Ducting Systems and Air Ventilation - Bathroom ventilation problem

I have a small bathroom, 40 sq. feet including shower that does not ventilate the steam out when showering. The current vent is in the ceiling, has a 3" duct outlet, rated at 70 cfm. I have replaced the old ducting with 4" rigid duct and connected to the roof vent which has about an 8 foot run. It still does not remove much steam. when testing the air flow when replacing the duct, it seemed to have good airflow before I connected it to the roof vent. What else can I do --replace the fan with a higher cfm unit? :confused:

70 cfm is not a lot for 40square feet. Id replace with 100cfm

Thanks. So when the manufacturer claims that 70 cfm is good for up to a max of 85 sq. ft. that is really not the case?

Not for good IAQ! Also fan should run for at least 20 minutes after a shower.

OK, thanks. I'll check into a new one. I know in my last house,I had a larger bathroom with a 70 cfm fan and it cleared steam within a minute of two - with the door closed. That's why I wondered is it also to do with the length of duck and location of the outside vent.

I put my shower vent fan on a timer switch. You just give the timer a twist when you go into the shower and then it turns off automatically after the time is up.

The switch (http://www.homedepot.com/h_d1/N-5yc1vZ1xr5/R-100396657/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053) mounts just like a normal on/off switch so they are easily retrofitted. Some older houses have very shallow boxes or if your electrical box is full of many wires you may not have room.