Roof Framing

References: NZS 3604: 1999 “Timber Framed Buildings”

Rafter Lengths and Bevels, Woods, Wilson House Building: A Practical Guide, R J Wilson BRANZ House Building Guide Roof Truss Erection Manual by Able Cooke Ltd Mitek product catalogue (http://www.mii.com/newzealand )

Introduction The main framing members of a roof are the rafters, ridges, hips, valleys, underpurlins, struts, collar ties and associated bracing. The functions of roof framing are: (i) To provide support and fixing for the roof covering (ii) To transmit the roof load to the walls and thus to the foundations (iii) To tie and stiffen the wall frame.

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GABLE ROOF

A gable roof is a very popular roof form, being comparatively simple in design and readily adaptable to a wide variety of different roof pitches. The actual part referred to as the gable is the vertical triangular section at the end of the roof, from the top plate to the rafters.

Figure 1. Gable roof

RAFTER LENGTHS AND BEVELS

Rafters for gable or lean-to roofs can be calculated by a number of methods including: 1. Rafter tables, (see Rafter Lengths and Bevels)

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2. Secants from a calculator, (Length = run x secant of the pitch) 3. Pythagoras, a2+ b2 = c2

4. Roofing square 5. Scaled or full size drawing, 6. Pre programmed calculators or computers Most of the methods mentioned give the length of the rafter from the outside of the plate to the centre line of the ridge.

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BASIC ROOF TERMS

Span The overall horizontal distance across the building to be roofed, measured from the outside of the framing on one side to the outside of the framing on the opposite side. (A D) Run Half of the span in a roof of equal pitches. It is measured horizontally from the outside face of the framing to a vertical cutting the intersection of the inclined surface. (AB) Rise The height of the roof measured from the top of the top plate to the apex. (B C) Pitch or Slope These terms express the inclination of a roof from the horizontal. The pitch is expressed in degrees, such as 35° pitch or 37° pitch etc

Figure 2. Roof measurement names

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Roof Bevels.

The angle used to form the plumb cut (bevel) is obtained from the triangle formed by the run and rise. The angle used to form the seat cut (bevel) is also obtained from the same triangle. A seat cut is used on a birdsmouth and overhang cut for soffit lining. The birdsmouth - In order to give a rafter seating on the top plate, a triangular piece is cut out leaving what is called a birdsmouth. The two cuts required to form a birdsmouth are: (i) the plumb cut (vertical) and (ii) the seat cut (horizontal). Note: Always cut one rafter accurately and use this as a pattern to mark the others.

Figure 3. Rafter parts named

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SETTING OUT PLATES AND RIDGES

The ridge board is a horizontal member positioned parallel to the top plates to which the top end of rafters are fixed. Where possible, set out the positions of rafters on the top plate before constructing the wall and erecting the frames. Lay the ridge board alongside the top plate and transfer the top plate rafter set out onto the ridge. Allow for overhang of ridge for the verges if required. Rafter positions can be set out at the maximum spacings or equal spacings (especially for exposed rafters). (refer to NZS 3604, table 10.2 rafter spacings) Where flying rafter construction is used, the last common rafters must be placed one spacing from the end wall to allow for fixing of the outriggers.

Figure 4. Roof frame members

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STRUTTING BEAMS, UNDERPURLINS, STRUTS, COLLAR TIES

Strutting beams (refer to NZS 3604, 10.2.11) Carry roof loads from the underpurlins via struts. Strutting beams must be at least 25mm above the ceiling joists and should have 65 mm bearing at each end. (refer to NZS 3604, fig 10.12 and table 10.7) Purlins Purlins are horizontal framing timbers fixed across the tops of rafters to support the roof covering.(refer to NZS 3604, 10.2.16 and table 10.9) Underpurlins Where rafters exceed their safe span, underpurlins are used to support the rafters. To make installation of struts easier, underpurlins should generally be placed as near as possible to the ridge (refer to NZS 3604,10.2.9, fig 10.10 and fig 10.11) Refer to NZS 3604, table 10.6 for the sizes and spans of underpurlins. Struts (refer to NZS 3604, 10.2.10, fig. 10.10, 10.11 and 10.12) Strut members transmit loads from the rafters or under purlins down to: (i) a strutting beam or (ii) a load bearing partition (refer to NZS 3604, fig 10.10) Struts supporting underpurlins can be placed: (i) In pairs, (refer to NZS 3604, fig 10.12) or (ii) Vertical, (refer to NZS 3604, fig 10.12) or (iii) At right angles to the underpurlin and roof, or (iv) Anywhere between (ii) and (iii) Collar Ties In standard roof construction (couple close roof) steeper than 100 (1 in 6 slope) pairs of rafters must be connected together with either collar ties or cleats. (refer to NZS 3604, 10.2.13.2, fig 10.13) Collar ties must be 150 x 25 mm or 100 x 50 mm fixed to every third pair of rafters or at 1.8m centres whichever is closer. Cleats Use 100 x 25 mm at the same centres as collar ties and tie opposing (pairs) rafters immediately below the ridge. (refer to NZS 3604, 10.2.13.3, fig 10.14)

