Find Landing Edge Lengths for Trim and Guards

Most DIYers assume every edge of a stair landing needs trim, but that is wrong. The side where the stair flight meets the landing is bounded by the stair structure, not exposed, so it does not belong in the takeoff. Including it overestimates material by the stair width, typically 36 inches, and wastes money. To find landing edge lengths for trim and guards, you measure only the exposed sides: those not against a wall or the stair itself. The 2021 IRC R311.7.3 requires a landing at least as wide as the stair and 36 inches long in the direction of travel, and R312 requires guards on open sides where the drop exceeds 30 inches. Exactly which sides of a stair landing belong in a perimeter takeoff are answered here.

A 36-inch by 36-inch landing at the top of a straight stair with three sides against walls and one open side has a perimeter of 36 inches, just the open side. The side adjoining the stair flight is not exposed, so it is excluded. For a 36-inch by 48-inch landing at the bottom with two open sides and two against walls, the calculation changes. The total perimeter is 36 + 48 + 36 + 48 = 168 inches, minus the stair width. If the stair is 36 inches wide, the perimeter is 168-36 = 132 inches, or 11 linear feet. The side where the stair meets the landing is always subtracted because it is bounded by the stair structure, not by air or wall.

Use the perimeter calculator when you need to convert edge measurements into linear feet for ordering trim or guard material. The calculation is straightforward: sum all exposed edges, then subtract the stair width from the side where the stair flight adjoins the landing. For a landing with all four sides open, such as a deck landing, the perimeter is the full sum: 36 + 36 + 36 + 36 = 144 inches, or 12 linear feet. No subtraction is needed because no side is bounded by a wall or stair. The 2021 IRC R311.7.3 defines the landing dimensions, but it does not specify which edges count for trim, that is a material estimation decision based on exposure.

Which Sides Belong in the Takeoff

Only sides that require finish material belong in the takeoff. These are exposed edges: open sides that need a guard or trim, and sides against a wall that still need trim to cover the gap. The side where the stair meets the landing is not exposed because the stair structure itself bounds it. This side is always excluded. The same applies to sides against a wall: they do need trim, so they are included in the perimeter, but the stair-side edge is never included.

Example With Two Open Sides

Take a 36-inch by 48-inch landing at the bottom of a straight stair. The stair is 36 inches wide. The landing has two open sides: one 48-inch side and one 36-inch side. The other 48-inch side is against a wall, and the remaining 36-inch side adjoins the stair flight. The perimeter is 48 (open) + 36 (open) + 48 (wall) = 132 inches, or 11 linear feet. The stair-side 36-inch edge is excluded. This matches the formula: total perimeter minus stair width.

The confusion around landing edge lengths comes from misidentifying which sides are exposed. The stair defines the landing geometry, and the perimeter supports material estimation. If you include the stair-side edge, you overestimate trim by the stair width. A 36-inch stair adds 3 feet of waste. For a landing with two open sides, the error is smaller but still significant: you order 11 linear feet of trim instead of 11, but you might buy a 14-foot length and cut off 3 feet. That 3-foot offcut is scrap you paid for.

Failure Case: Stair-Side Inclusion

The most common failure case is including the stair-side edge of a landing in the perimeter takeoff. This overestimates trim by the stair width, for example, 36 inches, leading to material waste and incorrect guard length. If you order a guard based on the inflated perimeter, the guard will be too long by the stair width. You then have to cut it, wasting material and time. The 2021 IRC R312 requires guards on open sides where the drop exceeds 30 inches, but the guard length matches the edge length, not the full perimeter.

Limit: Landing Surface Not Included

The landing perimeter takeoff does not include the area of the landing surface. It is a linear measurement for edge finishing only. The surface area is a separate calculation for decking or flooring material. This distinction matters when ordering materials: you need linear feet of trim and guards, but square feet of decking. Mixing the two leads to ordering errors that delay the job.