Building a Bullnose Starting Step: Kerf-Bending Technique for Staircase Carpentry

Understanding the Bullnose Starting Step and Its Role in Stair Construction

The bullnose starting step is the foundation of any well-built staircase. It is the first riser and tread assembly at the bottom of a stair run, distinguished by its curved ends known as bullnoses. These rounded profiles serve a visual and structural purpose: they create an elegant entry point to the staircase and provide a stable anchoring surface for the newel posts that support the handrail system.

In traditional stair construction, the starting step projects beyond the finished stringers on both sides, typically extending about 14 inches past the face of each stringer. This overhang accommodates the bullnose curve and gives the staircase a substantial grounded appearance. The overall length of a starting tread blank for a standard 42 1/2-inch wide stair measures approximately 70 1/2 inches from end to end before shaping.

While manufactured starting steps are available from stair-part suppliers, building your own offers several advantages. You can match the wood species and grain orientation to the rest of the staircase, control the quality of the joinery, and adjust dimensions to fit site-specific conditions. The technique for cutting stair treads accurately starts with careful stock selection and layout, which we cover in the sections below.

What Makes a Starting Step Different from Regular Treads

A standard stair tread is a simple rectangular board spanning between stringers. A bullnose starting step is a much more complex assembly. It combines:

• A wider tread blank with bullnosed ends that extend past the finished stringers
• A curved riser that is kerf-bent to match the bullnose profile
• A nosing detail that wraps around the front edge and both curved ends
• A structural connection point for the bottom newel post
• A return molding that finishes the underside of the overhang

The riser in a bullnose starting step cannot be cut from a single flat piece of stock. It must be bent around the semicircular profile of the tread ends through a precise kerfing process described in detail later.

Key Measurements and Terminology

Selecting Materials and Preparing the Tread Blank

Choosing the right material for your bullnose starting step is critical to achieving a professional result. The tread and riser should be made from the same species as the rest of the staircase to ensure color and grain consistency. Red oak is a popular choice for its workability, bending characteristics, and compatibility with common stair finishes.

Stock Selection and Grain Matching

For the tread blank, look for quartersawn or rift-sawn stock, which offers better dimensional stability than plainsawn material. A starting tread sees concentrated wear at the bottom of the stairs, so stability matters. If you have access to a piece wide enough for the full 12 3/4-inch depth, use solid stock. Otherwise, glue up narrower boards with careful grain matching to avoid a striped appearance.

1. Select stock at least 1 inch thick and 13 inches wide for jointing and planing
2. Joint one face and one edge square, then plane to final 1-inch thickness
3. If gluing up narrower stock, alternate growth-ring direction of adjacent boards to minimize cupping
4. Match grain patterns across glue joints for a seamless appearance
5. Cut the blank to 12 3/4 inches wide and at least 72 inches long
6. Mark the centerline of the staircase width and locations of both stringer faces

Notching the Back of the Tread

One of the most important layout steps is notching the back of the tread so it fits over the second riser. The bullnose ends create nosing on all three exposed edges: the front, the left curved end, and the right curved end. However, the back edge where it meets the second riser has nosing only on the bullnose ends. The central portion must be notched to sit flush against the second riser.

• The notch length equals the finished width of the staircase between stringers (42 1/2 inches in our example)
• The notch depth equals the nosing dimension: 1 1/8 inches
• Cut the notch using a sharp handsaw or a router with a straight bit and template
• Square the corners of the notch with a sharp chisel

This notch creates the 11 5/8-inch tread depth at the back center while maintaining the full 12 3/4-inch depth at the bullnose ends. The notched portion sits tight against the second riser while the bullnose ends project forward with the same nosing overhang as the front edge.

Kerf-Bending the Riser for Bullnose Ends

Kerf bending is the technique that makes a bullnose starting step achievable in a small workshop without specialized steam-bending equipment. By cutting a series of closely spaced saw kerfs across the back face of the riser stock, you remove enough material to allow the wood to bend around the bullnose radius without cracking.