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ROOF BRACING (Gable roof)

Bracing for roofs can be either: (i) A roof plane diagonal brace - A length of 100 x 25mm or a pair of diagonally opposed steel strips placed at 450 to the ridge and fixed at the ridge and top plate). (refer to NZS 3604, fig. 10.25, 10.5.2.3) and/or (ii) A roof space diagonal brace (one or two pieces of 100 x 50mm fixed from the ridge to ceiling joist level. (refer to fig. 10.26, 10.4., for heavy roofs see fig. 10.24, 10.4.5, 10.5.3) The number of braces required can be found in NZS 3604, 10.5 Note: Heavy roofs (concrete tile) require more bracing than light roofs (metal) and that both roof space and roof plane braces are required. (refer to NZS 3604, fig. 10.2, 10.4.5.)

SET OUT AND CONSTRUCTION OF GABLE ROOFS

A. GABLE END FRAMING (References from NZS 3604) Gable ends may be framed up in a number of ways: (refer to 10.15, 10.2.15, table 10.9) (i) Purlins cantilevered over the end common rafter (refer to fig.10.15) Jack studs are cut between rafter and end top plate. (refer to 10.2.15.2) (ii) Outriggers cantilevered over an end wall raking plate and supporting a flying rafter. Nogs (blocking) are cut between the outriggers over the raking plate to support the purlins. (refer to fig 10.15) Jack studs are cut between the raking plate and the end top plate. (refer to 10.2.15.3) (iii) The gable end boxed out over the end wall giving a horizontal verge soffit. In (i) and (ii) a barge board is fixed to the purlins or purlins and flying rafter, so that the verge lining can fit into the groove in the barge board. Where the gable end is boxed out, the barge board must fit over the gable end sheathing.

PURLINS AND BATTENS

Always place purlins or battens at centres to suit the roof cladding being used. Check manufacturers brochures if in doubt. For long run iron sheet roofing, reduce the spacing at the ridge and the gutter (end spacing) e.g. for corrugated iron sheet roofing use 800 mm crs at the ends, 1.000 crs for the others. Always butt join purlins and battens over a rafter or other solid support. For Purlin sizes, spacings and spans see table 10.9 Minimum purlin fixing requires 2/100 x 3.75mm nails skew nailed. Where purlins are fixed over sarking, extra fastenings may be required. (refer to 10.2.16, 10.2.17, fig 10.19, fig 10.20, table 10.10 nailing schedule for purlins or battens)

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SARKING AND UNDERLAY

Underlay must be provided under all metal and fibre cement roofing materials and is required under other roofing materials where the roof pitch is low. (see manufacturer’s information) Underlay should be breather type building paper laid on netting or other supporting materials. Sarking may be used over the top of the rafters as a ceiling lining (sheet material, tongue and groove timber, etc) or over the top of the purlins as permanent support for the underlay. In both cases the sheet material is often used as a ceiling/roof diaphragm to provide support for the outside walls (refer to 10.5.4(b), fig 10.27) Where diagonal “hit and miss”or sheet sarking is used over the rafters, roof plane diagonal braces are not required.

EAVES AND VERGE FRAMING

Eaves Construction (refer to 10.2.14) (i) Boxed Eaves Eaves bearers (sprockets) ribbon boards (soffit plates) and fascia are used. The bottom end of the rafter may need to be cut, to suit the fascia board used. (ii) Raking Eaves The eaves lining is parallel to the roof slope, placed over or under the rafters. If the lining is over the rafters, fascia board is often not used. (iii) Flush Eaves The fascia board is fixed directly to the wall framing Verge Construction (refer to 10.2.15.2) (i) Cantilevered purlins are used to support the barge board often with packers underneath (refer to 10.2.15.2, fig10.15) Outriggers of the same size as the rafters give support to the verge lining in the same position as the eaves lining (raking eaves) (refer to fig. 10.15). (iii) Gable end is often framed out to the barge board with a horizontal verge lining in line with the eaves lining.

