To correct the existing stringers, the author cut 3/4-inch OSB blocks that he then leveled and glued and screwed to the stringers. On this project, the stair is finished with drywall on its underside, which necessitated having to chamfer one corner of each block for clearance.
A 1-by and level are used to check the alignment of the corrective blocks across the stringers.
The author used a check-block template to register the corrective blocks, butting its stop-piece to the edge of the preceding block below. A stop-piece added to the template (shown on the right) helps register it against sequential blocks below.
A multi-tool was used to trim the stringers where they projected past the new blocks.
The author added 3/16-inch shims to the riser face of every correction block to allow room for a full 10-inch-wide top tread. (When the author reached the top of the stairs, he discovered the landing hadn’t been built parallel to the stair layout, which necessitated his having to trim 3/16 inch from the last block.)
The dropped landing was out-of-level, which the author corrected with progressively thicker shims on 8-inch centers to support the oak flooring added later by the flooring installer.