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CARPENTRY - HOUSING

©TAFE NSW Construction and Transport Division 47

Method used to calculate drop-off To calculate the amount of drop-off, it will be necessary to use the mathematical process known as trigonometry, which deals with the measurement of sides and angles of triangles, i.e. sine, cosine and tangent. The method required to calculate drop-off is tangent or simply Tan, when: • Tan = opposite side (Note: Use the Tan button on a scientific calculator) adjacent side • θ = the known angle of the triangle or the pitch of the roof, e.g. 30°, 22.5°, etc. Example 1: Find the drop-off measurement for a brick veneer cottage with a roof pitched at 30° and a required eaves width of 400 mm. Formula = Tan θ = opposite side adjacent side

To find the opposite side, transpose the formula by cross multiplying to allow the unknown measurement, i.e. opposite side, to be on its own:

Fig. 59 Detail of eaves

∴The total drop-off as measured from the top of the wall plate to top of the brickwork will be: 317 + 20 (which is the depth of the birdsmouth) = 337 mm

Tan 30° = opposite side 400 + 110 + 40

Tan 30° = opposite side

550

∴ Tan 30° 1

x opposite side 0.550

= (opposite side x 1) = ( Tan 30° x 550) To find Tan 30°, insert 30 into the calculator and then press the Tan button. The answer will equal 0.577350269. Reduce this to 3 decimal places and use it for the remainder of the calculation, i.e. 0.577

∴ opposite side = 0.577 x 0.550 0.317 m or 317 mm

400 110 40

Dro

p-of

f

Depth of birdsmouth

20

30°θ

BASIC ROOF and CEILING FRAMING


EAVES FINISHES There are a number of ways to frame and finish eaves. Types include simple boxed level eaves, lined on-the-rake eaves, eaves lined on top of rafters or combinations of these. The level framing members, running between the wall and fascia, are referred to as soffit bearers or eaves sprockets. They are spaced at 450 to 600 mm centres to provide fixing for the eaves soffit lining, which may be timber boarding or more commonly 4.5 mm thick fibre cement sheeting joined with a PVC jointer.

Fig. 61 Framing for raking gable end meeting boxed eaves

Fig. 62 Raked side eaves

Fascia

Lining to eaves soffit

Soffit bearers

Barge board Fibrous cement strip

Trimmer Trimmer

Rafter Tile batten

Anti-ponding strip

Fascia

Soffit bearer

Eaves soffit

CARPENTRY - HOUSING


Eaves soffit clearance Where timber eaves framing passes over face brick walls, as in brick veneer construction, an allowance for frame shrinkage must be provided. If no clearance is allowed, the top course/s of brickwork may be cracked or even dislodged when the timber shrinks causing the eaves framing to drop, which will allow the roof load to bear directly onto the brickwork. When unseasoned, or partly seasoned, timber is used it will continue to dry out causing a reduction in its section size. Timber shrinks in width, thickness and to a much lesser extent, its length. Unseasoned timber with a width of more than 175 mm will shrink excessively, i.e. up to 10 mm for every additional 25 mm of width in some cases. This shrinkage usually occurs in the width of bearers, joists, lintels and in the thickness of the top and bottom plates. (Stud length is relatively unaffected) Therefore, a clearance of 12 mm minimum is to be allowed between the underside of the eaves soffit bearer and the top of the brickwork. Note: No clearance is required for cavity-brick construction or where timber frame construction is used on its own or when the timber framing is fully seasoned. Shrinkage does not occur in steel framing or when manufactured products such as structural particleboard, ‘LVL’ or ‘Hyspan’ are used.

Fig. 63 Eaves soffit clearance

Fig. 64 Effect of frame shrinkage on brickwork

12 mm min to allow for shrinkage of framing Brick veneer

Material shrinkage causes frame to drop

End of soffit bearer drops with frame

Result of no clearance. Top course tilts under load



ERECTION PROCEDURE for the GABLE ROOF

After the ceiling frame is totally complete and the rafters have been set out and cut, follow the steps below:

Fig. 65 Completed ceiling frame

Fig. 66 Setting out rafter positions on the ridge board

STEP 1 Measure the length of the ridge and cut to length or join lengths together, as previously shown. Note: Allow extra length for gable overhang as required. Lay the ridge on flat and place the top edge flush with the ends of the ceiling joists. Using a square, transfer the rafter positions onto the edge of the ridge and square them down the face of one side.

