How to Find a Stringer in Stairs

Climb underneath the stairs with a flashlight and shine the light up on the underside of the stairway. Any diagonal boards you see with toothed upper edges that are connected to the backs of the treads and risers of the stairs are the stringers. Most stairways have a stringer on each side; wider stairways may have one in the middle as well.

Locate the stringers from on top of the stairs, if the bottom is inaccessible. The outer stringer will be visible unless the entire stairway is enclosed within walls. In enclosed stairs, assume the two outside stringers are within 2 inches of the walls. If a middle stringer is present, locate it by measuring the width of the staircase and dividing by 2, since the middle stringer will almost certainly be at the center of the stairs.

Confirm the location of the stringers by driving a small nail through one of the risers. To make the hole less visible, drive it through the top of the riser, just under the lip of the tread. If the nail goes through the riser, then slides through, you've missed the stringer. If resistance continues for the length of the nail, you've hit the riser.