Calculating Kerf Spacing and Depth

The number and spacing of kerfs depend on the radius of the bullnose curve and the thickness of the riser material. For a 3/4-inch thick riser bent around a 6-inch radius:

• Kerf depth: cut to within 1/16 inch of the face, leaving a thin skin of uncut wood on the visible side
• Kerf spacing: 3/8 inch between kerfs for tight radii, up to 1/2 inch for gentler curves
• Kerf width: equal to the thickness of your saw blade, usually 1/8 inch
• Total kerfed length: at least 24 inches for each bullnose end

Cutting the Kerfs

1. Set the blade height so it cuts to within 1/16 inch of the face of the stock
2. Install a sacrificial fence to prevent tear-out on the back side of the kerfs
3. Set the rip fence to space kerfs at 3/8-inch intervals
4. Cut one kerf, then advance the stock by the spacing distance and cut the next
5. Continue until you have kerfed approximately 24 inches of the riser end
6. Repeat for the opposite end of the riser board

The kerfed section will now have the flexibility to bend around the bullnose profile. Test the bend by hand before gluing. If the riser resists bending or shows cracks on the face, deepen the kerfs by raising the blade by 1/32 inch and recutting.

Bending and Gluing the Riser to the Tread

With the kerfs cut, the assembly process can begin. You will need the stair skirtboards properly installed before fitting the starting step, as they establish the reference plane for the entire assembly.

1. Apply a generous coat of wood glue to the kerfed surfaces of the riser
2. Position the riser against the back edge of the tread blank, aligned with the bullnose curve
3. Clamp the riser to the tread at the center of the straight section
4. Working from the center outward, bend the riser around each bullnose end, clamping progressively
5. Use cauls or bending forms to distribute pressure evenly across the kerfed area
6. Allow the glue to cure for at least 24 hours before releasing clamps
7. Trim the riser ends flush with the tread bullnose profile using a jigsaw or bandsaw
8. Sand the assembly smooth, focusing on the transition between kerfed and unkerfed sections

The glue fills the kerf gaps and, when cured, creates a rigid structure that retains the bent shape permanently. This is the same principle used in bentwood furniture and curved cabinet frames.

Installing the Starting Step and Anchoring the Newel Post

The final phase involves fitting the completed starting step assembly into the staircase and securing the bottom newel post. Proper installation ensures the step remains solid under foot traffic and provides a stable foundation for the handrail system.

Fitting the Starting Step into the Staircase

Before installation, verify that the rough framing is square and level. The process for installing manufactured stair parts follows a similar sequence, but custom-built components require additional fitting attention.

• Dry-fit the starting step into position to check fit against the stringers and second riser
• Apply construction adhesive to the top of the rough tread and the face of the second riser
• Set the starting step in place, ensuring the notched back seats firmly against the second riser
• Screw through the subfloor or rough framing into the underside of the starting step
• Countersink screw heads and fill with wood filler matched to the species
• Check for level in both directions and shim as needed before the adhesive sets

Anchoring the Bottom Newel Post

The bottom newel post bears the full load of the handrail and must be anchored securely through the starting step into the subfloor. A wobbly newel post is the most common complaint in staircase construction, almost always caused by inadequate anchoring at the starting step.

For the strongest connection, bore a hole through the starting step and subfloor, then install a threaded rod anchored to a floor joist or concrete slab below. The newel post is drilled to receive the rod and secured with a washer and nut recessed into the post base. A wooden plug conceals the hardware.

Adding the Return Molding and Finish Details

The underside of the bullnose overhang requires a return molding to create a finished appearance. This molding follows the curve of the bullnose and conceals the raw edge of the tread and riser assembly.

• Cut the return molding from stock matching the tread species, typically 3/4 inch thick and 2 to 3 inches wide
• Kerf-bend the molding using the same technique as the riser, with kerfs on the inside face
• Glue and pin-nail the return molding to the underside of the tread overhang
• Fill nail holes and sand smooth
• Apply stain and finish to match the rest of the staircase

If you are constructing the newel post yourself rather than buying manufactured, review the custom box newel post construction techniques for hollow post assembly, which pairs naturally with the kerf-bent riser approach described here.

Finally, seal the entire assembly with a durable floor finish. Apply at least three coats of polyurethane or conversion varnish, sanding lightly between coats with 220-grit paper. A bullnose starting step receives heavy traffic as the primary entry point to the staircase, so a high-quality finish is essential for long-term durability.