End of deep hanger strapped with loop iron and supported on a ceiling trimmer

Ceiling dogs on alternate sites of hanger

Smaller sectioned hangers over short spans

End of hanger bolted to gable stud to prevent twisting

Ceiling joists

Rafter positions marked onto ridge

CARPENTRY - HOUSING


Fig. 67 Fixing the first pairs of rafters to the ridge

Fig. 68 Fix off the remaining rafters

STEP 2 Erect a pair of rafters for each end of the roof. Nail the feet of each pair to the plate with the plumb cut ends butted together. Place a temporary nail at the top of each pair of rafters for stability. Lift the ridge up between the rafters until it is flush with the top edge, or to a marked straight line, then nail through from one side into the end of one rafter with 2/ 75 mm nails. Align the opposing rafter and skew nail from the opposite side using 2/ 75 mm nails.

STEP 3 Plumb one end and attach a temporary brace, to prevent racking, and then attach a string line along the top of the ridge to ensure it remains straight while the remaining rafters are nailed into position. Note: Provided all rafters are exactly the same lengths and the side wall plates are straight, then the ridge should automatically finish straight.

Temporary nail

Ridge

Push ridge up between rafters flush with tops

Skew nails

Ridge

This rafter is nailed first, from the other side

Ridge String line

Checking block Marked rafter positions

Ceiling joists Block

Rafter

Studs

Temporary brace

Top plate

Block



Fig. 69 Complete the assembly of the structural frame

Fig. 70 Types of permanent wind bracing

STEP 4 Set out and fix purlins into position as required, then cut and fix the struts for the whole roof. Set out, cut and fix collar ties on top of purlins, bolting or nailing them as required.

STEP 5 Fit permanent wind bracing. This may be in the form of opposing timber braces onto an internal wall or metal speed bracing over the surface of the rafters.

Rafter

Collar tie scarfed around rafter

Purlin

Inclined strut

Inclined opposing wind braces

Speed bracing

CARPENTRY - HOUSING


Fig. 71 Boxed gable studding and framing complete

Fig. 72 Gable end studding and framing complete for raked eaves

STEP 6 Cut and fix gable studding into place to suit wall stud spacings and sheet cladding joins. A pair of rafters and a bottom chord or tie forms the boxed gable frame. The studs are cut around the rafters and tie. Raked eaves on a gable end have the addition of a raking plate on either side, fixed under the line of the rafters. Outriggers are supported on these raking plates with short trimming rafters cut between them, for rafter continuity.

Gable studs checked out around the rafter and tie

Bottom chord or tie

Eaves trimmers

Gable studs placed at centres equal to wall framing or sheet cladding joins

Rafter trimmers cut between outriggers

Outriggers

Raking plate for fixing of cladding

Gable studs cut onto top plate

Gable studs spaced at centres equal to wall framing or sheet cladding joins



Fig. 73 Marking the width of the eaves

Fig. 74 Marking the ends of rafters to a line

STEP 7 To determine the eaves width, it will be necessary to calculate the drop-off position, unless these dimensions are given. Refer to previous details. Set out and mark the line of the overhang by measuring horizontally from the outside of the wall frame. Plumb a line down the face of the rafter ready to cut.

STEP 8 Plumb a line down, the same distance out, at the other end of the roof. Drive in two temporary nails on the top edge of the end rafters and attach a string line. Work along the rafters marking plumb down from the string line with a spirit level.

Mark the face of the rafter

550

String line plumbed down

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Fig. 75 Fitting the fascia ready to receive gutters

Fig. 76 Fitting the barge to enclose the end of the gutter

STEP 9 After the ends of the rafters have been cut plumb to a straight line and the eaves soffit bearers fitted, cut and fit the timber or metal fascia ready to receive the gutter. The top of the groove should be in-line with the top side of the eaves soffit sheet. The top of the fascia will project above the top edge of the rafters to provide a bellcast. The bellcast ensures that the first course of tiles will have the same pitch as the remainder of the roof and the distance above the rafters should be equal to the thickness of a tile batten plus the thickness of one tile.

STEP 10 Once the fascias are fitted and the gable ends are clad, cut and fix timber barges or Colorbond metal barge soakers. The bottom edge of the timber barge is fixed flush with the bottom edge of the fascia and run past the fascia to enclose the end of the gutter. They will require a timber bellcast infill piece to be attached to the top edge.

Soffit bearers fitted from fascia to wall frame

Fascia double skew nailed to prevent it from being easily pulled off

Top of groove flush with

Barge attached to gable rafter

End of eaves boxed

Timber fillet placed on top to form a bellcast

Quad gutter fitted to fascia



ROOF PITCH All pitched roofs are based on the same simple geometric shape, the right-angled triangle. In the case of the gable roof this shape is found on one side formed by the rise, run or half span and centre line length of common rafter top edge. The right-angled triangle shape contains one 90° angle and two complimentary angles, which make up another 90°. Therefore, the angles within a right-angled triangle will equal 180°.

Fig. 77 Proportions of the right-angled triangle in a gable roof PITCH The pitch or slope of the roof surface may be calculated using one of four common methods:

Pitch is specified as a ratio, i.e. 1:3, or Pitch is measured at the foot of the rafter in as the Rise per metre run, i.e. 333:1000 degrees.

