how to calculate roof rafter size

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I have extended my chalet style house to two tradition story house by getting rid of the dormant windows. the back of the house has completed now and ready for the roof. because I am still living in the house I want to cut the roof for the back and move to the back of the house and do the front.
house can I calculate the size of the wood that is needed for the roof, i.e the rafters
 
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Depends on the roof construction to start with. What about Building Regulations approval/ structural engineer's input for structural alterations etc.:?::!:
 
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I have done all the drawings for the extension and the house a part from the roof and my local authorities pass it on two conditions. extractor fan in each bathroom and roof design/drawings to be presented before commencing. However, I do not know how to do the calculation. an easy option I could go for ready make trusses. but the problem with that is, I need to take the whole house roof to fit the trusses which makes me exposed to the element at this time of the year.
is there any way to keep the house water tight while doing the roof?
 
Employ somebody to prepare construction drawings of a traditional site formed timber roof for you if that is how you prefer to do it.
 
Employ somebody to prepare construction drawings of a traditional site formed timber roof for you if that is how you prefer to do it.
what do they come under; architect, planner ..
 
If you've done the other drawings then you should be able to design the roof.

Google for " rafter span tables " or suchlike. But generally, it will be 5x2 rafters for a traditional gabled roof with purlins, or 4x2 for hipped roof.

But if possible, you need to seriously consider designing the roof to accept a room or a loft conversion at a future date - even if not for you but for a future buyer.
 
what do they come under; architect, planner ..
An architect or architectural technician would prepare construction drawings but as building control require structural engineer's calculations for structural work you might as well have an SE design the roof structure and members.
You would need to instruct the SE to design the structure accordingly if you anticipate a loft conversion at a future date as others mentioned.
 
An architect or architectural technician would prepare construction drawings but as building control require structural engineer's calculations for structural work you might as well have an SE design the roof structure and members.
You would need to instruct the SE to design the structure accordingly if you anticipate a loft conversion at a future date as others mentioned.
You don't need an engineer for a bog standard roof. My spaniel could probably do it.
 
You don't need an engineer for a bog standard roof. My spaniel could probably do it.
Woof. If building control still accepted roof timber sizes as given in the tables in superseded Part A ,yes. I don't think we know the configuration of Zehroni's proposed roof though do we , it might not be bog standard and Fido might struggle ?!
 

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