Designing and constructing a safe, structurally sound stairway requires a thorough understanding of tread dimensions and riser angles. Stair builders frequently need to maximize the stepping surface, known as the tread depth, without expanding the overall footprint of the staircase. This is typically achieved through the use of a nosing overhang, an undercut riser, or a combination of both. Understanding how to properly implement these methods is a fundamental skill for any professional framing carpenter or stair designer.
Standard Stairs Versus Overhangs
A standard stairway design utilizes a basic 90-degree angle between the riser and the tread. In this configuration, there is no overhang or undercut, meaning the physical tread board matches the exact layout depth of the stair stringer. For example, a standard layout might feature a 10-inch run resulting in a 10-inch tread.
To provide a more comfortable stepping surface, most builders incorporate a nosing. A nosing is an overhang that extends past the riser below it. Adding a one-inch overhang to a 10-inch stringer layout effectively creates an 11-inch tread space for the foot. Because it is highly efficient and relatively easy to execute during the trimming phase, adding a nosing remains the most popular method for building stairs.
Implementing Undercut Risers
An alternative to extending the tread board forward is to push the riser board backward. This technique is known as an undercut riser. Instead of installing the risers vertically at a strict 90-degree angle, the stair stringers are cut with a slight backwards angle.
By slanting the riser inward by one inch, the builder creates the exact same mathematical result as a one-inch overhang: an 11-inch stepping surface on a 10-inch layout. While this requires more complex calculations and precise circular saw work when laying out and cutting the structural stringers, it offers a sleek, modern architectural finish without a protruding lip.
Combining Nosings And Undercuts
In unique architectural scenarios, a designer may choose to combine both an overhang and an undercut on the same flight of stairs. If a builder utilizes a one-inch undercut on the riser and a one-inch overhang on the nosing, a standard 10-inch stringer run will yield a 12-inch overall tread depth.
While it is entirely possible to mix these methods, it is rarely structurally necessary. If a larger tread depth is required, the most efficient approach is usually to alter the angle of the riser directly on the stringer layout or simply increase the width of the tread material, rather than combining two separate modification techniques.
How To Measure The Maximum Projection
Regardless of the method chosen, builders must strictly adhere to maximum projection limits. In most jurisdictions, the maximum distance allowed for either a nosing overhang or an undercut slant is one and a quarter inches.
To accurately verify this measurement, place the flat blade of a framing square flush against the face of the riser below. Slide the square upward until it meets the underside of the tread nosing. The horizontal distance from the front edge of the nosing back to the square represents the total projection.
This measurement must remain under the maximum limit established by local building authorities to ensure the stairs are not a tripping hazard. Always consult the local building department to verify the exact structural codes governing the project area before cutting stringers.
THREE KEY TIPS FOR STAIR TREAD CONSTRUCTION
Maintain Strict Projection Limits: Never let a nosing overhang or an undercut riser exceed one and a quarter inches in depth. Pushing past this measurement creates a significant tripping hazard for individuals ascending the stairs, as their toes can catch on the pronounced lip.
Utilize A Framing Square For Verification: The most reliable way to check your overhang depth is to place a framing square firmly against the face of the tread riser at a 90-degree angle. This eliminates visual guesswork and provides a rigorous, inspector-ready measurement of the projection.
Prioritize Stringer Adjustments Over Complex Combinations: Instead of stacking an overhang on top of an undercut, simplify the engineering by adjusting the stringer layout. Modifying the pitch of the riser during the initial stringer layout phase is a structurally superior and more efficient way to achieve the desired tread depth.
BONUS QUESTIONS AND ANSWERS
To further expand on safe stair construction, here are a few advanced considerations regarding building codes and stairway design that professionals must keep in mind.
What Is The Minimum Required Tread Depth For Residential Stairs?
According to the International Residential Code, the minimum required tread depth for residential stairs is 10 inches. This measurement is taken horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread's leading edge. This ensures that the foot has adequate surface area to balance properly during descent.
Why Do Building Codes Require A Minimum Nosing Projection?
Building codes typically require a minimum nosing projection of three-quarters of an inch on stairways with solid risers. This slight overhang is crucial for safety during descent, as it provides a visual edge and allows the heel to properly clear the riser above it. The only exception to this rule is if the actual tread depth is 11 inches or greater, at which point the footprint surface is large enough that a nosing is no longer structurally mandatory for safety.
Can Open Risers Be Used Instead Of Undercut Risers?
Yes, open risers can be used to increase foot space, but they are subject to strict safety regulations. In residential construction, the open space between adjacent treads must not permit the passage of a 4-inch-diameter sphere. This specific building code exists to prevent small children from slipping through the gaps between the stairs while still allowing for a modern, open-concept architectural design.
