timber frame

[stmull]

Hi.

i could do with some help. i am looking to build an 8mtr x 5mtr garden room with a flat roof. The walls will be wooden frames with osb, insulation and internal plasterboard. wooden cladding on the outside.

I'm not a builder but feel i could manage this,. My concern is the load on the roof and the large open plan interior.

would the roof beams hold the structure and prevent the walls being pushed outward or would I needs some kind of ties length and width?

Also should I space the rafters at 400 or 600 for the roof.

Thanks for any advice. :?