The result will be different depending on the way the roof is framed. With an open attic, it will simply act to reduce to overall amount of airflow that results from the chimney effect, thus increasing the heat trapped in the attic, affecting the longevity of the roofing material and increasing air conditioning bills. In certain cathedral type roof framings, individual spaces between rafters could be cut off from any ventilation at all, causing severe buildup of heat.
The apex of roof is basically the highest point of the roof where ridge cap and ridge roll of a roof is located.
To seal the ridge from weather. If a ridge vent is used it is just that, a vent for the roof.
The highest part of the roof is the ridge.
The highest point of each roof area.
peak or ridge or ridge cap
The top of the house had a ridge on it.Our house has a ridge on the top of the roof.
Ridge
The highest point, the peak of each gable or hip.
For a typical gable roof you have a ridge board at the top with rafters running from the ridge to the top plates of the walls
roof, peak, ridge
The ridge.
You would not have to add a ridge board, particularly if you are planning to build the addition roof directly over the existing roof.