The rise is given as an actual measurement,

Specified as a fraction of the span, e.g. Rise = 2700 mm. e.g. Pitch = 1/3

Fig. 80 Roof pitch or slope given as a fixed rise Fig. 81 Roof pitch or slope given as fractional pitch

Fig. 78 Roof pitch or slope given as a ratio Fig. 79 Roof pitch or slope given in degrees

Same size, shape and proportions on both sides of the roof

HE

IGH

T or

Ris

e of

roof

90°

HYPOTENUSE

or Centre line rafter length

Complimentary angles

BASE or Run of rafter

Half span = 3 units

Rise 1 Unit

30°

Rise

Half Span

Span Span = 3 units

Rise 1 Unit

60°

30°

CARPENTRY - HOUSING


CALCULATING ROOF PITCH The two main methods used to determine roof pitch are: 1. Degrees; and 2. Pitch ratio. Degrees This method involves the same process as for calculating the drop-off, i.e. • Tan = opposite side (Note: Use the Tan button on a scientific calculator) adjacent side • θ = the known angle of the triangle or the pitch of the roof, e.g. 30°, 22.5°, etc. The calculated proportions are then used to determine the length of the rafter when used with the Pythagorean formula, i.e. a² = b² + c² Example 1: Find the length of the hypotenuse, or rafter centre line length, when the pitch of the roof is 30° and the plan length, half-span or run of the rafter is 2700 mm.

∴ The length of the opposite side or the rise of the of the triangle = 1.558m

STEP 1 Formula = Tan θ = opposite side adjacent side

= Tan 30°

= opposite side 2700

STEP 2 To find the opposite side, transpose the formula by cross multiplying to allow the unknown measurement, i.e. opposite side, to be on its own:

∴ Tan 30° 1

x opposite side 2700

= (opposite side x 1) = ( Tan 30° x 2700)

To find Tan 30°, insert 30 into the calculator and then press the Tan butThe answer will equal 0.577350269. Reduce this to 3 decimal places andit for the remainder of the calculation, i.e. 0.577

= opposite side = 0.577 x 2.700

= 1.558m

To find Tan 30°, insert 30 into the calculator and then press the Tan button. The answer will equal 0.577350269. Reduce this to 3 decimal places, i.e. 0.577, and use it for the remainder of the calculation.



∴ The centre line length of the rafter or the hypotenuse of the of the triangle = 3.117m

Note: The centre line length of the rafter is taken from the centre of the ridge to plumb over the birdsmouth. The length of the eaves overhang may be calculated in the same way and then added to 3.117 or the original run or plan length of the rafter may be increased to include the eaves width, which will allow the total cutting length to be calculated in one go. Pitch ratio This method involves the use of a ratio, i.e. rise : metre run, to provide the proportions of the triangle or roof. The ratio is equal to the pitch of the roof, e.g. 30° = 1: 1.732, which means for every 1.0m of rise there will be 1.732m of run or plan length. This ratio, of 1 : 1.732, is converted to a rise in millimetres to a run of 1 metre, as follows: This means that for every 1.0m of run there will be 577 mm of rise. This is the same as a roof with a 30º pitch, as Tan 30º = 577 mm. Fig. 82 Proportions of the ratio shown on the triangle

STEP 3 Use the Pythagorean formula to calculate the centre line length or hypotenuse of the triangle, i.e. a² = b² + c², when: • a² = hypotenuse; • b² = run or plan length; and • c² = rise.

= a² = 2.700² + 1.558²

= a² = 7.29 + 2.427

= a² = 9.717

STEP 4 To find ‘a’ on its own, it will be necessary to find the square root (√ ) of 9.717. Therefore, enter 9.717 on the calculator and then press the √ button.

= √ 9.717

= 3.117m

= 1.000 1.732

= 0.577m or 577 mm.

30°

1.000 (1.732) (run, plan length of rafter or half span)

0.57

7 (1

.000

) (R

ise)

CARPENTRY - HOUSING


The benefit of using a pitch ratio, rather than working from degrees, is that a true length of rafter per metre run of rafter can be established and used as a constant for calculating the length of any rafter, with any half span or run, having the same pitch ratio: Example 2: Find the length of the hypotenuse, or rafter length, when the pitch of the roof is 1:1.732 and the plan length, half-span or run of the rafter is 2700 mm.

Therefore, for every 1.0m of run or half span the hypotenuse or rafter length will be 1.155m

Note: The answer should be the same as for the method using degrees, within 1 or 2 mm depending on how the numbers were rounded off to 3 decimal places.

