1. Gable Roof
BCGCA3007B

2. Gable Roof
Flush with no eaves
Flush with raked eaves
Boxed

3. Flush Gable (No Eaves) Rafter finishes in line with end wall
Barge fixed to outside wall
No Overhang
Studwork to take sheeting or support brickwork
Not required to support roof

4. Flush Gable with Raked Eaves
Ridge Extends to Form Eave
Verge Trimmers to Support Gable Rafter
Trimmers to Support Verge Trims and Roof Battens
Gable Stud supports
Second Rafter
Brick Work or Wall Lining
Second Rafter to Support Verge Trims
Roof Extends to Form eave
Top Plate may be extended to support Rafter

5. Boxed Gable Verge Rafter Gable Studs & Sheeting
Purlin & Lintel Extended to Support Verge Rafter
Minimum Back span
2 x Overhang
Design 3 x Overhang
Base of Gable Lined & Level with U/S of Eaves

6. Verges Is the Junction of the Roof and the Barge/Verge Board
Verge Detail for Tiled Roof

7. Verge Detail for Tiled Roof

8. Principles of Roofing Ridges are Level Rafters run at 90
Hips & Valleys bisect all internal & external Corners
Roof members are set out along centerlines

9. Roofing Members Ridge Common Rafter Collar Tie Top Plate Hanging Beam
Purlin
Top Plate
Strutting Beam
Strut
Ceiling Joist
Strut

10. Roof Members Common Rafter – Main Sloping Roof Member
Spacing 450mm or 600mm for Tiled Roofs
900mm for tiled roofs
Supports Roof Battens which in turn support roof coverings
Must be in single lengths or joined over supports
Fixings 2 x 75mm Skew & 2 x 75mm into Ceiling Joist (if joist is > 38, 90mm Nails)

11. Roofing Members Ridge Common Rafter Collar Tie Top Plate Hanging Beam
Purlin
Top Plate
Strutting Beam
Strut
Ceiling Joist
Strut

12. Ridge Highest Part of the Roof Will run full length of Gable Roof
Runs Parallel to TOP PLATE
Fixes Rafters at Top of Roof
Rafters are nailed either side and not offset by more than 1 thickness
In Uncoupled Roof, they act as beams
Nailed to Rafters with 2 x 75mm Nails

13. Joining Ridge Boards

14. Roofing Members Ridge Common Rafter Collar Tie Top Plate Hanging Beam
Purlin
Top Plate
Strutting Beam
Strut
Ceiling Joist
Strut

15. Purlin Also known as Underpurlin Fixed to Underside of RAFTER
Runs parallel to ridge and wall plates
Reduce span of RAFTER
Will run full length of Gable Roof

17. Roofing Members Ridge Common Rafter Collar Tie Top Plate Hanging Beam
Purlin
Top Plate
Strutting Beam
Strut
Ceiling Joist
Strut

18. Struts
Transfer Loads to Load Bearing Walls

23. Roofing Members Ridge Common Rafter Collar Tie Top Plate Hanging Beam
Purlin
Top Plate
Strutting Beam
Strut
Ceiling Joist
Strut

24. Collar Ties

25. Common Rafter Principles
Birdsmouth Max 1/3 Depth of Rafter

26. Common Rafter Principles
Span – From Birdsmouth to Birdsmouth
Half Span – From Birdsmouth to Centre

27. Common Rafter Principles
True Rise –From
Horizontal line that passes thru the Intersection of Birdsmouth Plumb Line &Top of Rafter
To : vertically plumb up to the Apex
NOTE – True Rise is not measured from TOP PLATE

28. A Closer Look Top Of Rafter Birds Mouth Top Plate Ceiling Joist
NOTE – True Rise is not measured from TOP PLATE

29. Common Rafter Principles
If Rise is taken from Top Plate to Apex pitch will be incorrect

30. Common Rafter Principles
Pitching Line x-y line

31. Common Rafter Principles
Measured to edge of Ridge
Measured to centre of Ridge

32. Common Rafter Principles
Centre Line Length
True Length
True Length is usually what we need

33. Determine Rafter Length Mathematically
Determine Span = 2700mm
Determine Half Span = 2700/2
= 1350mm
Determine Rafter Run = 1350-
Half Ridge Thickness = 19/2 = 9.5
Rafter Run =
True Length Rafter = / Cos 25
= 1480
Rafter 90 x 45
Ridge 125 x 19

34. Overhang Plan always from external wall

35. Determine Rafter Length Mathematically
In this case Timber Famed Wall
O/H = 450 / cos 25
= 497mm
Roofing calculations are always measured from Birdsmouth or pitching point
Rafter 90 x 45
Ridge 125 x 19

36. Determine Rafter Length Mathematically
In this case Brick Veneer Wall
O/H =
cos 25
= 662mm
Total Rafter Length
Timber Frame = = 1977mm
Brick Veneer = = 2142mm
Rafter 90 x 45
Ridge 125 x 19

37. Determine Rafter Length Graphically
Draw Roof Full-size
Measure members directly
Avoid using scaled drawing
Scale use only for angles
Rafter 90 x 45
Ridge 125 x 19

38. Determine Rafter Length Using Roofing Square
Use Calculator
Press Tan 25 – what does this give you
Therefore for every 1 metre run there is 0.466m rise
Using the principle of similar triangles we half the size of the triangle

39. Press Tan 25 – what does this give you
Use Calculator
Press Tan 25 – what does this give you
Therefore for every 1 metre run there is 0.466m rise
Using the principle of similar triangles we half the size of the triangle