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CEILING JOIST

A horizontal framing timber which supports ceiling linings. Refer to NSZ 3604 table 10.4 regarding ceiling joist sizes and maximum spans. Ceiling joists must: • Have a minimum landing on their support of 32 mm. • Not support roof framing members other than ceiling runners. • Join over supports and shall either: be lapped not less than 300 mm, or butted and flitched with timber of the same dimension as the joist and extending not less than 225 mm on each side of the joint. (refer to NZS 3604, fig 10.8)

CEILING RUNNER

A timber beam supporting ceiling joists that exceed the permissible span in standard roof construction. Runners are used in roof truss construction to provide lateral support to the bottom chord and support the intermediate ceiling joists. Ceiling runner are laid in straight lines and on edge. Refer to NZS 3604 table 10.5 regarding ceiling runner sizes and maximum spans. Ceiling runners must have a minimum landing of 65 mm on a packer, which is directly supported by the top plate of a loading bearing wall, provided that either: • The ceiling runner lands directly over a stud, or • The packer spans between the studs on each side of the ceiling runner. Ends of ceiling runners may be chamfered, but the depth of the ceiling runner at its support shall remain at least 50%. Ceiling runners are fixed to the ceiling joists by: • Purpose made steel hangers which alternate on opposite sides of the ceiling runner, or • 50 x 50 mm timber hangers. (refer to NZS 3604, fig 10.9)

TRIMMING OF RAFTERS AND OPENINGS IN CEILINGS

(refer to NZS 3604 section 13 – Ceilings) Where possible, position rafters to avoid any cutting i.e. planning for rafters on each side of a chimney is better than cutting rafters later. (ii) Trusses must not in any circumstances be cut. A truss must be placed on each side of the opening and ordinary rafters used in between if required.

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(iii) Ensure the trimmer/s and the rafters supporting the trimmer/s are strong enough to support the increased loading. It may be necessary to double the rafters, used as the trimming joist ,i.e. each side of a dormer window. (iv) Access to the ceiling space shall be provided through a clear opening not less 600 mm x 500 mm, giving easy unobstructed access of at least 600 mm in height between the top of the ceiling joists and other roof members. (refer to 13.3.1) (v) Openings in ceilings shall be bounded by trimmers and trimming joists. (refer to fig 13.2, fig 13.3) Trimmers shall be the same depth as the curtailed ceiling joists. (refer to 13.3.3, 13.3.4)

CEILING BRACES

Any ceiling area exceeding the maximum spacing lines will require: • A ceiling diaphragm, or (refer to NZS 3604 section 13.5, fig 13.4) Ceiling lining materials for diaphragm refer to NZS 3604, 13.5.2) • A dragon tie Consist of a continuous length of 100 x 50 mm timber, or a steel angle of equivalent compression strength.Fixed at 450 to both external walls and not more than 2.5m from the corner. Fixed directly to the top plates or fixed to blocking pieces which are not deeper than 100 mm and are at least 75 mm wide. At the external wall being considered, the dragon ties shall also be fixed to a joist, truss or rafter located within 100 mm of the top plate.

CEILING BATTEN

A horizontal timber or metal member fixed below ceiling joists or truss bottomchords to which the ceiling linings or ceiling tiles are fixed.Refer to NZS 3604 table 13.1 Ceiling battens

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3. HIP AND VALLEY ROOF CONSTRUCTION

The hip roof is one of the most popular roof forms in use in residential house building. The roof is pitched on all sides of the building and if the pitch is the same on all sides, it is termed equal pitched.

