I got just what you want...
The carpenter bees are actually pretty easy to eliminate. They don't make hives, as do many bees, they LIVE and breed in the holes you are seeing. They prefer unpainted wood with low moisture content, and they usually bore into the wood from the bottom (dry) side to prevent getting wet when it rains.
Easy to treat:
1. Paint the wood, especially the bottom sides of the joists. Easy on garage rafters, totally sucks on ground level front porches.
2. Get any common, over-the-counter insecticide, and spray it into every hole you find. EVEN last year's holes. Solvents like brake cleaner work well, but they evaporate and go away. Put something into the hole that will be there next year, too. They re-use old holes, unless you make it a bad place to live. One gallon "pump upo" yard sprayers work real well for this.
3. Plug any holes you find with whatever is easiest. Normal houshold caulk works well. It is most satisfying if you do it after dark, when they are inside the holes.
4. If the problem persists, "nuke" the wood surfaces with a harsher insecticide from somebody with a pesticide applicators license or spray the wood with something really vile like creosote or used motor oil [THINK FIRE HAZARD AND HAZARDOUS WASTE issues].
Be aware that some of the little buggers might just be buzzing through, looking for a good home. They might be overcrowded at your neighbors house.