It's one thing to know the theory behind roof shingle replacement and quite another to be able to judge the signs yourself. However, the best way for you to figure out when to replace the shingles on your roof is indeed to visually look for the signs yourself. If you do this on a regular basis (i.e. once every twelve months or so), you'll definitely be able to pinpoint when the time is right for shingle replacement.

When you do the inspection, you can't just settle for looking at the shingles from the ground in front or behind your house. You actually need to get on the roof and inspect them down to the smallest detail for this plan to work. You can get into the roof with a simple ladder and after a few tries you should be able to adjust to the incline of the roof in terms of your balance.

The first thing that you want to do when you get onto the roof is to inspect the shingles from an outward point of view. This means that you should look around the entire roof and see if there are any broken shingles. Once you've done that, your next step should be to check along the face of each shingle and see if there are any ripples in the shingles or waves across different lines of the roof. If there are, this is evidence that the shingles are starting to change shape under the erosive pressure and therefore might need to be changed depending on what else you can see.

Look closely along the different intersections as well to see if there is separation between the shingles and if there are any shingles that are starting to lose their adhesive touch. A good sign of the latter is loose black grit that can sometimes be a warning factor that shingles need to be replaced. If there is a lot of lose grit around your roof be very careful about walking, since old grit can go from sticky to slippery very quickly and present a walking hazard along the roof.

Finally, pick a few shingles at random and apply pressure to them. Start with light pressure and see if any of the shingles break apart as a result of that pressure. If light pressure is enough to break apart some of the shingles you test, your roof is very much in need of replacement. If light pressure fails, switch to medium pressure on a few of the tested shingles, noting that if any shingles break under medium pressure your roof may need to be placed depending on what you saw on the visual inspection.