∴ Total cutting length of all the rafters will be 3.567m

STEP 1 Rise per metre run = 1.000 1.732

= 0.577m

STEP 2 Length of hypotenuse

= a² = b² + c²

a² = 1.000² + 0.577²

a² = 1.0 + 0.333

Therefore, a = √ 1.333

= 1.155m

STEP 3

Centre line rafter length

= 2.700 x 1.155

= 3.118m

To find the centre line length of the rafter, simply multiply the run or half span by the constant, 1.155:

STEP 4

Cutting length of rafter = [(2.700 + 0.400) - 24] x 1.155 2

= 3.088 x 1.155 = 3.567m

To find the cutting length of the rafter, i.e. including eaves overhang, simply add the eaves width to the run or half span, deduct half the ridge thickness, then multiply the answer by the constant 1.155. The ridge is 24 mm thick:



SETTING OUT AND CUTTING RAFTERS The common rafters for a gable roof are all the same length, have only two bevels and may be set out from one pattern rafter. The length of this pattern rafter may set out by calculation, as previously mentioned in calculating roof pitch, or be set out using a steel roofing square, which will also have the two bevels required for the rafter. Proportions of common rafters Some of these proportions were dealt with earlier in this unit. The critical elements are shown below:

Fig 83 Common rafter proportions

Centre line set-out length of rafter

True length of rafter

True length of

overhang

True length of reduction for ½ thickness of ridge

Plumb bevel

½ Thickness of ridge on plan

PLUMB BEVEL LEVEL BEVEL

Birdsmouth notch

Allowance for plumb cut

Level bevel

CARPENTRY - HOUSING


Plumb and Level bevels The simplest method used to establish the plumb and level bevels is the ‘pitch board method’. Once the rise per metre run is known it is simply a matter of reducing the full measurements to a scaled size, say by dividing both measurements by 10. This means that a roof with a pitch of 577 mm to 1000 mm, or 30°, divided by 10 would be:

These smaller measurements are then set out to look like half the roof on a piece of timber or board material. A sliding bevel is then laid against the edge and adjusted to suit the angle:

Fig. 84 Pitch board method

Fig. 85 Setting sliding bevels to the plumb and level bevels

Note: These bevels may be transferred to the rafter or any other roof member, which requires a plumb or level bevel.

577 10 = 57.7mm

and

1000 10 = 100mm

STEP 1 Set up the pitch board by drawing the half roof shape, using the scaled measurements.

STEP 2 Lay a sliding bevel against each edge and adjust the blade to suit the angles formed.

100

57.7

Level bevel

Plumb bevel

Sliding bevel set to the plumb bevel angle

Sliding bevel set to the level bevel angle



THE STEEL SQUARE A common method used to set out the length of a rafter is to use the steel roofing square. This is a very versatile tool as it may be set up with the plumb and level bevels within the 90° triangle formed by the square and the adjustable fence or buttons. Again, the pitch of the roof is set up on the square using scaled measurements, which in this case are usually half the full size proportions. Example 1: If the pitch ratio is 1 : 1.732 or 30° it is firstly changed to a rise per metre run, which equals 577 to 1000 mm, then these measurements are halved to become 288.5 to 500 mm. Setting up the square

Fig. 86 The steel square

Fig. 87 Adjustable timber fence

Fig. 88 Alternative buttons, clips or guides

STEP 1 The proportion for the rise, i.e. 288.5 mm, is placed on the tongue of the square and the run or half span, i.e. 500 mm, is placed on the blade of the square.

STEP 2 A timber fence may be used to link the measurements and form the right-angled triangle. The timber fence sits on the top edge of the rafter to be set out. This allows it to slide along the rafter edge travelling in increments of 500 mm, until the desired distance is reached.

STEP 3 An alternative to the timber fence is the use of patent type steel buttons or clips. They are attached to the tongue and blade measurements and then the square is used the same as for the timber fence. Note: When the square is turned upside down it may be used to set out risers and treads for stairs.

40

Tongue Blade

50

600

Heel

400

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Patent buttons, clips or guides There are a number of brand types available for use with the steel square. One common type is the steel or brass button type produced by ‘Paulcall’ and manufactured in Australia. Another common type is the steel clip type produced by ‘Starrett’ and manufactured in the USA.

Fig. 89 Common buttons and clips Graduations Steel squares are marked off around the inside and outside edges of the tongue and blade in 2 mm graduations. Also, there are 10 mm and 100 mm graduations to allow for larger dimensions to be identified.

Fig. 90 Graduations on the steel square

'Paulcall' type

'Starrett' type



Using the steel square to set up a pattern rafter Once the pitch of the roof has been determined, set the scaled dimensions on the square, select a straight length of timber to use as a pattern rafter and step along the required number of times until the run or half span distance has been travelled. Example 1: Set up a steel square with a pitch ratio of 1 : 1.732 or 30° and set out a pattern rafter, which has a run or half span of 2700 mm and an eaves width of 400 mm.

