How To Calculate Total Rise And Run For Deck Stairs On Uneven Terrain
Building exterior stairs off a deck requires precise measurements to ensure every step is safe, comfortable, and structurally sound. The most critical stage in the planning phase is determining the total rise, which is the exact vertical distance from the upper deck surface to the landing area on the ground. While flat ground allows for straightforward measurements, sloping or uneven terrain introduces significant layout challenges. Establishing accurate vertical and horizontal measurements is the only way to calculate individual stair risers and treads properly.
Establishing Measurements On Flat Ground
For sites with a relatively level landing area, finding the total rise requires basic layout tools. Extend a long, straight board horizontally from the top of the deck surface, ensuring it is perfectly level. From the approximate end of this horizontal projection, drop a second straight edge vertically to the ground.
Ensure this vertical board is plumb, then drive a wooden stake into the ground and secure it firmly. The measurement from the ground to the bottom of the horizontal board represents the total rise. With this number, the exact quantity of steps and the height of each individual riser can be calculated.
Navigating Sloped Or Uneven Terrain
When a severe slope prevents the use of a single horizontal straight edge, a stepped framework approach is necessary. Instead of spanning the entire distance at once, construct a series of level and plumb guide boards that step down the hill. By adding the vertical measurements of each staggered section together, the total rise is determined.
Similarly, adding the horizontal measurements between each vertical drop yields the approximate total run. This modular approach allows for precise tracking of the elevation drop over highly irregular landscapes.
Keeping The Layout Square And Accessible
Accuracy dictates that the stairway descends perfectly square to the deck framing. A temporary square template, such as a factory-cut sheet of plywood, can be placed on the ground and aligned with guide strings to ensure the layout remains true. It is highly recommended to build this measuring framework offset from the actual stairway location by several inches. Offsetting the template keeps the work area clear for digging footings and cutting stringers while preserving a rigid reference point to double-check measurements during the entire construction process.
Conclusion
Taking the time to establish a solid, level, and plumb measuring framework eliminates guesswork when cutting stair stringers. Whether working on a flat backyard patio or a steep hillside, securing accurate total rise and run dimensions guarantees a professional and structurally sound staircase installation.
Three Key Tips
Tip 1: Offset The Measuring Framework
Construct the temporary measuring jig about six inches to the side of the planned stairway path. This allows continuous access to the reference points for double-checking stringer cuts without the framework blocking soil excavation or stringer assembly.
Tip 2: Verify Plumb And Level At Every Connection
When building a stepped framework on a steep hill, use a high-quality bubble level or laser level on every single horizontal and vertical board. A slight degree of error at the top will compound over the distance, resulting in inaccurate total rise calculations and uneven stair steps.
Tip 3: Square The Layout To The Deck
Use a known factory-square edge, such as a full sheet of plywood, combined with a string line pulled from the deck rim joist to establish a perfectly square layout. Failing to square the stairs to the deck will result in awkward landing angles and difficult stringer attachments.
Bonus Questions And Answers
What Is The Maximum Riser Height Allowed By Building Codes?
Once the total rise is calculated, it is time to divide that number by the desired amount of steps, but local building regulations strictly govern these dimensions for safety. Under the International Residential Code, the maximum allowed riser height is typically seven and three-quarter inches, and the maximum variation between the tallest and shortest riser within a single flight cannot exceed three-eighths of an inch. Ensuring the risers stay within this limit prevents tripping hazards and creates a predictable rhythm when walking up or down the stairs. Always verify exact dimensions with the local building department, as some municipal codes restrict riser heights to an even tighter seven inches.
How Should The Ground Be Prepared For The Bottom Stair Landing?
After the temporary measuring framework helps identify where the bottom step will land, a solid foundation must be prepared to support the stair stringers. Digging a trench and pouring a concrete landing pad is the most reliable method for exterior deck stairs. The concrete pad should extend below the local frost line to prevent the stairs from heaving upward during freezing weather. Setting the stringers on a solid concrete base rather than directly on topsoil prevents settling and keeps the entire staircase from sinking or shifting under heavy foot traffic.
How Can Premature Wood Decay Be Prevented Where The Stairs Meet The Ground?
Even with accurate measurements and a poured concrete landing pad, the bottom of the stair stringers will eventually decay if they constantly absorb standing water. To extend the life of the wood framing, apply a liberal coat of copper naphthenate wood preservative to the bottom end-grain cuts of the stringers before installation. Additionally, elevating the stringers slightly off the concrete using a galvanized standoff post base or a hidden scrap piece of composite decking acts as a capillary break, stopping ground moisture from wicking up into the vulnerable lumber.
