Method for attaching new 3x2 timber to existing 3x2 roof timber for Insulation

[Mosschops72]

Hi All - I am converting my garage and adding 100 PIR insulation board between the pitch roof rafters. The existing timber is old fashioned 3x2 and so I need to increase the depth by adding another piece of 3x2 to the underside of existing. This gives the 50mm ventilation behind the boards and the 100mm depth for the insulation.

My question then is what method would people advise for attaching the new 3x2 to the underside of the existing ? Nails seems to be the preferred or a bolt perhaps ? Any advice welcomed

Thanks,
Andy

 

[^woody^]

Screws

 

[Ian H]

Rather than adding 75mm of wood, it might be better to fully cover the rafters with your 100mm PIR.

Or if you covered them fully with 75mm PIR that might be better then 100mm between them.