A rain diverter is a shaped slit of metal design that fits at the edge of your roof and keeps rain from falling in front of doorways or other areas you want to protect from streams of water. Rain diverters are only used on houses that do not use gutters, since gutters are designed to protect the entire house in areas that experience frequent rain or snowfall. Rain diverters are only designed to protect certain parts of the house in climates that do not experience frequent rainfall.
Rain diverters can be bought at local hardware stores, but they are essentially pieces of sheet metal shaped in an L. You can make your own using a piece of sheet aluminium and clamping the first 2 inches down at a right angle from the rest of the piece of the metal (which should be about 8 inches across). In total length, the diverter should be about 2 feet longer than the door on either end.
For installing the rain diverter, you will need a plumb bob, a putty knife or similar tool, roofing nails and a hammer, and sealants like caulk and roofing cement. Be sure you also have a stable ladder to climb up and install the rain diverter, along with the necessary implements to clean the diverter from time to time, such as a long broom or a brush.
Rain diverters are very easy to install, but you first need to peel back some of your roof tiles. Asphalt shingles work best with this process, and other types of tile may be difficult to put back. To find out what shingles to move, plot the position of your diverter using a plumb bob and whatever other tools you need, such as a level and tape measure. A plumb bob is a weight tied to a string that allows you to measure direction and angles based on the vertical line it creates. By positioning the plumb bob on the diverter, you can centre it at the position on the roof above the door or other area you want to protect.
Once you have figured out the spot you want the diverter to rest, focus on the layer of shingles directly above that position. Use a putty knife to loosen enough tiles in that row to slide the long part of the diverter underneath the shingles---use care so that the shingles do not break and you do not damage the roof underneath the shingles.
Once the diverter is positioned underneath the shingles, nail it in place with the roofing nails. Position the nails a few inches above the diverter's angle, where they will be hidden by the shingles. Now you need to make sure no water can leak underneath the shingles. Use the roofing cement and daub it on each of the nails, then line the borders of shingles in the gaps and press the shingles back down. Be sure to do this project when the air is dry and you aren't expecting rain!