The USDA recommends perishable food be kept safely out of the "danger zone," which means temperatures between 40 and 140 degrees Fahrenheit, while being stored, thawed or prepared. A frozen pork roast left out overnight will thaw from the outer layer on through, potentially leaving the outside of your roast in the temperature range where it can be exposed to harmful bacteria.

The best way to thaw a frozen pork roast is in the refrigerator. A small roast will thaw at about 3 to 5 hours per pound, while a larger roast may need 4 to 7 hours per pound. If you need the roast to thaw more quickly, place it in an airtight package to prevent leaking and set in a cold-water bath, replacing the water about every 30 minutes. A 3-to-4-pound roast can thaw using this method in 2 or 3 hours.

While it may seem counterintuitive, it is perfectly fine to cook a roast that is frozen -- it will just take longer. According to the USDA, a frozen roast will take about 50 percent more time to cook than if it were thawed.