Light fixtures come in a variety of sizes, but they have several things in common. They need to connect to your home’s electrical supply. Most come with a mounting plate, a flat surface that connects to your exterior wall. If your home is covered in wood planks or relatively smooth covering, the mounting plate fits neatly against the side of your house. If you have vinyl or wood siding, the mounting plate can be too large to fit on one slat of siding, leaving a chunk of it hanging loose. If it does fit on the slat, the light fixture will end up sticking out away from your home at an angle, ruining its appearance.
The mounting block forms a collar around anything that needs to penetrate siding, like a water spigot or a light fixture. It consists of a flat face, usually surrounded by a trim border to make it look more attractive. If you are installing new vinyl siding on your home, the manufacturer will make mounting blocks in various sizes the same color and texture as your new siding. You can drill holes in a mounting block to allow access to electrical wiring or plumbing.
Using the mounting block is easier than trying to cut out the exact size hole you need to affix a mounting plate to the side of your house -- you can use a smaller mounting block for a smaller plate and a larger block for a larger fixture. A mounting block protects the electrical wires and exterior of the house from water, dirt and other elements better than a light fixture installed without one.
Vinyl siding expands in warm weather and contracts in cold. If you install the mounting block flush with the siding, they will rub as the vinyl siding shifts. Over time, this can loosen the siding and destabilize the light fixture. Leave a ¼-inch gap between the mounting block and the siding. When installing new siding on your home, put the mounting block in place first, then install the siding slats around it.