Staircase handrails rely on the newel post — the large post that anchors the handrail at the bottom of the staircase — for support. If the post is loose, the railing becomes wobbly and offers little support for those climbing the stairs.
To repair a loose newel, bore a 1-inch-deep hole into the base of the post with a 1-inch-diameter spade bit. Angle the bit toward the staircase framing behind the post. Next, bore a 7/32-inch-diameter pilot hole through the post and into the framing. Switch to a 5/16-inch-diameter drill bit and bore a screw-shank clearance hole only through the post.
Use a ratchet wrench to drive a 5/16-inch-diameer X 4- or 5-inch-long lag screw with washer through the post. Check to be sure the post is level before tightening the screw all the way. Conceal the screwhead with a wooden plug cut from a 1-inch-diameter dowel rod.
One lag screw will steady most posts, but if it's still a little wobbly, drive in a second lag. This technique can also be used to bolster shaky handrail posts on a deck, porch, and gazebo.