This is designed to provide convective airflow through the space with the air entering through the soffit and venting out the gable or ridge vents. The vents are required to be on opposite sides from each other and within 2 of a corner. In addition there are adjustments to these ratios to take into account the airflow restrictions caused by the grills and mesh characteristic of all vents. All attic spaces are required to be accessible with a specific size access panel not less than 22 inches by 30 inches when the attic is 30 inches or more in height. All crawl spaces require an access panel of not less than 18 inches by 24 inches in size.
All new roofs require ice dam protection.
Make sure you indicate this in your design for two reasons, our approval, and so your contractor is aware and can include the cost in his bid or proposal. If you have an existing hot air furnace and plan to extend the ductwork to heat or cool the new addition, be careful not to ruin the balance of the existing system. If you take too much air from a main trunk or branch line, you might be cold in the far edges of the house. Be sure the furnace has the BTU's and can handle the additional CFM requirement the new rooms will require of the system. If all is okay then you need to provide a drawing showing the new ductwork with size and outlet CFM capacity.