How high do you need to go to do your job? For jobs up to 20 feet in the air, you can get by with a stepladder (a ladder consisting of two sides hinged at the top). A general rule of thumb to decide the height of stepladder you need is to take the height you need to reach, and subtract 4 feet. So, if you're trying to get 14 feet in the air to clean your rain gutters, you need a 10 foot stepladder. If you're trying to paint your bedroom with an 8 foot ceiling, you need a 4 foot step ladder. If you are tall, or have an exceptionally long reach, you could get away with a shorter ladder, but why risk it?
For an extension ladder, things are a little different. Extension ladders are ladders with two or more sections that fold or slide to make the ladder longer. These will generally get you high enough to do jobs about 37 feet above the ground. A good rule of thumb with extension ladders is to take the height you need to reach and ADD one to three feet, so if you need to paint your eaves at 27 feet above the ground, look for an extension ladder that is 28 to 31 feet long. The added length is especially important as you get up higher, so while you may only need a 16 foot ladder to work at 15 feet, you'll want the full 40 foot ladder to get up to 36 or 37 feet.
Load Capacity
The next consideration is load capacity. It's not one that people think about a lot, but it's really important especially if you are carrying tools or supplies up on the ladder with you. A typical household ladder is rated to hold about 200 lbs. That's not very much if you're a man over 6 feet tall carrying a full tool belt, gallons of paint, heavy electric tools, or a piece of plywood. On the other end of the spectrum, the heaviest duty commercial grade ladder will hold up to 375 pounds. Take your own weight, the weight of your tools into consideration when choosing the right ladder.
Materials
A final consideration for choosing a ladder is the materials the ladder is constructed of. Ladders are generally made of one of three things: wood, aluminum, or fiber glass. Let's state first, that if you are doing electrical work, you must choose a fiberglass ladder, as it is the only one that is non-conductive at all times (wood will conduct electricity when it's wet.)
Wood ladders are cheap and common. Go with wood when price matters, you don't weigh very much and you don't need to get very high. Don't go with wood if you can't store it indoors as it will weather quickly.
Aluminum ladders are strong and light weight. They resist corrosion and are probably the most common ladders these days. They will however, conduct electricity, so don't use them near power sources (outside near power lines or when doing any wiring.)
Fiberglass ladders are strong and non-conductive. They are a great value and won't conduct electricity.
Check all three types out before selecting the material you prefer.
