Finishing the area beneath a staircase transforms unused space into a functional part of the home. Covering the exposed framing with drywall creates a clean, integrated look, whether the space is used for storage or as part of a finished room. While the task might seem daunting due to the complex, non-standard angles involved, achieving a professional finish is possible with methodical planning and precise execution. The unique challenge stems from securing and finishing panels along the sloping stringer. Successfully completing this project depends on proper preparation of the underlying structure and accurate transfer of the geometry to the drywall sheets.
Preparing the Underside Framing
Before securing any drywall panel, the underlying structure must provide a consistently flat and stable surface for attachment. Framing members should be spaced no more than 16 inches on center throughout the area. This spacing ensures that the drywall, typically 1/2-inch or 5/8-inch thick, remains flat and resists movement or cracking after installation.
The main stringer, the large angled beam supporting the stairs, often presents the biggest challenge because its narrow edge does not offer a wide enough surface for securing the drywall’s edge. To correct this, wooden blocking must be secured perpendicular to the stringer’s side. This creates a continuous, flat mounting surface that follows the exact pitch of the staircase, providing the necessary backing for the angled cut of the drywall panel.
Inspect the void for any mechanical or electrical components before the surface is sealed. Any low-voltage wiring, such as for lighting, or plumbing lines should be routed and secured away from where the drywall screws will penetrate the framing. Addressing these utilities now prevents the need for disruptive repairs or rerouting after the drywall installation is complete.
Measuring and Cutting Angled Panels
The most unique aspect of dry walling the underside of stairs is accurately determining and transferring the slope of the stringer to the panel. Measure the pitch of the staircase using a long, straight edge placed against the wooden blocking and an adjustable angle finder. This tool provides the precise degree of the slope, which must be replicated on the drywall sheet before cutting.
To transfer this angle, mark the measured degree on the sheet. Use a T-square or long straightedge to score the paper face of the panel with a utility knife. Scoring must be done with firm, continuous pressure to ensure a clean break along the intended line once the sheet is snapped backward. For straight, perpendicular cuts meeting the floor or adjacent walls, a standard T-square guided cut maintains crisp edges.
For areas with complex geometry, such as irregular corners, creating a paper template can simplify the transfer process. Heavy construction paper or thin cardboard can be pressed against the framed area, marked, and used as a pattern to trace the shape onto the drywall panel. When measuring overall dimensions, account for the thickness of the drywall itself, especially where the new panel will butt against existing walls or ceiling surfaces. Failing to account for the 1/2-inch or 5/8-inch material thickness can result in panels that are too long or create an ill-fitting joint.
Installation and Seam Finishing
Once the panels are cut to the correct dimensions and angles, the installation phase focuses on securing them firmly to the prepared framing. Lifting the large, angled piece into the tight space often requires two people, ensuring the panel is tightly butted against all adjacent surfaces before fastening begins. Drywall screws are the appropriate fastener, and they should be driven into the wooden framing every 7 inches along the perimeter and every 12 inches in the field.
When driving the screws, dimple the paper surface slightly without tearing through it, using a screw gun with a depth-setting nose. This slight recess allows the screw head to be easily covered by joint compound, creating a smooth plane. Along the angled cut where the panel meets the stringer blocking, fasteners must be placed close to the edge to prevent the drywall from bowing or flexing away from the framing.
The project concludes with seam finishing, which ensures a uniform and smooth transition. For standard flat seams, a thin layer of joint compound, often called “mud,” is applied over the seam. A strip of paper tape or fiberglass mesh tape is then embedded into the wet compound. Paper tape is preferred for its strength in flat seams, while fiberglass mesh offers faster application without a bedding coat.
The long, angled corner where the underside of the stairs meets the adjacent wall requires specialized treatment to maintain a sharp, straight line. For this external corner, a flexible vinyl or metal corner bead should be secured and covered with mud to protect the edge and define the line. Internal corners, such as where the drywall meets a vertical wall, require paper tape folded down the center crease before being embedded in compound. Apply multiple thin coats of joint compound, allowing each to dry completely and then lightly sanding before the next coat, creating a feathered edge that blends seamlessly into the surrounding surface.