Mastic under external door threshold plate?

[gavztheouch]

Hi I have some external doors to fit. My frame is 245mm studs with 60mm external wood fibre and then rainscreen cladding. My foundation is insulated raft and the concrete in the raft is the finished floor.

 

I’m going to use flashing tape, it’s pro clima extoseal to flash the threshold. The door will sit on top of this flashing on packers. There will be a gap I was planning to fill. The architect suggested packing with structural grout but I was thinking something like sikaflex structural glue/silcone would be better. Lay it down on a bed of the stuff, and hope once it’s sets it should be solid underfoot.

 

 How do you seal your external doors to the weather under the door frame?