When my house was built it seems the designer did not realize the importance of having a well ventilated attic.  As such, the attic over my garage/family room  has only one gable vent.  On the other end of the attic is a wall, and there are no other vents in the roof.   The lack of additional vents renders the single gable vent useless, as there is nowhere for air to come in from.   The lack of ventilation causes two serious problems.  First, the attic becomes extremely hot, as all the energy from the sun beating on the roof has nowhere to go.  This will increase your energy bill in the summer, as the increased temperatures will cause an increase of heat transfered to the conditioned space in the house.  In addition the increassed temperatures will decrease the life of the roofing material.

Secondly, the lack of ventilation will cause a buildup of moisture in the attic, as the humid air leaks into it from the rest of the house.   Moisture buildup is never good, and in the attic it can cause a handful of problems including mold growth on the rafters (which was noticed by our home inspector).

With the attic all open and easily accessable I’ve taken the opportunity to add in some soffit vents.  These vents will allow cool, dry air to flow up into the attic, and go out the gable vent.  This still isn’t ideal though, as the gable vent is only at the one end.  Therefore when we redo the roof, I’ll also be adding a roof vent and that will greatly increase the air flow.  Adding the soffit vents is quite easy, although all of the work is done over your head, which makes it much harder.  All i did though was to cut some square holes with a jig saw and then screw some vent plates over them.