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Flat Roofs & Introduction to Gable Roofs

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 following copy on blank paper. Joists 150 x 38 sawn Oregon @ 600mm centres, max. span 3.6m

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Flat Roofs & Introduction to Gable Roofs

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  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 followingcopy on blank paper • Joists 150 x 38 sawn Oregon @ 600mm centres, max. span 3.6m • Trimmers 100 x 38 sawn Oregon @ 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 belowallow 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 spacing

  6. Joists 8.350 – 1 .600 = 13.916 – 1 = say 14 – 1 = 13 Order 13 @ 6.0m

  7. Hanging beam 4.150 + .200 = 4.350 Order 1 @ 4.5m

  8. Trimmers 5.800 – 1 = 9.666 – 1 .600 Say 10 – 1 = 9 Trimmers at two ends plus internal wall 9 x 3 = 27, 27 x .600 = 16.2 lin/m Order 2 @ 6.0m, 1 @ 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

  12. 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

  13. Two ceiling designs

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

  15. Lean – to – Roof constructionP 26 • This type is constructed against an existing wall or other roof • Simple extensions • Carports • Verandahs • Awnings

  16. Rafter / Ceiling variations • Common on carports, open exposed rafters

  17. Other types

  18. 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

  19. Answer – length of rafter √ 2.7² + .900² = √ 7.29 + .81 = √ 8.1 = 2.846 lin/m Order 3.0m lengths 900 2.700 4.200

  20. Number of rafters (4.200) + 1 .600 = 7 + 1 = 8 Order 8 @ 3.0m rafters

  21. Gable Roofs and Components

  22. Name the components (handout)

  23. 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

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

  25. Definitions Continued • ½ Span : The Plan length of the rafter

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

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

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

  29. Pitch : The amount of slope or gradient in a roof, typically expressed in degrees 30°θ

  30. Continue on roofs Ridge Rafter Strut Purlin Hanging beam Ceiling joist

  31. 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

  32. 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

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

  34. Set out pattern rafter • By either stepping out with square, or by calculation

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