Garage roof joist sizing and span - does this seem ok?

[happypeter]

Hi,
2.6M span with 125mm X 47mm C24 @ 600mm centres. Topped with 18mm OSB and epdm rubber. Its a sectional concrete garage (2.6M X 4.8M) looking to replace current roof with a new flat one. Does this sound ok?
many thanks

 

[Alastairreid]

 

[happypeter]

Ok thanks looks like it will have to be 125mm @ 400mm spacing. Seems like overkill especially when my old shed is only using 50mm thick joists!

 

[happypeter]

Now you got me looking at tables it does seem to be open to interpretation Is there much difference between c16 and c24? tanatlised VS untreated?

 

[Alastairreid]

Treated v untreated relates to its protection against insect, rot, mould attack etc.
For a garage project with a 2.6m span I wouldn't loose any sleep over c16-24.
I would use the deeper joists to prevent sagging under snow loads and foot traffic.

 

[happypeter]

Treated v untreated relates to its protection against insect, rot, mould attack etc.

Thats exactly what I thought but some wood suppliers suggest it is also stronger as below https://www.daviestimber.co.uk/blogs/news/pressure-treated-timber

"Strength. Pressure treated timber has stronger properties than untreated timber, so can stand much more force. When installed it will provide a sturdy construction."

 

[Alastairreid]

Thats exactly what I thought but some wood suppliers suggest it is also stronger.

Treated v untreated has the same strength.
Salesmen are economical with the truth.