Rungs, cleats, and steps of portable ladders (except as provided below) and fixed ladders (including individual-rung/step ladders) shall be spaced not less than 10 inches (25 cm) apart, nor more than 14 inches (36 cm) apart, as measured between center lines of the rungs, cleats, and steps.
Keeping this in consideration, what length of ladder do I need?
An extension ladder needs to be several feet taller than the height you need to reach. Keep these points in mind: The highest safe standing level is four rungs from the top of the ladder. The sections must overlap at least 3 feet on ladders up to 36 feet in length and as much as 5 feet on longer ladders.
How tall is the average ladder?
A person's maximum safe reaching height is approximately 4' higher than the height of the ladder. For example, a typical person can safely reach an 8' ceiling on a 4' ladder*. Extension ladders should be 7 to 10 feet longer than the highest support or contact point, which may be the wall or roof line.