Fig. 91 Steel square ready for use

STEP 1 Set the fence or buttons on the steel square to suit the scaled proportions, which will provide the pitch and bevels for the roof, i.e: • Pitch = 1 : 1.732 • Rise per metre run = 1.000 1.732 = 0.577m (Therefore 577 mm : 1000 mm) • Divide both measurements by 2 = 577 = 288.5 mm, and 1000 = 500 m 2 2

STEP 2 Place the square over the rafter with the fence on the top edge. Mark a plumb cut line across the face to represent the centre line of the ridge. Measure back half the thickness of the ridge and mark a firm plumb cut line on the face. This will be the cutting line for the top of the pattern rafter. Now move the steel square back to the original centre line ready to start stepping along the rafter.

1155

1000

577

30°

500

288.

5

400 300 200 100

100

200

CARPENTRY - HOUSING


Fig. 92 Detail of the reduction for the ridge

Fig. 93 Setting out the length plus overhang

STEP 3 Divide the measurement on the blade, i.e. 500 mm, into the run or half span: = 2700 500 = 5.4 ∴ The square will be moved along 5 times plus an additional 200 mm to equal the run or half span of 2700 mm. Note: The overhang is marked by placing the edge of the tongue on the birdsmouth plumb cut and marking off 400 mm from the blade

Plumb out True length of reduction for ½ thickness of ridge

Plumb bevel

½ Thickness of ridge on plan

Plumb cut set back ½ thickness of ridge

Ridge position

Run or half span of rafter plus eaves width = 3100

Run or half span of rafter = 2700 400 Eaves width

200

500

500

500

500

500

C L Total cutting length of common rafter = 3580

Centre line set out length of common rafter = 3118 = 462 Eaves Overhang

Y Plumb cut for fascia

Centre line for ridge Ris

e of

roof

= 1

.560

288.

5



Forming bevels on the square Once the scaled measurements are set on the square, with the fence or buttons fixed in place, the plumb and level bevels will automatically be formed in the complimentary angles of the square. When the square is laid over the edge of a rafter these bevels may be easily transferred as the square slides along the rafter.

Fig. 94 Rafter bevels on the square

Setting out with a pattern rafter A straight length of timber is selected for the pattern rafter, it is set out by measurement or using the steel square. It is then cut to form a finished rafter with a plumb cut at the top, plumb cut at the bottom and the birdsmouth checked out. A short length of batten is then nailed directly above the plumb cut at the top and the birdsmouth at the bottom. The pattern rafter is now ready for use. Lay the rafters to be cut on top of a pair of saw stools with the spring uppermost. The pattern rafter is then laid over each rafter, making sure the top edges are hard up under the short batten, and then the plumb cuts and birdsmouth positions are transferred by marking with a pencil. Note: The purpose of positioning each rafter hard up against the short battens is to ensure all the top edges will be in-line to maintain a straight roof surface. Although the plumb cut at the bottom of the rafter may be marked and cut at this time, it is usually better to leave the ends and cut them to a string line once the roof frame is complete. After the first pair of rafters is marked and cut, they should be tried in place to ensure the length and bevels are correct before proceeding with the remainder.

Fig. 95 Positioning the pattern rafter for marking

Plumb bevel

Level bevel

Rafter

Stop batten Rafter to be marked out Stop batten

Pattern must be straightest rafter possible Keep round edge of rafter to top always

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Fig. 96 Transferring pattern set out to the rafters Alternative pattern method A modified version of the pattern rafter is preferred by some tradespersons to mark out the common rafters. It is called a ‘rafter boat’, which is a short template having a plumb cut at both ends, a birdsmouth and a cleat on top to act as a guide when the boat slides along the top edge of the rafter. All the rafters are laid across saw stools and placed together on edge so the length and position of the birdsmouth can be marked. These positions are squared across the top edge of all the rafters. The boat is moved along the top edge of each rafter to align with these marks and then the plumb cuts and birdsmouth shapes are traced onto the rafter, ready to be cut. Note: When metal fascias are used the ends of the rafters don’t have to be exactly in-line, therefore they may be pre-cut, as the fascia brackets are adjusted to a string line before they are fixed.

Fig. 97 Using the rafter boat to set out rafters

Position 1 Position 2

Fascia cut (Pattern only)

Bird's mouth

Stop batten

Rafter

Rafter

Stop batten Plumb cut

Plumb cut for fascia

Eaves overhang

Birds mouth

Plumb cut for ridge

Timber guide

Centre line length of rafter Eaves overhang

Mark plumb cut for ridge Mark birdsmouth Mark plumb cut for fascia



CALCULATING FRAME QUANTITIES The following example outlines a method used to calculate framing member lengths, quantities and costs.