40. Press Tan 25 – what does this give you
Use Calculator
Press Tan 25 – what does this give you
Therefore for every 1 metre run there is 0.466m rise
Using the principle of similar triangles we half the size of the triangle

41. Using Similar Triangles
1341/500 = or
= 2 r 341

42. Using Similar Triangles 1341/500 = 2.682 or = 2 r 341
Step out 2 full triangles
Select Start Point Allowing for O/H

43. Intersection of Top of Rafter & Edge of Square

44. Using Similar Triangles 1341/500 = 2.682 or = 2 r 341
Step out 341 & use square to extend
Step out 2 full triangles
Select Start Point Allowing for O/H

45. Determine Roofing Angles used in Gable Roofs
Plumb Cut
Foot Cut

46. Determining Angles with Roofing Square
When we set out rafter previously we determined plumb cut

47. Determining Angles with Roofing Square
Plumb Cut
Foot Cut

48. Determining Angles Mathematically
Extend Line 90⁰ From Rafter

49. Determining Angles Mathematically
Extend Line 90⁰ From Rafter
Extend Plumb Line

50. Determining Angles Mathematically
Extend Line 90⁰ From Rafter
Extend Plumb Line
Angle Formed is same as roof pitch

51. Determining Angles Mathematically
Extend Line 90⁰ From Rafter
Extend Plumb Line
Angle Formed is same as roof pitch
Offset = Tan (pitch) x width

52. Determining Angles Mathematically
Extend Line 90⁰ From Rafter
Extend Plumb Line
Angle Formed is same as roof pitch
Offset = Tan (25⁰) x = 42

53. Determining Angles Mathematically
Next we can Determine our Birdsmouth
Width across plumb cut = 90/ cos 25
= 99
Therefore max Birdsmouth = 33
Distance from top plate to top of Rafter = 66

54. Complete Q5 in Workbook

55. Determine Roof Angles Graphically
Plan – View
We can only see rafter run

56. Determine Roof Angles Graphically
Extend Top Plate & Ridge

57. Determine Roof Angles Graphically
Extend Top Plate & Ridge
Mark Rise

58. Determine Roof Angles Graphically
Plumb Cut
Extend Top Plate & Ridge
Mark Rise
Draw Hypotenuse
Foot Cut

59. Steps In Construction Gable Roof
Physically Confirm Span & Plates are Parallel
Calculate True Rafter, Rise & Plumbcuts
Mark out ceiling joists, rafters & ridge
Install Ceiling Joists
Cut Pattern Rafter & test to confirm
Cut required rafters & install

60. Estimating Gable Roof - Rafter From Previous Ceiling Estimate
Pitch = 25⁰
Determine No of Ceiling Joists
12 250/ 600 = = =
Therefore 22 set of Rafters
= Verge
= 48

61. Determine Rafter Length
Span = 6900

62. Determine Rafter Length
Span = 6900
Half Span = 3450

63. Determine Rafter Length
Span = 6900
Half Span = 3450
Run = 3450 – 9.5 (Half Ridge) =

64. Determine Rafter Length
Span = 6900
Half Span = 3450
Run = 3450 – 9.5 =
True Length = / cos 24
=3766

65. Determine Rafter Length
Span = 6900
Half Span = 3450
Run = 3450 – 9.5 =
True Length = / cos 24
=3766
Overhang = 450/cos 24
= 493

66. Determine Rafter Length
Span = 6900
Half Span = 3450
Run = 3450 – 9.5 =
True Length = / cos 24
=3766
Overhang = 450/cos 24
= 493
Total Rafter = 4259

67. Estimating Sheet

68. Estimating Gable Roof - Purlin
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof
Ridge 140 x 19
Purlin 90 x 70 F7
Purlin
Run full length of Roof
Do we need purlins

69. Span Required = 3766
Max Span 2600
3766/2600 = 1.4
Therefore 1 row required each side
In First Instance we check single span
Max Span = 1900

70. Estimating Gable Roof -Purlin
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Purlin
Run full length of Roof
Do we need purlins
Purlins Run Full Length
x 150 (Joins) =

71. Estimating Sheet

72. Estimating Gable Roof - Ridge
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Ridge Runs Full Length
x 450 (O/H)
x 300 (allow for joins)
=14000mm
Note –
For Flush eaves there is no O/H ridge = 12500
Boxed eaves will require O/H ridge = 13400mm

73. Estimating Sheet

74. Estimating Gable Roof - Struts
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Struts are Difficult to Estimate
Develop a Best Guest Method
Determine No Required
12500/2700 = 4.6 = 5 = 6 Each Side

76. Estimating Gable Roof - Struts
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Struts are Difficult to Estimate
Develop a Best Guest Method
Half Rise x √2 x √2
Tan 24⁰ x 3441 x 2
= 3064
Determine No Required
12500/2700 = 4.6 = 5 = 6 Each Side

77. Estimating Sheet

78. Estimating Gable Roof – Collar Ties
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Collar Ties 90 x 35
Collar Ties on Every 2nd Rafter
On top of Purlins
22/2 = HALF SPAN
3450

79. Estimating Sheet

80. Estimating Gable Roof – Verge Trims
Pitch = 24⁰
Overhang 450
Rafter 90 x 45 F7
600 c to c
Tiled Roof with Flush Gable & Raking Eaves
Ridge 140 x 19
Purlin 90 x 70 F7
Span 2700
Collar Ties 90 x 35
True Length of Rafter

81. Rafter True length / 600
= 6.28 = 7 (Ridge Closes)
= 33.6
Say 33.9

82. Estimating Sheet

83. One Row of Purlin Each Side