Whatever your downspout and gutters are made of they will need replacement in time. Basic maintenance requires that you clean the gutters and downspouts twice yearly, removing excessive debris and making sure rust is not accumulating and weakening them. However, even with this maintenance, replacement is a matter of when, not if.
When to Replace Galvanized Steel Downspouts
If you notice that your downspouts are leaking every time it rains, rust may have built up to the point where holes in the downspouts have appeared. This means they need to be replaced in order to maintain their effectiveness. As a rule, if you can see rust on the outside of the downspout when looking at it from the ground, they probably could stand replacing.
When to Replace Plastic Downspouts
Plastic downspouts will wear down after time and weather damage as well. Look for discoloration and fading of the downspout coating. Just because they do not rust does not mean they aren't worn down.
When to Replace Aluminum Downspouts
Aluminum lasts longer than steel, but it dents easily. If you notice cracking, chipping or curling at the edges, that could mean the downspouts should be removed and replaced.
Galvanized steel downspouts need to be replaced more frequently than those made of plastic or aluminum. However, if the downspouts on your home are 40 years old or more, whatever the material, they should be inspected and possibly replaced.