Rafter size and ledger plate on single storey extension

[xdrive]

Hi.

I have a 4 metre extension. I'm trying to work out rafter size. I'm trying to achieve a 15 degree slope.

Will be using interlocking double roman tiles.

The clear span from ledger to wall plate is 3.68 metres.

I will be using c24 rafters with no purlin.

There will be 2 sky windows.

The ceiling inside will be lowered to 2.4m height and finished with plasterboard.

The span table suggests 2x7 c24.

1. Would the deadload be no more than 0.5 or between 0.5 to 1?

2. Is 2x7 about right for that span and pitch? I've been told by a builder 2x6 would suffice.

Will use 2x7 ledger plate also if 2x7 rafters.

3. Is rafter span horizontal length from ledger plate to beginning of wall plate start of birds mouth? Or end of wall plate after seat cut?

4. Do the rafter sit on top of ledger plate or sit in hangers? That would determine height of ledger.

5. Are air bricks necessary on side of roof?

Cheers

 

[23vc]

The top plate doesn’t need to be as deep as you birdsmouth the rafters onto it so most of the rafter plumb cut sits against the wall
Span is distance along the pitch not horizontal

 

[tony1851]

Open (vaulted) ceiling or conventional flat ceiling with separate ceiling joists?

 

[xdrive]

Open (vaulted) ceiling or conventional flat ceiling with separate ceiling joists?

Conventional flat ceiling with separate ceiling joists

 

[tony1851]

Your dead load will be in the 0.5-1.0 range; 7x2 C24 @ max 400 centres would be needed.

6x2 would deflect too much.

 

[^woody^]

The clear span from ledger to wall plate is 3.68 metres.

3. Is rafter span horizontal length from ledger plate to beginning of wall plate

Span is diagonal distance. Is your 3.68 "span" diagonal or horizontal?

 

[xdrive]

Span is diagonal distance. Is your 3.68 "span" diagonal or horizontal?

The above Span is horizontal. The diagonal clear span is 3.8m