Outdoor steps, kerbing and retaining walls are classic landscape gardening work with their own dimensioning, foundation and drainage requirements. Here is the step length formula for comfortable steps, frost-free foundations, kerbs in mortar and the key rule for retaining walls: drainage behind the wall.
Introduction
Steps, kerbs and walls are the fixed structures that hold a landscape together: steps absorb level changes, kerbs hold the paving in place, and retaining walls hold the soil. All three must be founded properly and withstand frost and water pressure — otherwise they slip, crack or heave.
Outdoor stairs — rise and tread
A pleasant stair follows the step-length formula (also called the stair formula): 2 times the rise plus the run should be around 61–64 cm, typically 63 cm. The rise is the step's height, and the run is the step's depth. Outdoor stairs are made flatter than indoor — low rise and deep run — because you walk differently outside, and because steps must be safe when wet or iced.
| Rise | Grund (2s + g ≈ 63 cm) | Grade |
|---|---|---|
| 13 cm | 37 cm | Very flat, staircase |
| 14 cm | 35 cm | Flat, pleasant garden steps |
| 15 cm | 33 cm | Standard outdoor |
| 16 cm | 31 cm | Steeper, compact |
On long flights of stairs, landings (rest areas) are inserted so you do not have to go too many steps in a row. Steps and landings are given a slight fall (NfA: 15 ‰) for drainage so water does not stand and turn to ice.
Frost-free foundation
Freestanding structures — stairs, walls and kerbs — must be taken to frost-free depth or frost-proofed, so they are not lifted by frost heave when water in frost-susceptible soil freezes and expands. Frost-free depth is typically calculated at around 0.9 m below heated buildings and around 1.2 m at freestanding foundations, depending on soil type (Building Regulation guidelines). On frost-susceptible (fine-grained, silty) virgin soil, the soil is replaced with frost-safe material or insulated, where it is not practical to dig to full depth.
Step materials and construction
- 01Block steps in granite or concrete are set in a compacted frost-free base layer and held in concrete/mortar.
- 02Steps built of stone and tiles: backed and set with bonding so they lock and do not slip.
- 03Give the step a slight forward fall so water runs off and avoid pockets where water and ice collect.
- 04Slip-resistant surface (rough/flamed natural stone or structured concrete) on steps that get wet.
When the stairs are access to a building
If the stairs are an access route to a building, the Building Regulations (BR18) apply to, among other things, tread, rise, handrail and guarding beyond the purely landscape gardening standards. Check the current BR18 requirements for access areas on the specific job — they may require a handrail and guarding where there is a risk of falling.
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