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Flat Roofs & Introduction to Gable Roofs

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Presentation on theme: "Flat Roofs & Introduction to Gable Roofs"— Presentation transcript:

1 Flat Roofs & Introduction to Gable Roofs
M. S. Martin / March 2004 / Reviewed & Combined March 2007, Reviewed March 2010

2 Review Ceiling Frame Calc’s

3 Spec for example following copy on blank paper
Joists 150 x 38 sawn 600mm centres, max. span 3.6m Trimmers 100 x 38 sawn 600mm centres Hanging beam 240 x 45 hardwood, max. span of 5.4m On sketch following, draw in joists and hanging beam to scale and indicate trimmer positions also.

4 Sketch picture below allow 100mm for wall frames (not shown)
Internal room sizes shown 4.100 3.300 5.800 8.350

5 Formula's No. of joists = width of room - 1 spacing Length of hanging beam = length of room + 2 x wall thickness Trimmers = length of wall – 1

6 Joists 8.350 – = – 1 = say 14 – 1 = 13 Order 6.0m

7 Hanging beam = Order 4.5m

8 Trimmers 5.800 – 1 = – Say 10 – 1 = 9 Trimmers at two ends plus internal wall 9 x 3 = 27, 27 x .600 = 16.2 lin/m Order 6.0m, 4.2m

9 Flat roof construction
Basic Roof & Ceiling Frame text – from P23 Flat roofs combine the roof and ceiling frame in one Generally described as one with a pitch less than 10 deg. To allow for extra loads the rafters / ceiling jiosts are increased in sectional size

10 Ceiling types Ceilings can be either lined on the rake or rafters laid flat and use graded battens to give fall to roof

11 Variations to ceilings



14 Skillion roofs – P 25 Single sloping roofs
Walls at either end of roof at different heights Internal walls parralell to end walls will be at different heights Walls at 90 deg. to end walls will taper in height under the sloping rafters / joists

15 Two ceiling designs

16 Simplest method of marking
With skillion roofs the simplest thing is to scribe the rafter over the supporting plates

17 Lean – to – Roof construction P 26
This type is constructed against an existing wall or other roof Simple extensions Carports Verandahs Awnings

18 Rafter / Ceiling variations
Common on carports, open exposed rafters

19 Other types

20 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

21 Answer – length of rafter
√ 2.7² ² = √ = √ 8.1 = lin/m Order 3.0m lengths 900 2.700 4.200

22 Number of rafters (4.200) = = 8 Order 3.0m rafters

23 Gable Roofs and Components

24 Name the components (handout)

25 Answers Ridge Roof Tiles Rafter Tile Battens Collar Tie Hanging Beam
Purlin Anti-ponding Board Strut Purlin Strut Top Plate Strutting Beam Ceiling Joist Top Plate Soffit Bearer Stud Block Cornice Ceiling Lining Internal Wall Fascia Soffit Lining

26 Gable roof definitions
Roof span: Is the horizontal distance measured overall the wall plates Span Plate Stud Section A-A

27 Definitions Continued
½ Span : The Plan length of the rafter

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

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

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

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

32 Continue on roofs Ridge Rafter Strut Purlin Hanging beam Ceiling joist

33 Set Out for Common Rafter
Plumb cut is at the top of the triangle We reduce the centre line length of rafter by ½ the thickness of the ridge to get our true length

34 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

35 Set Out on Steel Square Calculate rise per / m. run
Transfer to steel square Reduced rise Reduced run

36 Set out pattern rafter By either stepping out with square, or by calculation

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