If it's been a while since you did any roof maintenance, you may notice a nice collection of leaves, needles, moss and / or mold building up there. Cleaning your roof will always present some interesting challenges, especially when there are asphalt shingles involved.
First, use a soft broom, rake or blower to remove as much debris off the shingles themselves. This will help you make sure that any cleaning solution you use gets down to where it really counts. Just brush carefully so that you do not detach any granules or loose shingles and end up needing to replace them.
To keep those shingles in place and make sure you remove any debris, moss, or stains, you can create a mix of trisodium phosphate and water. Once this mix has been combined and sprayed onto the shingle, let it sit for a while. It should start to fade the stains, and then you can carefully brush off any mold or licenses that might have been building up.
If you're going to make your own cleaning solution, you can use regular bleach and water. This should be effective to kill any bacteria that are present and get rid of the stains that are on the shingles.
Regular soap and water is also an option if you're worried about the chemicals washing away or causing the granules to fall off. There is typically a warranty included with the asphalt shingles, but if you power washed them or other damage them with incorrect treatment, the warranty will be voided. The safest cleaning solution is soap and water, but there is nothing wrong with using a little diluted bleach to remove stains and algae builup.
While it's not technically a cleaning product, a copper strip can be installed to reduce the amount of future cleaning you'll have to do. Once you have cleaned your roof, install a strip of copper at the apex of the roof for both sides. Then, whenever it rains, this copper will leak down onto the shingle and help prevent algae from growing in the first place.
As an alternative to the copper strip, which can be expensive, many asphalt shingle manufacturers will suggest having a preventive chemical treatment applied to keep moss and at bay.
It's important to keep your roof regularly or you could spend a lot more replacing it. Make sure you either check it yourself or have a professional come in annually to determine if any trouble spots exist. Catching these problems when they're small and inexpensive is much more effective than waiting until you have a major problem on your hands. The manufacturer will also be a good source of cleaning information for you and can tell you exactly what products are safe to use.