Flat Roofs & Introduction to Gable Roofs

M. S. Martin / March 2004 / Reviewed & Combined March 2007, Reviewed March 2010

Review Ceiling Frame Calc’s

Spec for example followingcopy on blank paperJoists 150 x 38 sawn Oregon @ 600mm

centres, max. span 3.6mTrimmers 100 x 38 sawn Oregon @

600mm centresHanging beam 240 x 45 hardwood, max.

span of 5.4mOn sketch following, draw in joists and

hanging beam to scale and indicate trimmer positions also.

Sketch picture belowallow 100mm for wall frames (not shown)

Internal room sizes shown4.100

3.300

8.350

5.800

Formula'sNo. of joists = width of room - 1 spacing

Length of hanging beam = length of room + 2 x wall thickness

Trimmers = length of wall – 1 spacing

Joists8.350 – 1.600= 13.916 – 1= say 14 – 1= 13

Order 13 @ 6.0m

Hanging beam4.150 + .200 = 4.350

Order 1 @ 4.5m

Trimmers5.800 – 1 = 9.666 – 1.600Say 10 – 1 = 9Trimmers at two ends plus internal wall9 x 3 = 27, 27 x .600 = 16.2 lin/m

Order 2 @ 6.0m, 1 @ 4.2m

Flat roof constructionBasic Roof & Ceiling Frame text – from

P23Flat roofs combine the roof and ceiling

frame in oneGenerally 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 typesCeilings 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 – P 25Single sloping roofsWalls at either end of roof at different

heightsInternal walls parralell to end walls will

be at different heightsWalls at 90 deg. to end walls will taper in

height under the sloping rafters / joists

Two ceiling designs

Simplest method of markingWith skillion roofs the simplest thing is

to scribe the rafter over the supporting plates

Lean – to – Roof constructionP 26This type is constructed against an

existing wall or other roofSimple extensionsCarportsVerandahsAwnings

Rafter / Ceiling variationsCommon 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

Order 3.0m lengths

2.700

4.200

900

Number of rafters (4.200) + 1 .600= 7 + 1 = 8

Order 8 @ 3.0m rafters

Gable Roofs and Components

Name the components (handout)

AnswersRoof Tiles

Tile Battens

Top Plate

Fascia

CorniceCeiling Joist

Ceiling Lining

Strutting Beam

Internal Wall

BlockTop Plate

Soffit Bearer

StudSoffit Lining

Anti-ponding Board

Collar Tie

RafterRidge

Purlin

Strut

Hanging Beam

Purlin

Strut

Gable roof definitionsRoof span: Is the horizontal distance

measured overall the wall plates

SpanPlate

Stud Section A-A

Definitions Continued½ Span : The Plan length of the rafter

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Rise: Is the vertical height measured from the x – y line to where the upper edge of the rafters would intersect

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Centre Line Length : The hypotenuse in the right angled triangle formed by the ½ span and the rise

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Common Rafter : Is the member fixed to the ridge with a plumb cut and birdsmouthed over plate, extending to an eave.

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Fig 22 Section through a gable roof

Half span or Run

Top plateCeiling joist

Common rafterStrut

Collar tieEqual

Plumbbevel

Ridge

Equal

Centre-lin

e

length

Actual le

ngth

Gauge linedepth

Ris

eH

eig

ht

DETAIL AT RIDGE BOARD

DETAIL AT PLATE

Birdsmouthnotch not greater than D/3

Centre-lin

e length of ra

fter

Overhang

Eave width

Level bevel

Plate line

Ris

e H

eig

ht

Pitch : The amount of slope or gradient in a roof, typically expressed in degrees

30°θ

Continue on roofsRidge

Rafter

Purlin

Hanging beamCeiling joist

Strut

Set Out for Common RafterPlumb cut is at the

top of the triangleWe 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 SquareCalculate rise per / m. runTransfer to steel square

Reduced rise

Reduced run

Set out pattern rafterBy either stepping out with square, or by

calculation