silicone alone will do it!

Silicone does not stick to wet surfaces, the roof as well as the wall will become wet, at least damp due to condensation. Expansion and contraction of material will lead to leaks.
If building 'green' one has to build airtight and well insulated.None of this can be done with the proposed construction method as far as I see.

The best solution for your lean to would be to use polycarbon sheets, giving some insulation and NO condensation. I have used 16mm "7 layers" and they are easy to install, and no leakage either. For sealing against the brick wall, the best method is to let a flashing into the bricks at a joint, and continue the flashing down over the plastic roofing a good 6-8"

@fridihem:
How did you manage the airtightness?

heinbloed,
 
Airthightness was not a MUST in my case.
 
My project was for a conservatory, with high spec windows and doors, insulated in the floor and walls. It was intended to be only used during the months, March to October, but with the winters being much much milder up here, I am using it even in the dead of winter, fun to be sitting "outside" with snow around.

Thanks
 
You can get plastic corrugated flashing designed for this style of roofing I think this is the way forward the cost just keeps rising!!

There is a perma-flexible material called 'compri-band', a self-adhesive tape which expands when laid into cavities, sealing them.
This tape allows for movements and still seals.
It looks like rubber foam,comes on rolls of 5-10m length and 10-100mm wide and is suitable for these jobs. Note that some of these compribands aren't suitable for interior usage due to off-gassing(the cheaper ones). But there are also many brands available which are fully certified for interior usage.
If you can't find it in the DIY market ask your door/window supplier.

screwfix do a flashing tape, been on for a year for my woodshed. elsewhere have used mortar with plasticisers think this will eventually fail but might get you through 10 years