What happens with soffit vents but no ridge vent, or a ridge vent with no soffit vents?

Soffit vents, no exhaust

Some homes have soffit vents and nothing else, or only a couple of small exhaust vents. Wind moves a little air, but heat that collects at the peak stays there. Signs include a very hot upper attic in summer, mildew on the upper deck, and shingles that age faster on the upper part of the roof. The fix is adding exhaust high on the roof: a ridge vent where the ridge is long enough, or static vents near the peak where it is not.

Ridge vent, no intake

This is more common than it should be, especially on houses with no overhangs or where blown insulation has buried the soffit vents. With no intake, the ridge vent pulls air through ceiling light cans, the attic hatch and gaps at the tops of walls, which means venting cooled air in summer and drawing humid air through the house. In storms it can pull wind-driven rain in through the vent. The fix is opening intake: clear and baffle buried soffit vents, cut vents into solid soffits, or install edge intake vents on homes without overhangs.

Getting the balance right

The code's 1/300 option assumes 40 to 50 percent of the vent area is high and the rest is low, and most manufacturers ask for at least as much intake as exhaust. Check the attic on a sunny day: with working intake, you should see daylight or a clear baffle channel at the eave in each rafter bay. More in roof ventilation basics and ridge vent vs box vents.