If the outside stair carriage has bowed outward, use a 2×4 jammed against a near wall to push the carriage back into place. You can instead use an adjustable screw column horizontally to push the carriage back, but nail the column’s top plates so it can’t fall.
Where a carriage has separated from its stringer, clamp the pieces together; then add two J4-in. lag bolts, staggering the pairs of bolts every 18 in. along the length of the boards. If a carriage has pulled free from a stud wall, reattach it with washered lag bolts. Where a carriage is attached to a masonry wall, drill through the carriage into the masonry, using a carbide-tipped masonry bit. Slip a lead sleeve into the hole and expand the sleeve by tightening a washered 3/8-in. lag bolt into it. To forestall rot, slip a piece of 30-lb. building paper behind the carriage before bolting it down.
Occasionally, stringers or carriages come loose at the top and bottom. In a well-built staircase, the upper ends of carriages are nailed to the inside of the header above; the lower ends of those carriages sit on, and are nailed to, the doubled joists of the rough opening below.
However, sometimes the lower ends of center carriages are mistakenly nailed to the inside of a RO header; in time the nails pull free and the carriages slip down. Jack up the fallen carriages, using a plumbed, adjustable column securely footed on the floor or atop a 4×8 beam on edge. To create a flat jacking surface for the top of the column, screw a triangular piece (with the same slope as the stairs) to the underside of the carriage. Should old nails resist your effort, cut through them with a metal-cutting blade in a reciprocating saw. Goggles, please.
Jack up the center carriage and join its lower end to the header with steel connectors or f4-in. right-angle mending plates. Secure the plates to the header with 18-in. lag bolts and through the bottom of the carriage with й-in.-diameter carriage bolts. It’s not usually necessary to use mending plates on the upper end of the carriage because the lower end is bearing most of the weight.
Inadequate support for the middle of a staircase can lead to split treads or major failures. Where a center carriage is not sawtoothed to receive treads, add plywood supports beneath each step. Cut support blocks from scrap plywood h in. thick. Then glue and screw them to alternate sides of the carriage—one per tread. If the stair sags in the middle and has no center carriage, add one.
You can replace finish surfaces after the carriages are bolstered and reattached and the stairs and balusters are reinstalled. Be sure that the nailing plane on the underside of the carriages is flat, shimming as needed. To reattach plaster lath or drywall, use type W drywall screws (hammering drywall nails can crack surrounding materials).
