The answer is a resounding “it depends.” The theory is that the gable vents may disrupt the airflow from the soffits to the ridge. In theory yes, it should work. So why is it not working? The ridge vents are exhaust vents and the soffit vents are intake vents. The goal of a venting system is to remove heat and moisture from the attic. Let’s start with the ridge vent and work our way down.
1. The ridge vent is rarely installed properly. If the roofer remembers to cut the plywood at the ridge, more often than not the cut is not big enough, or he/she will only cut one side. (fig 1)
2. When the actual ridge vent is installed, it is typically a rolled style vent that completely collapses when the roof caps are installed providing little if any exhaust. (fig 2)
Soffit vent chutes: For whatever reason someone either forgets to install the vent chutes, or they collapse the chutes or they block them with insulation. (fig 3)
Soffit vents: (fig 4) The soffit vents provide intake air. The example shown is a standard two inch aluminum strip vent.
Here is an experiment for you. Take an ordinary box fan and place it in your open bedroom window. Turn it so that the air is being exhausted out of the room instead of blowing into the room. Open the bedroom door. Now turn the fan up all the way. Now shut the door. The fan speed slows down. It’s trying to exhaust, but there is no place to draw air from. The purpose of the intake or soffit vents is to draw or allow air in. Everyone installs a ridge vent when a new roof is installed, but no one is installing soffit vents.
If an attempt is made at installing soffit vents they usually fall the way of the ridge vent and are almost always installed incorrectly. (Fig 5) This is due to a lack of training and simple economics. A good quality and correctly installed venting system costs more money.
So now back to the original question. The answer is; the gable or roof vents may be making up for the improperly installed soffit and ridge vent system and in most (but not all) cases should be left in place and uncovered. Every house is different and no one procedure will work on every home.