Staircase Parameters
📊 Technical Analysis & CTE Compliance
Designing a staircase in Spain is heavily regulated to guarantee safety, usability, and accessibility for occupants. The primary regulatory framework is the Spanish Technical Building Code (Código Técnico de la Edificación - CTE), specifically the Basic Document for Safety of Use and Accessibility (DB-SUA 1). This standard establishes strict bounds for step dimensions and their proportions.
To design a safe and comfortable stair climb, architects use Blondel’s Law. This ergonomic formula relates the tread (huella - the horizontal step depth) to the riser (contrahuella - the vertical height of each step). The rule dictates that the effort to lift a foot twice and move forward once should equal a normal stride: (2 × Riser) + (1 × Tread) ≈ 60 to 64 cm. The CTE legalizes this proportion by accepting values ranging from 54 cm up to 70 cm.
This online calculator allows you to enter the total floor-to-floor height and step dimensions to verify if your staircase complies with Spanish building codes.
⚙️ Key parameters under CTE DB-SUA 1
Spanish building codes distinguish between two usage categories for staircase design:
- Private use (domestic staircases): The maximum allowed riser (C) is 18.5 cm (up to 20 cm in specific renovation cases) and the minimum tread (H) is 22 cm.
- Public use (commercial and communal areas): The maximum riser is 17.5 cm and the minimum tread is 28 cm.
- Flights: Single flights must consist of a minimum of 3 steps and a maximum of 12 steps for public buildings (or up to 15 in residential environments).
📐 The math behind Blondel’s Law
The calculation to find the perfect step configuration follows these stages:
2 × Riser (R) + Tread (T) = Stride Length (Blondel)
- Calculate the step count: Divide the total height (in cm) by the target riser height and round to the nearest whole step: Steps = round(Height / Target Riser).
- Determine the exact final riser: Divide the total height by the step count: Exact Riser = Height / Steps.
The resulting Blondel calculation ($2R + T$) must fall between 54 cm and 70 cm to pass Spanish building code checks.
📊 Worked staircase layouts
We analyze two typical project scenarios under Spanish regulations:
Example 1: Indoor staircase for a two-story residential house (private use)
- Step count calculation: **280 cm ÷ 18 cm (target) = 15.55 → Rounded to 16 steps**
- Exact final riser height: **280 cm ÷ 16 = 17.50 cm**
- Blondel ratio: **2 × 17.50 cm + 28 cm = 63.00 cm**
- Code checks: **17.50 cm ≤ 18.5 cm (Pass) | 28 cm ≥ 22 cm (Pass) | Blondel in range (Pass)**
Example 2: Staircase inside a restaurant or retail unit (public use)
- Step count calculation: **150 cm ÷ 17 cm (target) = 8.82 → Rounded to 9 steps**
- Exact final riser height: **150 cm ÷ 9 = 16.67 cm**
- Blondel ratio: **2 × 16.67 cm + 30 cm = 63.34 cm**
- Code checks: **16.67 cm ≤ 17.5 cm (Pass) | 30 cm ≥ 28 cm (Pass) | Blondel in range (Pass)**
⚠️ Critical errors in stair design
- Measuring raw concrete instead of finished floor heights: You must measure from finished floor level to finished floor level. Forgetting the thickness of tile adhesives, mortar beds, or parquet underlays results in uneven first or last steps, which is illegal and a major tripping hazard.
- Using treads under 22 cm: Very short step depths prevent you from placing your heel fully when walking down, leading to slips.
- Uneven riser heights in a single flight: The human brain builds mechanical muscle memory when walking stairs. A variation of even 0.5 cm between steps breaks this rhythm and causes trips.
❓ Frequently Asked Questions (FAQ)
The ergonomic sweet spot for residential risers is between 17 cm and 17.5 cm. It is comfortable for children and seniors and keeps the stair footprint reasonable.
Under the CTE code, the minimum usable width (anchura útil) for a private residential staircase is 80 cm. For public properties or community blocks, this increases to 100 cm.
Invented by French architect Nicolas-François Blondel, the rule ensures step ratios match the natural stride. By keeping the sum of two rises and one tread near 62 cm, climbing and descending feels effortless.
Yes, spiral stairs (escaleras de caracol) are permitted for private domestic use. They must still guarantee a minimum tread of 22 cm at 30 cm from the inner handrail, and risers must not exceed 20 cm.
Handrails are mandatory for any staircase with a drop height exceeding 55 cm. Flights wider than 120 cm require handrails on both sides to ensure safety.
A comfortable climb angle ranges from 30° to 37°. Any slope steeper than 40° is considered a utility or attic ladder, which is too steep for daily domestic use.