has to be checked against the recommendations of the manufacturers of the Roof Cladding material
if any as their figures would normally apply.
TILE BATTEN SIZE = ______ x _____ mm tile battens @ ______ crs.
Note also the fixing type and description.
Fixing Type = ________________________________
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Alt. Fix. Capacity = _________ kN
Roof Framing – Design Criteria and Process
Design Ceiling Joists: Table 10.3 NZS 3604:2013
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Box 6
Depending on the roof type structure proposed, CEILING JOISTS may be necessary so check the building
section and if they are required, size in this box. .
CEILING JOIST SPAN = _________m.
To determine the spacing of the joists we must consider the ceiling battens which they support. Ceiling battens
can span 600, 900, or 1200 crs. max. Typically, spans of 900 or 1200 are commonly used. Check the criteria to
see if the ceiling joist spacings are given, or make a decision.
CEILING JOIST SPACINGS = _________mm
Using Table 10.3, find a span figure which equals or exceeds the CEILING JOIST SPAN.
CEILING JOIST SIZE = _______ x _______mm ceiling joists @ ________ crs.
Check your proposed solution in relation to rafter depths and seating on top plates. If necessary employ ceiling
runners to reduce spans and section sizes.
Design Ceiling Battens: Table 13.1 NZS 3604
Box 7
Ceiling battens are typically used for fixing ceiling linings to, for both sloping and flat ceilings. Use Table 13.1
(Section 13 for Ceilings) to size these battens.
Consider the spacing of the structure that the ceiling battens will be fixed to. This may be rafters, trusses or
ceiling joists. STRUCTURE = _________________________ (This spacing equals the ceiling batten span).
STRUCTURE SPACING (CEILING BATTEN SPAN) = _________mm
Determine the spacing of the ceiling battens. This will depend on what ceiling lining is selected as the ceiling
batten spacing equals the distance the ceiling lining can span. Typically, 10mm or 13mm thick gypsum plaster
sheets are used, with 10mm spanning 400mm, and 13mm spanning 600mm.
CEILING LINING = __________________________
CEILING BATTEN SPACINGS = _____________ mm
From Table 13.1, the size of the ceiling battens can now be determined.
CEILING BATTENS = _______ x _______ ceiling battens @ _______ crs.
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