Metal-Era commissioned Cermak Peterka Petersen (CPP) of Ft. Collins, CO (a wind engineering consulting and testing firm) to conduct this study and produce the resultant calculator based on their findings. The following are some of the key assumptions and limitations of this study.

Assumptions of the study include:

• Extreme summer heat and melting snow were considered two primary design situations.
• This calculator is based on assuming a worst-case scenario where conditions are least conducive to ventilation.
• It is assumed to be a very sunny and calm day with no wind.
• It is assumed that the nailbase air gap is unobstructed, in other words the spacers in the nailbase are not accounted for.
• The temperatures of the roofing nailbase and air are considered to be in equilibrium, therefore the heat capacity of the building is not considered in the calculations.
• The K value for the eave entry and ridge exit are based off of Metal-Era products of a K 5.

The limitations of the study are:

• It does not apply to situations such as when a sloped roof meets a vertical wall.
• The calculation is for a single face of a roof and the design of any airflow system for an entire roof was beyond the scope of this study. However, the calculations can be used to estimate the performance of each face or portion of a roof.
• Hipped roofs with ridge/eave length ratios under .4 experience a significant reduction in airflow and in general are difficult to ventilate. Also the performance of vented nailbase for r < .2 is so poor that some alternate scheme is necessary for the triangular sides of hipped roofs. Suggested schemes include allowing the flow to exit the air space below the ridge (possibly into a plenum or small attic space) or increasing the gap height of the gap near the ridge. As a result of this difficulty, care should be taken when applying this calculator to hipped roofs.
• The calculation procedure has not been validated against experimental data. However, each of the sub-models such as heat transfer and pressure drop are based on well-established physical principles.
• There are dozens if not hundred of potential roof nailbase configurations. It was not the goal of this study to examine all of these, but rather to provide a tool which can be used to help with the design of individual vented roof systems by predicting their performance for various configuration options.