roof fall on a garden room

[Jupiter01]

I am building a garden room which will be 6.5m long and 3m wide. The walls will be built from 6x2 stud walls.
I was planning to lay the roof joists along the width of this room.

Can I use 4x2's for this or does it need to be 5x2's?

Secondly, I want to have a slight fall on this roof (towards the back). I've not previously had to do a bird mouth cut and wonder if the easiest solution would be to build all the walls to be level and then fit a small wooden strip on top of the front wall - to raise this very slightly - this will then give me the fall and I could use joist hangers to fit the joists on the front and back. Could this approach work?

I'd appreciate your thoughts on this. Thanks in advance.

 

[stevie888]

You really need 5s and even C24s would be overspanned strictly by TRADA tables, but fine for a garden room.

Falls so slight wouldn't be birdsmouthed: stick your packing piece on top of your topplate and land the joists on top. 50mm will give you 1in 60 which is perfick.

 

[Jupiter01]

and even C24s would be overspanned strictly by TRADA tables

Sorry, but not sure what this means. Can you elaborate please?

Happy to use 5x2s if that's necessary. 600 or 400 centres for this?
I can use a 50mm packing piece. Do I need joist hangers or shall I just angle drill the joists?

Thanks for your help.

 

[stevie888]

Overspanned is builderspeak for a longer span than listed in the tables. These spans are determined by deflection with a massive snow load so in the real world you're fine.

I'd stick to 400crs as chances are you'll want to go on the roof once in a while and you don't want excessive deflection in the decking.

Your joists will simply sit on top of the sides with a nice overhang to keep weather of the sides. 3.6m timbers will suit nicely and dead easy to find.

 

[Jupiter01]

just one more - i assume there is no need for joist hangers? Just angle screw them?