Example 1: Calculate the roof frame members for the tiled, boxed gable roof shown below. The pitch is 1 : 1.732 or 30°. The member sizes and costs are as follows: TABLE 2

Fig. 98 Typical boxed gable roof

MEMBER SECTION SIZE

MATERIAL SPACING STRESS GRADE

COST

Common rafters 90 x 35 Radiata pine 600 c/c F8 $2.10/m

Ridge 150 x 32 Oregon - F5 $4.00/m

Purlins 125 x 75 Oregon 1800 c/c F8 $9.00/m

Collar ties 70 x 35 Radiata pine 1200 c/c F8 $1.90/m

Scissor struts 250 x 50 Hardwood 2400 c/c F14 $15.00/m

Gable studs 70 x 35 Radiata pine 600 c/c F8 $1.90/m

Gable tie 90 x 35 Radiata pine - F8 $2.10/m

Soffit bearers 70 x 35 Radiata pine 600 c/c F8 $1.90/m

Fascia and barges 200 x 25 Primed Radiata pine

- $7.00/m

Eaves soffit sheets 1800 x 1200 x 4.5

Fibre cement sheets

- $15.00/ sheet

5400

400

400

300

8400

300

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Order length of rafters Calculate the ordering length of rafters for a pitch of 1 : 1.732.

Therefore, for every 1.0m of run or half span the hypotenuse or rafter length will be 1.155m

∴ Total order length, to the nearest 300 mm increment, of all the rafters will be 3.9m

STEP 1 Rise per metre run = 1.000 1.732

= 0.577m STEP 2 Length of

hypotenuse = a² = b² + c²

a² = 1.000² + 0.577²

a² = 1.0 + 0.333

Therefore, a = √ 1.333

= 1.155m

STEP 3

Centre line rafter length = 2.700 x 1.155

= 3.118m

To find the centre line length of the rafter, simply multiply the run or half span by the constant, 1.155:

STEP 5

Order length of rafter = 3.567 + 100 = 3.667

To find the ordering length it will be necessary to add 100 mm to the cutting length to allow for the plumb cut at the foot of the rafter:

RAFTERS

STEP 4

Cutting length of rafter = [(2.700 + 0.400) - 24] x 1.155 2

= 3.088 x 1.155 = 3.567m

To find the cutting length of the rafter, i.e. including eaves overhang, simply add the eaves width to the run or half span, deduct half the ridge thickness, then multiply the answer by the constant 1.155. The ridge is 24 mm thick:



Quantity of rafters The formula for the total number of rafters will be: Number = [ (total length of ridge + 1 ) x 2 sides] Rafter spacing

Order = 90 x 35 Radiata pine F8 – 32/ 3.9

The length of the ridge will be equal to the total roof length plus 300 mm joint length, for ridges over 5.4m long. Note: Lengths greater than 5.4 become difficult to handle, therefore it is common practice to join the ridge as previously described on page 13 of this unit.

Order = 150 x 32 sawn Oregon F5 – 2/ 4.8

STEP 1 Ridge length = length of roof + verge overhang for both ends

= 8400 + 300 + 300

= 9000 mm or 9.0m

STEP 2 Number of rafters

= [ (total length of ridge + 1 ) x 2 sides] Rafter spacing

= [ ( 9.000 + 1 ) x 2 sides]

0.600

= [ (15 + 1 ) x 2 sides]

= [ 16 x 2 sides]

= 32

STEP 1 Ridge length = length of roof + verge overhang for both ends + jointing 2

= 8400 + 300 + 300 + 300 2

= 9300 2

= 4650 mm or 4.650m

RIDGE

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Purlins are spaced at 1800 mm c/c, therefore there will be only one row along

each side. Each row will run the full length of the roof to the outside of the boxed gable ends. An allowance of 150 mm is added for every 5.4m of length for jointing.

Order = 125 x 75 sawn Oregon F8 – 4/ 4.8

Collar ties are spaced every second pair of rafters, i.e. 1200 mm c/c, and it is assumed they will lie on top of the purlins at the centre of the roof rise. They will only be necessary for the roof frame within the length of the wall frames.


STEP 1 Purlin length = length of roof + verge overhang for both ends + jointing 2

= 8400 + 300 + 300 + 150 2

= 9150 mm

STEP 2 Number of purlins

= 2/ total length 2

= 2/ 9150

2

= 2/ 4575 mm, say 2/ 4.800 per side

STEP 1 Collar tie length

= Width or span of the roof 2

= 5400 2

= 2700 mm or 2.7m

STEP 2 Number of collar ties

= Length of wall frames + 1 spacing

= 8400 + 1

1200

= 7 + 1

= 8

PURLINS

COLLAR TIES



The scissor struts will rest on the external wall plates, run under the purlins and cross over in the centre of the roof, where they will be bolted together. Allow the same length as the rafters, as they are set at a lower angle the additional length will be used for bolting. They are spaced, at maximum centres of 2400 mm, within the roof for the length of the wall frames only.

