kitchen ducting

Hi all So we have just installed our new kitchen which is at the rear extension of the house. I have a 80cm Elica Canopy cooker hood Unfortunately no access for the ducting to go horizontally and out through a wall, has to be through the roof. The roof is a low pitch roof, approx 12* degrees...

We are installing a fan which has a 150mm ducting outlet. Unfortunately the path will only allow a 100mm hole to be drilled. After the hole there will be a 90 degree bend followed about a 1.7 meter straight path out. I was wondering if it would be better for that 1.7 meter length to stay as...