# Forum Home Renovation Structural Renovation  what size timber for skillion roof rafters

## hoss

hey guys we are planning on ripping down a lean to laundry and adding an extention. It will be pretty much one room 10m x 4m with a skillion roof the rafters running the 4m with a colourbond roof.Ive looked at the span tables posted on here but not sure what im looking at. im just wondering on what size timber we would need to use for the rafters i was thinking 120 x 45 or so?

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## ausdesign

140*45 mgp12 @ max. 600 centres or 190*35 mgp10 @ max. 900 centres.

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## shauck

Depends on roof and ceiling mass kg/m2 (below are only "typical roof construction" examples and may differ in your situation (i didn't include tiled roofs which are higher numbers) 
Metal sheet tiles or medium gauge corrugated iron, battens, 12mm softwood ceiling lining, sarking, lightweight insulation = 20kg/m2 
Steel sheet roofing .75mm thick, 13mm plaster ceiling, roof and ceiling battens, sarking and lightweight insulation = 30kg/m2 
Steel sheet roofing 0.75mm thick, battens, graded purlins and high density fibreboard ceiling lining = 40kg/m2 
Then on to terracotta tiles etc, etc 
For F17 SHWD (or LVLs) - to span 4m if rafters are 900 spacing, single span (roof not supported by internal wall to reduce span)  
120x45 will support 4100 if 20kg/m2 (so only just to minimum standard)
140x45 will support 4000 if 40kg/m2 (so only just to minimum standard) 
Many say, go up from minimum standard.

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## hoss

thanks guys i ask this because i have limmited room with my ceiling height as the extention will be comming off an existing skillion roof. i have 2400 to 2500 from top of proponed slab to the the colourbond on the existing roof.Now not really knowing how to go about this as an owner builder do i get a drafty to draw my plans up and i then submit this to to a building inspector. And will the drafty include all material specs like the rafter size etc or do i have to go through the building inspector for this. prob a stupid question i know but i have been owner builder for a kit shed once before and the shed came with all the drawings was much easier than a custon extention as we plan on this time.

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## shauck

A drafty can do this but you'll need to discuss with them what is important, such as your height problem. Your floor to ceiling height depends on use of extension so to get it past council you will have to consider this first. You say "proposed" slab. Is there room/possibility to excavate somewhat before the slab goes down if required heights can't be met with current plan. Below is BCA info from 2011 but is the same in 2012 edition 
Clause 3.8.2.2 of BCA Volume Two sets out the minimum requirements  for ceiling heights in dwellings. BCA 2011 now includes a diagram and  accompanying notes that makes it very clear how to measure ceiling  heights for the different areas in a house. 
   Please note the actual standards for measuring ceiling height have  not changed. This diagram has simply been provided to give clarity as to  where and how the measurements are to be applied.  
   The letters represent the following minimum dimensions: 
   A = 2.4m Habitable room (excluding a kitchen  2.1m)
B = 2.4m Habitable room - two-thirds of the floor area
C = 2.1m Non -habitable room two-thirds of the floor area
D = 2.2m An attic to be habitable - two-thirds of the floor area 
E = 1.5m Calculating the floor area of a room or space, any ceiling height of 1.5m is excluded
F = 2.0 Stairs (measured vertically above the nosing line)
The combined dimensions of G must not exceed one-third of the floor area (See E above) of the room or space.

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## hoss

the first 2m in of the extention will be a small hallway and laundry and from there we have the height to go up as high as we like so think we might be just ok.Will contact a drafty to have a look i think

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## shauck

Cool. Best of luck. Not all draftys are created equal. A trusted recommendation.

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## barney118

> Depends on roof and ceiling mass kg/m2 (below are only "typical roof construction" examples and may differ in your situation (i didn't include tiled roofs which are higher numbers) 
> Metal sheet tiles or medium gauge corrugated iron, battens, 12mm softwood ceiling lining, sarking, lightweight insulation = 20kg/m2 
> Steel sheet roofing .75mm thick, 13mm plaster ceiling, roof and ceiling battens, sarking and lightweight insulation = 30kg/m2 
> Steel sheet roofing 0.75mm thick, battens, graded purlins and high density fibreboard ceiling lining = 40kg/m2 
> Then on to terracotta tiles etc, etc 
> For F17 SHWD (or LVLs) - to span 4m if rafters are 900 spacing, single span (roof not supported by internal wall to reduce span)  
> 120x45 will support 4100 if 20kg/m2 (so only just to minimum standard)
> 140x45 will support 4000 if 40kg/m2 (so only just to minimum standard) 
> Many say, go up from minimum standard.

  Shauck where did you get 0.75mm steel sheet roofing? It is 0.42mm is standard.  :Biggrin:

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## shauck

> Shauck where did you get 0.75mm steel sheet roofing? It is 0.42mm is standard.

  Just quoting from the standards. I've not got tin that thick. Imagine what it would cost!   :Biggrin:

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