Flat Roofs & Introduction to Gable Roofs Ref: M. S. Martin / March 2004 / Reviewed & Combined March 2007

Review Ceiling Frame Calc’s

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.

Sketch picture below allow 100mm for wall frames (not shown) Internal room sizes shown

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

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

Hanging beam = Order 4.5m

Trimmers – 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

Flat roof construction Section 8 – from P306 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

Ceiling types Ceilings 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 - P308 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

Two ceiling designs

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

Lean – to – Roof construction P309 This type is constructed against an existing wall or other roof Simple extensions Carports Verandahs Awnings

Rafter / Ceiling variations Common 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 = √ = √ 8.1 = lin/m Order 3.0m lengths

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

Gable Roofs and Components

Name the components (handout)

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

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

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

Continue on roofs Ridge Rafter Purlin Hanging beam Ceiling joist Strut

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

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 Square Calculate rise per m. run Transfer to steel square Reduced rise Reduced run

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