Timber Cantilevers

[addt]

Hi Lads/Ladies

I'm looking at attaching a supplementary member alongside existing roof rafters to create a cantilevers up to 1200mm. I'm wonder what is the required back span/overlap required. In this instance i have 200mm deep rafters and proposing to stitch 90mm deep overhangs. I wonder at what overlap length the rafters start to act as one. I have seen required back spans such as 1.5xL to 2.0xL (L is the Cantilever)from truss manufacturers. This seems a little excessive as the overhang is being fully supported along the overlap. Are these 1.5 and 2.0xL just guideline and rules of thumb or is there a design or analysis that can be carried out to determine the required overlap length and fixing requirements?

Cheers
AddT

 

[XR250]

1200mm seems excessive for 90mm overhangs. In US speak, this is a 4 ft. overhang on 2x4 rafters - which is alot depending on the snow load.
From a connection standpoint, usually going back 1x the cant length is more than sufficient.