Setting out top plates

Mark the positions of all rafters on the top plate before wall frame is erected. Mark the rafter positions on to the ridge board at the same time. JR CR CR CR CR Top plate run JR TR centre line ridge board run JR CR CR CR CR JR CR CR CR CR Top plate run

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Setting out and cutting common rafters (CR) (50mm thick)

(i) Determine rafter run (half the span) (ii) Calculate the length of the common rafter (refer to rafter tables or secants) (iii) Set out a pattern common rafter using bevels from rafter tables, roofing square or other means. Common rafter - deduct 13mm (half thickness of ridge ( 25mm thick)

(square off the plumb cut) Thrust rafter - deduct 25mm (half thickness of common rafter square off the plumb cut. (iv) Cut out pattern common rafter. Take special care when cutting the birdsmouth. (v) Use the pattern common rafter to set out all other rafters. deduct half thickness of ridge PATTERN COMMON RAFTER- square off plumb cut calculated rafter length overhang length D 3/4 D or65 mm min. x seat cut 32 mm minimum bearing overhang 100 mm maximum run rafter run

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Setting out and cutting hip rafter (HR) (25mm thick) (i) Calculate the length of the hip rafter Deduct 36 mm (square off plumb cut) (refer to Hip roof intersection pp. 20) (ii) Rafters may be lengthened by butting and flitching each side of the joint with timber 25 mm thick and 450 mm long minimum. (iii) Hip rafters that project 600mm or more beyond the corner of the building must be either 50 mm thick or be flitched on both sides at the birdsmouth with 25 mm thick and 900 mm long timber. (iv) Cut out pattern hip rafter. (v) Use the pattern hip rafter to set out the other hip rafters. Note An on site measure can be used to check accuracy of set out and squareness of building PATTERN HIP RAFTER calculated hip rafter length edge cut overhang length 50 x 75 purlin 36 mm plumb cut measurement ? x + 50 (50 mm approx. deeper than C/R) overhang run (diagonal of C/R hip run (diagonal of C/R run) over hang)

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Setting out and cutting valley rafters (VR) (50mm thick)

(i) Although the valley rafter has the same calculated length on the centre line as the hip rafter, it has a different birdsmouth and deduction. Deduct 18mm (half diagonal thickness of the ridge) (ii) Use cut valley rafter as a pattern for others if required. VALLEY RAFTER calculated valley rafter length ( same as hip rafter ) overhang length deduction 18 mm 50 mm thick to centre of edge cut valley rafter x add on 25 mm overhang run over hang valley run

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Setting out and cutting jack rafters (JR)

(i) Calculate lengths individually (run x secant pitch) or use shortening (spacing of rafters x secant pitch). (ii) Plumb and seat cuts are the same as the common rafter (iii) Deduct 18mm (half diagonal of hip rafter) or take the calculated length to the long point (not exactly right but close enough!) JACK RAFTERS jack rafter edge cut calculated length 2 calculated length 1 Jack rafters can be set out on the pattern common rafter Note: Jack rafters must be cut in pairs. Valley jack and cripple jack rafters are set out in a similar way.

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ROOF INTERSECTIONS common rafter hip rafter (25 mm) 50 mm thick major ridge (25 mm) thrust rafter (50 mm) centre line 36 mm broken hip rafter (25 mm) hip shortening valley jack rafter or common rafter (50 mm) common rafter (50 mm) valley rafter (50 mm) 18 mm VR shortening minor ridge board (25 mm) 7 Setting out and cutting purlins (i) Fixed approx. 100 mm away from the ridge and all hip rafters, alongside the valley boards, at the fascia board and at regular spacing to suit the roofing being used. (ii) Set out, cut, and fix the bottom purlin first which must cut tight to the hip rafters. (iii) Set out cut and fix hip purlins, then cut and fix all intermediate purlins. (See sketch in “Rafter Lengths and Bevels”). 8 Scotch valley For additions to an existing roof, a scotch valley using lay boards may be used. (See sketch in “Rafter Lengths and Bevels”.) 9 Underpurlins

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Where the span of the rafter exceeds the maximum allowed for that size either: (i) Increase the depth of the rafters until they can span from the top plate to the ridge unsupported. (refer to 10.2.9, table 10.6) or (ii) Set out, cut, and fix underpurlins as required with support from isolated struts or paired struts. (refer to 10.2.10, fig 10.10)

SELECTION AND FIXING OF ROOF BRACING

1 (i) Determine the number and type of braces required from table 10.1 (ii) Cut and fix braces as shown in fig.10.22, 10.23, 10.24, 10.25, and 10.26. (refer to 10.5.2) 2 (i) Where underpurlins are used, cut and fix collar ties as shown in fig. 10.13 (ii) Where underpurlins are not used, cut and fix cleats as shown in fig. 10.14

