Which sealant for this job, please?

[ToughButterCup]

Morning  folks.

For this week's DIY challenge I need to make my own flat roof. The roof bit is not too difficult. I have almost settled on a design.  Its the box gutter that concerns me.

 

The closest I can find for the design is this YT video. (0:08)

See the black stuff that's between the layers of OSB - what is it?

 

Ta!

[Construction Channel]

I would have said it’s gutter seal or similar. Loads of movement and sticky as hell. 

 

Everbuild ROOF Roof And Gutter Sealant, Black, 310 ml https://www.amazon.co.uk/dp/B003OVCCU8/ref=cm_sw_r_cp_api_rUhTBbMVYM3YY

[ToughButterCup]

Ta.