Order = 250 x 50 sawn hardwood F14 – 10/ 3.9

The length of each gable stud may be determined by reducing each one, in sequence, by multiplying the horizontal distance out from the fascia line by the rise per metre run. The horizontal distance out is calculated by deducting the stud spacing for each stud.

Fig. 99 Gable stud spacings

STEP 1 Length = Rafter length

= 3.9m each side

STEP 2 Number of struts

= Length of walls +1 spacing

= 8400 +1

2400

= 3.5 + 1

= 4 + 1

= 5 per side

Run of boxed gable rafter

Equal stud spacing at 600 c/c

No 5

No 4

No 3

No 2

No 1

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577

577

577

577

SCISSOR STRUTS

GABLE STUDS

CARPENTRY - HOUSING


When the rise per metre run = 1.000 = 0.577m 1.732

Note: Use an off cut to make up the last short stud position in this case.

Add the total length of all the shortened studs together for each boxed gable end, i.e. No. 2 to No. 5, and then add the centre stud, i.e. No. 1.

Order = 70 x 35

Radiata pine F8 – 4/ 3.9, 1/ 3.6

These will be equal to the full span plus two times the eaves width, for each

end.


STUD No. 1 Distance out = Half span + eaves width = 3.100

∴ Length of stud = 3.100 x 0.577 = 1.789

STUD No. 2 Distance out = 3.100 – 0.600 = 2.500

∴ Length of stud = 2.500 x 0.577 = 1.443


∴ Length of stud = 1.900 x 0.577 = 1.096


∴ Length of stud = 1.300 x 0.577 = 0.750


∴ Length of stud = 0.700 x 0.577 = 0.404

Total length for each side of gable end

= 1.443 + 1.096 + 0.750 + 0.404

= 3.693m, allow 4/ 3.9m

Total length for centre studs = 1.789 x 2 = 3.578m, allow 1/ 3.6m

Total length of tie = 5.400 + (400 x 2)

= 6.200, allow 2/ 6.3m

GABLE TIES



Divide the length of each side by the spacing of the bearers. Treat the overhang of the boxed gables separately from the sides, as they have a smaller eaves width. The formula for the total number of soffit bearers for the sides will be: Number = [( length of side ) + 1] x 2 sides bearer spacing ∴30 x 0.500 = 15.0m, say 3/ 5.1 (allow 0.650 for brick veneer construction) The formula for the total number of soffit bearers for the boxed gables will be: Number = [( length of gable end) + 1] x 2 ends bearer spacing ∴20 x 0.400 = 8.0m, say 2/ 4.2 (allow 0.550 for brick veneer construction) Order = 70 x 35 Radiata pine F8 – 3/ 5.1, 2/ 4.2

= [(8.400 )+ 1] x 2 0.600

= [14 +1] x 2

= 15 x 2

= 30

= [( 5.400 + 1] x 2 ends 0.600

= [9 + 1] x 2

= 10 x 2

= 20

SOFFIT BEARERS

CARPENTRY - HOUSING


The fascia will run the full length of the roof, equal to the ridge length.

Order = 200 x 25 Primed Radiata pine – 2/ 4.8, 2/ 4.5

The barges will be the same length as the common rafters plus an additional 300 mm to allow for the thickness of the fascia, the plumb cut at the bottom end and the extended section past the end of the gutter. Allow for two lengths per gable end.

Order = 200 x 25 Primed Radiata pine – 4/ 3.9

The eaves soffit sheet strips will be cut from full 1800 x 1200 mm wide sheets. Allow the gable end sheets to run the full width of the boxed gable and butt the sides into them. Note: The joint between the external wall cladding or brickwork and the soffit sheets will be covered with a 25 mm quad, or similar, during the Exterior cladding/ finishing stage. Number of sheets for sides = (length of side ) ÷ 3 length of sheet

Fascia length

= Length of wall + gable end overhang + jointing

Fascia length

= 8.400 + (300 x 2) + 0.100

= 8.400 + 0.600 + 0.100

= 9.1m, allow 1/ 4.8, 1/ 4.5m per side

Barge length

= Cutting length of rafter + 300 mm

= 3.569m + 0.300

= 3.869m, allow 3.9m

Number of soffit strips per side

= 8.400 1.800

= 4.666

= Say 5 strips

FASCIAS

BARGES

EAVES SOFFIT SHEETS



Number of sheets for gable ends = (length of end ) ÷ 4 length of sheet

Order = 1800 x 1200 x 4.5 mm Fibre cement sheets – 6 off

Number of sheets (both sides)

= (5 x 2) ÷ 3

= 10 ÷ 3

= 3.333, allow 4 sheets

Number of soffit strips per end

= 5.400 + (0.400 x 2) 1.800

= 5.400 + 0.800 1.800

= 6.200 1.800

= 3.444

= Say 4 strips

Number of sheets (both ends)

= (4 x 2) ÷ 4

= 8 ÷ 4

= Allow 2 sheets

CARPENTRY - HOUSING


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GLOSSARY OF TERMS

Adjacent - Means it is placed next to or found beside something. Apex - This is the very top or point of something, like the apex of a roof,

meaning where members come together at a common top position. Bisect - This means to cut, separate or divide something exactly in half, such as

when a 90° angle is bisected it becomes two 45° angles. Circa - This means around, approximately, round about, etc. Usually refers to

dates when estimating the age of a building or structure such as circa 1854, also written as (c 1854).