ROOFING SQUARE

Before the roofing square can be used to set out rafters, the rise per metre of run (rise/m.run) must be found. 1 Pitch of the roof can be expressed as: (i) ratio of rise to span, (rise ÷ span) (See “Stanley” metric square booklet). (ii) slope of roof in degrees (iii) rise (mm) to metre of run(rise ÷ run). Example (i) Roof rise 2.400, roof span 8.000 pitch = 2.4 ÷ 8 = 0.3 pitch (ii) 310 slope = 600mm rise per metre run (tangent) (iii) 600mm rise/metre run (equals 0.3 pitch)

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Setting out rafters using the steel square

(1) Common rafter Total length of CR equals run times length of common rafter plumb 600 mm rise/metre run per metre run. cut (120 mm) Common rafter length (see roofing square table) (Rise) seat cut C.R. set point (200) (Run) (2) Hip & valley rafters Total length of HR (&VR) equals run times length of hip or valley per metre 600 mm rise/metre run (120 mm) run (see roofing square). plumb Hip rafter length cut (Rise) seat cut H.R. set point (283) (Run) (3) Side (edge cut) cuts (i) Jack rafters edge cut 233 from table jack rafter (side cut of jack use) (Rafter length) (Run) 200 mm (ii) Hip or valley rafters edge cut 217 from table (side cut of hip or hip rafter valleys use) (Hip length) (Run) 200 mm

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ROOF TRUSSES

Roof trusses shall be specially designed in accordance with NZS 3603 and shall be fabricated in controlled factory conditions, however the Engineer may approve the assembling of trusses on site. Drawing and Specifications shall be provided for all trusses. These shall contain all information necessary to fabricate and erect the truss. (refer to NZS 3604, section 10.3)

PRINCIPLES OF TRUSS DESIGN

All truss construction is based on a system of triangles which when connected together can be supported at two points only, refer to Terms illustration on page 23. Advantages of using trusses 1 No central supports are needed. All partitions can be non load bearing. 2 Roof structure is stronger than standard rafter construction. 3 Long spans are possible. 4 Roof structure can be erected very quickly. 5 Trusses can be spaced up to 1200 mm centres for light weight roofing and 900 mm centres for heavy weight roofing, or can be spaced further apart if required. 6 Trusses can be designed and built to cantilever over porches etc.

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Nail plates Plate sizes are expressed in centimetres . e.g. A 3 x 10 connector is 30 mm wide by l00 mm long. (ii) Short plate teeths are all high density (iii) Longer plates incorporate medium and low density teeth near the centre.

FIXING ROOF TRUSSES

Set out (i) Check truss set out drawing supplied by the fabricator before doing any setting out. Mark out the truss locations on top of the top plate referring to the truss layout plan (roof truss layout plan is provided by the truss fabricator)for reference. (iii) Trusses are always designed to be spaced at regular intervals. For light roofs, the truss spacing shall not exceed 1200 mm and for heavy roof 900 mm. Girder and truncated girder trusses should be set out first as other truss positions are usually measured from them. Dual purpose girders i.e. truncated girder/girders, need special attention as they may appear similar in profile to others. It is important they are located accurately. Standard trusses should then be set out, taking care not to exceed the design spacing.

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Handling of trusses

(i) Trusses must always be lifted in a manner that minimises lateral bending. It is usually recommended to keep them strapped in bundles until they are erected as this reduces the likelihood of damage. Trusses may be lifted either by crane, hiab hoist or manually, depending on the span and wall height. Trusses should always be kept vertically when handled individually. (iii) Trusses lifted by crane requires sling or spreader bars. Note: Never hoist a truss into position on its flat.

SLING SPREADER BAR

(iv) When erected by hand, slide the truss over the side walls on skids, spread at 3 m intervals. Rotate into position and supported at apex and panel points or web points to ensure they do not distort or sag between supports. (v) Provided internal partitions are in place, trusses can be laid on their flat temporarily before standing into correct position.

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Erection of trusses

Take care when standing up steep trusses. Where possible tie ropes to the panel points and hold or tie temporarily until temporary bracing can be fixed. (ii) Set trusses to a line, or mark the top plate position on each truss. (Top plate must be straightened first!) (iii) Have temporary ties ready to fix truss at its correct position at the apex and at least the half way point on each side. (Preferably at the panel points). Purlins or roofing battens will hold trusses in permanent position. (iv) Put one or two ties onto the bottom chord of each truss. Ceiling battens generally supply permanent tying of bottom chords. (refer to NZS 3604,