It can not be stressed enough how important ventilation is to the longevity of your roof and how energy efficient your home is. If you want to get everything you can from your roof then the first step is to check if you have sufficient ventilation. The proper amount of airflow through your attic will accomplish multiple goals and when done properly will have a return on investment.
During the summer, heat will rise to the top of your attic without a way to dissipate. That lack of airflow will cause hot moist air to rise and accumulate. If that moisture cannot escape it collects and will cause damage to studs, insulation, roof decking and anything else in the attic. If you have shingles with nails popped then that is typically a symptom of bad airflow. Add in some heavy winds and you will be losing shingles or at the least those shingles will be compromised. Without the balance of cooler air flowing in and warmer air flowing out the possibility of a buildup of mold and mildew could occur. This buildup of mold and mildew could affect the health of your family and if it gets out of control could be expensive to alleviate. If you think that your HVAC system is working too hard then it could be because of the amount of heat that is leaking into the house from the attic. Heat rises but when it is rising against an envelope of warmer air than it settles. It settles back into your home causing your air conditioner to work harder and longer which creates higher energy usage.
During the winter, the problem with moist air rising, as mentioned above, is exacerbated because that warm moist air will condense on cold surfaces. Rotting insulation, rotting wood, and mold and mildew are all exaggerated but winter does present other unique complications. Bad ventilation causes warm moist air to collect at the top of the attic. The combination of that and the heat of the sun on the roof will melt the snow which runs down the roof to freeze again at the eaves. This cycle creates ice damming. The ice damming will force melted water up under shingles and leak behind siding rotting out roof decking and leaking into your home. The leaks are rarely noticed right away and the origin can be hard to find even for an expert roofing contractor. Thousands of dollars of damage can have occurred before the leak is even noticed.
If you are not sure your attic space is properly ventilated then you should call a roofing contractor and have it checked. The cost for such an inspection is minimal if not free. The cost to repair or replace a roof or repair the damage from water leaking into your home could cost you thousands.