To answer this question, you first need to determine what type of gutter system exists on your house. Do your downspouts drain into an underground tile system, or empty onto a splash block? Or, do you have bare gutters without, or too few or thin downspouts?
If your house has bare gutters, rainwater is likely draining next to your walls and pooling there. Over time, this water can lead to structural damage, such as cracked walls and ceilings. It can flood your basement and crawl spaces.
If you have too few downspouts, adding extra ones can back up the current one(s). If the downspouts drain into an underground tile system, there may be backup, leading to overflow at the gutters. Over time, soil movement and tree roots can cause blocked or leaking tiling.
Underground Tile System
If you have a house built around the turn of the 20th century, odds are you have an underground tile system for draining rainwater. You can check the point at which the downspout enters this system for pooling and backup.
To avoid structural damage, the best option is to cover these tiles with a prefabricated concrete piece. This eliminates possible joint damage between tiles. Otherwise, it is best to simply change your drainage system, as tiling is not effective and prone to problems.
If your downspout empties onto a splash block, make sure the block is large, and high enough off the ground to carry the water away from the foundation of the house. Make sure these blocks are not broken or deteriorating. You can increase the efficacy of this system by adding a length of PVC, prefabricated, or other flexible piping as an extension.
The key to this method is to be sure that the water is draining onto your property (your local building inspector can tell you the required minimum distance to drain from your neighbors property), and that it drains at least 10 feet from your foundation.
First, gather at least the following tools: for cleaning the downspouts and gutters that are already there, gloves, a ladder, a brush, and a plumbers snake will be handy.
For the installation, you will need a hacksaw, a line level, and a metal crimper. Accessories you may need (depending on the type of installation you choose) are screens, flexible piping, cabling, and extender pieces.
If you simply want to extend an existing downspout, you can add a hinged or automatic extension to allow it to be moved out of the way when not in use. To add a hinged extension, the end of the downspout must be clear of any elbows. First, use your hacksaw to cut the end off a length of downspout at a 45-degree angle.
Be sure to use a miter box to guide the saw, or a mitre saw. Next, drill pilot holes for ¼ inch aluminum sheet-metal screws or rivets. Finally, join the cut end to the bottom of the existing downspout with a hinge.
If you want to keep your underground tile system, you can extend it as well. First, dig a trench. Put an elbow and drain adapter on the house end of a 4 inch PVC drainpipe. Connect it to the downspout. Next, insert the other end into a catch basin or surface bubbler. The bubblers grate lies flush with the surface and allows water to spill onto the ground.