Round soffit vents?

[HipGnosis]

My house doesn't have much attic ventilation.
It needs a new roof, so I'll have the roofer put in/on ridge venting.
It also needs soffit vents.
The round vents seem much easier to install - just buy the right size hole saw and figure out where the joists are.
Do they vent enough?  Or will I need to install so many that they're not practical?

[zolotiyeruki]

Can you post a link to the vents you're thinking of installing?  Generally, you want to make sure that the area of the exhaust is equal to the intake area.  The airflow will be limited to the smaller of the two.

[farmerj]

If I can piggy back -- what is this sort of work categorized as? I have some soffit work I'd like performed but the roofers I usually work with told me that it's not their thing.

[HipGnosis]

If I can piggy back -- what is this sort of work categorized as? I have some soffit work I'd like performed but the roofers I usually work with told me that it's not their thing.

Adding vents to existing soffit is handyman work.
Adding (or replacing) vents as the soffit is installed is siding work.