What is the difference in a custom gable vent and a standard gable vent?

In the world of new construction, there is an advantage of using standard size gable vents that are readily available in most hardware stores. These vents are usually rectangle or triangle and primed white for painting. New construction has only the issue with proper ventilation and having enough net free air flow. Unfortunately, the standard gable vents found local may be too small for adequate venting of most projects and roofs. Common sizes include the 12 x 18 x 1 with a front nail fin. Usually the largest will be a 24 x 30 x 1 if it is a stock size. Contractors will buy these in bulk to save on remodels and new builds. Keeping this in mind, the common measures of yester year may differ from today. 50 years ago, a contractor grade vent was usually 24 x 30 for example. And today these are commonly 23 1/2 x 29 1/2 inches. These will still fit if they can be found. The real issue is when an old vent is smaller slightly than the new stock gable vent options. So the new options become a choice of cutting the hole larger to install a standard vent or to save the time and trouble of extra work and just order a custom size gable vent that will fit and work properly.

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