How is the Net Free Area of a Ridge Vent Determined?

Determining Net Free Area for Ridge Vent Doesn't Have to be Hard

By Andy Swan, Roof Saver

I had a great phone call recently from a carpenter teaching high school students about roofing in her building trades class. One of her students challenged her to explain how Roof Saver® ridge vent could possibly offer 15.2 square inches of Net Free Area per linear foot. The instructor was puzzled so she called me.

The students understood that the area filled by the ridge vent between the cap shingle and the roof deck is ¾” high by 12 inches per foot of ridge. “Picture the rectangle formed by that cap shingle suspended ¾” above the roof deck with nothing but air in between.”  The computer key strokes were loud enough that I could tell she was calculating the area of the imaginary rectangle and she said, “OK, that gives me 9 square inches.”

“Yes, I replied, and you have that on each side of the shingle for a total of 18 square inches of open area per linear foot.”

“But your literature says 15.2 square inches per foot?!” she groaned.

“Right, so now we apply the vent media open volume factor calculated by an independent testing lab to get the gross area and further reduce that figure by 10% per the International Building Code requirements for mesh-type vents and arrive at 15.2 square inches of Net Free Area.”

“Oh, now I see it. The two rectangles formed between the cap shingle and roof deck offer 18 square inches of open area for air to flow. But the fibers in the vent media reduce the open area so the actual Net Free Area is 15.2 square inches per linear foot.”

I could hear her formulating the question for the next quiz.

Andy Swan is president of Blocksom and Co., manufacturer of Roof Saver Ridge Vent. For more information on Roof Saver visit www.roofsaver.com.