Upgrading the decking material on a residential deck is a common home improvement project. However, swapping out thinner materials, such as three-quarter-inch plywood, for thicker dimensional lumber will alter the total height of the deck structure. This seemingly minor elevation change creates a major problem for the attached staircase: the final step leading onto the deck becomes taller than the rest of the steps.
Uneven stairs are more than just a nuisance. They present a significant tripping hazard and can cause a staircase to fall out of compliance with standard building safety guidelines. Fortunately, a complete staircase rebuild is not always necessary. By carefully calculating the new total rise and strategically adjusting the existing treads, homeowners and contractors can restore uniformity to the staircase without cutting new stringers.
Recalculating The Total Rise
When the top deck elevation increases, the overall total rise of the staircase changes. The first step in correcting the issue is to measure the new total rise from the bottom landing up to the newly finished deck surface.
Once the new total rise is established, divide that measurement by the total number of risers in the staircase. For example, if the previous total rise was thirty inches across four steps, the original riser height was exactly seven and one-half inches. If thicker decking material adds one inch of height, the new total rise becomes thirty-one inches. Dividing thirty-one inches by four yields a new target riser height of seven and three-quarter inches for every single step.
The Incremental Shimming Method
With the new target riser height calculated, the existing treads must be adjusted to bridge the gap. Because the height discrepancy compounds as you move up the staircase, you cannot simply add the same thickness of material to every step. Doing so would only shift the uneven step to the bottom of the staircase.
Instead, the treads must be carefully removed so that incremental shims can be installed underneath them. Starting at the bottom step, add a quarter-inch of material to raise the first tread. Because that quarter-inch alters the distance to the second step, the second step will require a half-inch of material to achieve the correct height.
The third step will then require a three-quarter-inch shim. By increasing the added material on each consecutive step, the new, taller riser height is evenly distributed across the entire flight, resulting in a safe and uniform staircase.
Conclusion
Adjusting stair treads to accommodate a modified deck height is a practical, labor-saving alternative to tearing down and rebuilding structural stringers. As long as the newly calculated risers do not exceed maximum height limits, this incremental adjustment process will successfully level out the stairs, providing a seamless and secure transition to your newly upgraded deck.
Three Key Tips
Always calculate the new total rise from the ground landing to the very top of the newly installed decking material before modifying any existing stair parts.
Increase the thickness of your spacer material incrementally as you move up the stairs. Adding the exact same thickness to every tread will not fix the uneven height problem.
This specific adjustment technique is only viable when the uppermost riser is taller than the rest of the steps. It will not work if the top riser has become shorter due to the installation of thinner deck boards.
Bonus Questions And Answers
To ensure your staircase modification is completely compliant with modern safety standards, here are a few additional structural guidelines to keep in mind.
What Is The Maximum Allowable Height Difference Between Stair Risers?
According to standard residential building codes, the greatest riser height within any flight of stairs shall not exceed the smallest by more than three-eighths of an inch. This strict tolerance exists because human muscle memory anticipates consistent steps. A variation larger than three-eighths of an inch easily disrupts natural walking rhythm and creates a severe tripping hazard, making exact riser equalization critical for safety.
What Is The Maximum Allowed Riser Height For Residential Stairs?
The standard building code dictates that the maximum allowable riser height for a residential staircase is seven and three-quarter inches. When adding shims to your treads to fix an uneven top step, you must ensure your newly divided mathematical calculation does not force any individual riser to exceed this strict limit. If your new calculation demands a riser taller than seven and three-quarter inches, the current stringers cannot be saved, and the entire staircase must be rebuilt with an additional step to keep the height compliant.
Do I Need To Adjust My Handrail When Changing Tread Heights?
Yes, any significant alteration to the height of your stair treads may require you to reposition the staircase handrail. Building codes require handrails to be placed between thirty-four and thirty-eight inches above the front nosing of the stair treads. If you raise your treads by installing shims, your handrail will effectively become lower relative to the walking surface. You must verify that the existing handrail still falls within the legally mandated height range, and remount it higher if it does not.
