So I am going to install a new fan and duct in a relatively small bath (8x6.5x8.5). I am looking at an 80cfm Panasonic whisper. Currently there is very poorly installed 4inch flexible duct that I plan to replace. Run is about 3 feet horizontal, 90degree and about 5 feet to exhaust out roof vent. Here is my plan and couple of questions. All input appreciated. My thinking is to use flexible metal from the fan to the elbows stretched tight with no bends. That will lead into 2x45* elbows to make the 90* to the roof. From the elbow to the roof, use rigid metal to the roof vent. Mastic and tape on all seams. Questions Fan supports 4in or 6in duct. To use 6 I would need to use a hole saw to expand the roof vent. Is this worth it or stick with 4 and just upgrade the vent? For the horizontal run should I use a 1/4" slope downward, keep flat or slight slope upward? My thinking is the slight slope down to prevent runback of condensation into the bath. Any other thoughts?