Cluster - This is a term, which refers to a group or gathering of a number of members in a frame, such as a roof cluster, which consists of the end of the ridge, two centering rafters, a crown end rafter and two hip rafters.

Complimentary - In this case it means any two angles, which make up a right angle. Constant - In this case it means a number, quantity or amount, which is used as the

basis for several calculations. For example the length of 1.414m is a constant, which may be used to calculate the 45° hypotenuse length of a right-angled triangle, once the length of one side is known.

Corbelled - Refers to stepped out brickwork used to support other members, such as corbelled eaves.

Dihedral - This is the angle formed between any two surfaces where they meet along a common length, such as a ridge in a roof. A dihedral angle is formed between the underside of the two roof surfaces or where a roof surface meets a parapet wall, etc.

F.C. - This is an abbreviation for fibre cement, similar to the product ‘Hardiflex’.

Hypotenuse - This is the angled side of a right-angled triangle. Inclined - Means to be at an angle to something, such as an inclined strut or brace.

In-situ - This is an abbreviated version meaning in situation or position, such as pouring concrete in-situ, meaning to pour in place into the formwork.

In-to-over - This is a method used for marking the spacings of members. It literally means marking from the inside face or edge of one member to the outside face or edge of the next member. It is equal to working centre-to-centre but is more practical for fixing purposes as it allows one edge to be lined up with the mark, so it is easily seen, ready for fixing.

Lined on-the-rake - This is a term used to describe the ceiling lining of a pitched roof, which is fixed to the underside of the rafters. There is no access to the roof structure as there is no roof space formed.

Parapet - This is a vertical wall or gable, which extends past the line of the roof to enclose the roof from view. The parapet is usually constructed of brickwork or timber framing and clad with sheet material.

Patent - This is a term used to describe a product which has had its design registered with the Patents office. It is the original idea of a person or persons, which cannot be copied without consent.

Primed - This is a protective white or pink paint coating applied to timber before it is fixed into place. It seals the timber and provides a surface ready to take undercoat paint prior to the finishing coats. It helps to prevent timber decay from occurring.

CARPENTRY - HOUSING


Scribe - This is where the shape of one piece is to be fitted to the surface shape of another. It may also refer to anything placed in-situ and marked to suit the final resting position of that member, such as scribing a hip in position.

Slat - Refers to a thin narrow piece of timber or several thin arrow pieces put together to make something else, such as slats of timber used to make a sheet of lattice.

Soffit - This is the horizontal under face of a structure or lining. The eaves soffit is the underside face of the eaves sheeting.

Trapezoid - This is an irregular quadrilateral with only two parallel sides. The shape may be found on the side roof surface of a hip roof made up of the fascia, ridge and two hips.



FURTHER READING

Australian Standards Committee, 1992, AS 1684 – National Timber Framing Code, Standards Association of Australia Homebush, Sydney. Bloomfield, F. C. and E. Peterson, Revised by B.S. Brown and H. A. Slatyer, First Edition 1958, Fifth edition 1985, The Australian Carpenter and Joiner – volumes 1 & 2, Standard Publishing Co. Pty Ltd., Naremburn, NSW. Simpson, Charles & Barry Hodgson, 1995, Building a house – framing practices, Macmillan Education Australia, South Melbourne. Staines, A., Reprinted 1987, Owner Builders & Renovator, Pinedale Press, Caloundra, Qld. Staines, A., First Edition 1988, The Australian Roof Building Manual, Pinedale Press, Caloundra, Qld. Staines, A., Reprinted 1988, The Australian Owner Builders Manual, Pinedale Press, Caloundra, Qld. Stapleton, M. and Ian Stapleton, 1997, Australian house styles, The Flannel Flower Press Pty Ltd, Mullumbimby, NSW Teachers of Building, 1996 Reprinted 1997, 1998, Second Edition 1999, Basic Building and Construction Skills, Addison Wesley Longman Australia Pty Ltd, South Melbourne. Ward-Harvey K., 1984, Fundamental Building Materials, Sakoga Pty Ltd, Mosman NSW.

VIDEOS Construction and Transport Division, Roofing Pattern Rafter (CTV18), Roofing Pattern Fixing (CTV19) available from Resource Distribution, Yagoona.

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