Moss grows in situations where there is shade and damp conditions. In some cases, it might be in a location that is perfectly fine, such as a wooded garden. In other cases, however, moss is in a location like a patio or sidewalk that might cause a dangerous situation when people walk across the sometimes slippery vegetation. To remedy this, it is best to kill the moss and then ensure that it does not return.
Spray pre-mixed Roundup on the moss-covered area. Soak it well, at a time that you know that it will not be washed away by rain.
Wait for 24 to 48 hours for the moss to die. If it doesn't completely die out with one application, spray on a second time and wait again.
Wash the dead moss off the patio or paving stones with a pressure washer. Use a low pressure setting so that you do not damage the surface of the stone or concrete. It is easier to fully remove the moss after it is dead.
Remove limbs and other objects, if possible, to let more light into the area so that the moss doesn't grow back.