Patio design dilemma

Happy Easter All

Here is one for all of the thinkers out there .

I am trying to build a patio / roof extension to my house. 4.2 m wide 8m long .
I need the new roof to match the old one in pitch and thickness .

The pitch is the easy part but i am having trouble with the thickness The existing roof is 162 mm from top of battern to bottom of rafter .
But i am planning to use 76x38 x 2.0 m patio tube and the batterns will be the same and top mounted but his adds up to 152 mm .

My question is what is the best way to get the extra 10 mm i need to make the two rooves to match?

I am using patio tube to keep the wieght and cost down .

Would it be best to use spacers when screwing the tin roof down which seems a bit hard, add 10mm wooden batterns to the top of the patio tube batterns or add 10mm wooden batterns to the bottom of the rafters???

The bottom of the raffters will be covered in villa board once to roof is finished .

Thanks,
Rob

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