If you're contemplating using a step ladder on your stairs, take a step back and reconsider. The bottom line is that conventional A-frame ladders simply aren't designed for use on stairs, and using them improperly often results in accidents. Instead of risking your safety, you'll want to use ladders made for use on stairs or purchase special equipment that can help you use an A-frame ladder safely in a stairwell. If you do it properly, you can also make your own miniature deck to spread across two ladders, providing a firm platform to stand on while you work.
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Purchase a Stair Ladder
If you find yourself working in stairwells often, consider investing in a stair ladder. Specially designed, these ladders allow you to easily adjust the ladder's legs, making one side of the ladder longer than the other as needed. Some models even allow you to adjust all four legs independently so you can safely work in tight spaces, like spiral staircases, where you may not be able to get two ladder legs on a single step.
When using a stair ladder, you must make sure you lock the legs in place once you have them properly positioned. This will help ensure that the ladder doesn't shift beneath you while you're working. As always, when working on a ladder, avoid reaching too far. Keep your hips between the uprights of the ladder at all times. If you can't do that and reach where you need to reach, reposition the ladder so that you can.
Use Ladder Aids and Tools
If you're not prepared to replace your ladder, you can use one of two available tools to make your current ladder safer. One option is to use a ladder leveler. A leveler is a set of telescoping legs that attach to the outside rails of your ladder. You can adjust their length as needed and then lock them safely into place before climbing the ladder. These work well on stairs and uneven ground.
Another option is to use an extension ladder support. As you've likely guessed based on the name, extension ladder supports are used with extension ladders rather than A-frame ladders. This device looks like an inverted letter "L." The top of the unit is a shelf on which the feet of the ladder sit. This shelf has a raised lip around the edge that keeps the feet of the ladder from sliding off.
Beneath the shelf and off to one side sits an adjustable metal arm, or prop. The shelf moves up and down along this arm so that you can adjust the support to the desired height. Simply place the leg of the ladder support on a step and then adjust the shelf so that it rests on another step at a 90-degree angle. Lock the shelf in place and you can sit your ladder flat on the shelf and work safely.
Build a Ladder Deck
Another option is to build your own scaffolding. In order for this trick to work, you need two ladders and an overhanging wall. First, grab an A-frame ladder and set it up on the landing at the top of the stairs. Next, lean an extension ladder from the steps onto the overhanging wall above the stairwell. Now, measure the span between the two ladders.
Build yourself a small, wooden deck narrow enough to fit between the ladder rails and long enough to span the distance between them. Underneath each end of the deck, install wooden stops so that the deck won't slide off the ladder but instead catches on the rung. Carefully position your deck between the two ladders and stand on it while you work.