How to Fix Squeaky Stairs from Underneath with Shims and Glue Blocks

From underneath the staircase, have an assistant walk up and down the stairs. Carefully listen to pinpoint the exact location of the squeak. You'll want to identify the tread (the horizontal part you step on) and the riser (the vertical part between treads) that are moving against each other or against the stringer (the support beam for the stairs). Mark the area with a pencil if necessary for easy reference.

Examine the area where the squeak originates. You'll likely see a small gap between the tread and riser, or between these components and the staircase stringer. Using a utility knife or a handsaw, cut thin pieces of wood (like scraps from 1x lumber or pre-made wood shims) that are slightly longer than the gap. The goal is to create pieces that can be tapped into these gaps to eliminate movement.

Once you've identified the rubbing points (usually where the tread meets the riser or the stringer), apply a generous amount of wood glue to these surfaces. If you're addressing a gap, apply glue to the edges of the shim material and the surfaces it will contact. This glue will help to bond the parts together and further reduce friction and noise.

Carefully tap the glued shims into the identified gaps using a hammer. The goal is to create a snug fit that eliminates any looseness. Once the shims are in place and the movement is reduced or eliminated, you can add glue blocks. Cut small triangular blocks of wood and glue them in the corners where the tread meets the riser and stringer, providing extra support and further preventing movement. You can also drive short screws through the stringer into the tread or riser if needed, but be careful not to split the wood or penetrate through the visible side of the tread.

Once all shims and glue blocks are in place and any screws are secured, have your assistant walk on the stairs again. Listen carefully from underneath to confirm that the squeak has been resolved. If the squeak persists, you may need to repeat the process with slightly different shim placement or try adding more shims and glue.

Wipe away any remaining excess wood glue with a damp cloth. Sweep or vacuum up any wood shavings or dust created during the process. Organize and put away your tools and materials.