Channel drainage installation

[Danbarratt7]

I’m looking to install 3m of channel drainage to pick up the rainwater from a new pitched roof and take to an existing gulley.
I’ve got a 9 inch grinder to cut concrete, looking for advice on how far to dig down and if I have to put a concrete bed below the channel?
I’m not a plumber but not shy of DIY.
And advice is welcome

 

[bobasd]

A pic please?
How thick is the concrete?
You can get advice on here but "pavingexpert.com" is worth a look for his great diagrams and pic.