making a 3x12 from 3x6

Are you sure you are reading the plans and the instructions right. Its usally to double up the rafter side by side around a Velux roof light, two 6" x 3" making a total width of 6" x 6" not 12" x 3". Theses are just nailed together with 4" or 5" nail from both sides. The extra rafter sometimes goes from ridge tree to centre perlin and somtimes from wall plate to ridge tree. or wall plate to centre perlin.

Keith

I'm not using velux - way way too expensive. The velux has depth, these windows I have orders don't they are a galzing bar system (like a conservatoory roof) that sit on a frame that I build, 50-75mm above the finished roof level. As I intend to have 100mm of kingspan "over rafter" I have a lot of height to gain. I can't have too much kingspan "between rafter" as I have exposed rafters.

The velux also look a bit dappy when you have too many, my window is

Rick

How about building up using three 3x2 sections, that way you could nail or screw using standard sizes.

Alternatively , counterbore the top 3x6 from above and screw down to the rafter - big counterbore/countersink and getting the angle right would be quite important.

Never tried anything like this BTW, but:

How about dowels/biscuits on each internal edge?

A bit esoteric but how about long dowels pegs instead of bolts, you will still be able to see the ends, but they will look nice.

Or recessed long bolts/studding and dowel plugs hiding them ... which might be easiest?

HTH,

Alex.

Use 9" Spax screws. B&Q stock them.

Thanks Rick