## 2 Criteria Affecting Stair Safety and Sustainability in a Single-Family House
While horizontal fillers in balustrades can pose a climbing hazard for children and pets [45], it is recommended to choose vertical fillers (balusters). Intermediate landingsThese are an important safety feature in staircase construction, as they may help to minimise the consequences of falling from stairs. To reduce the staircase flight length, house owners or architects frequently decide to add additional treads to the intermediate landings. This decision transforms the U-shaped staircase into a spiral S-shaped staircase, with variable width. The consequences of falling from an S-shaped staircase are more severe compared to the pure U-shaped alternative; therefore, the numberof treads installed on the intermediate landings might be associated with the stair safety aspect. The size of the intermediate landing can be determined, based on the geometric parameters of separate steps. Stair widthThe stair width has no straightforward impact on stair safety and sustainability, but it is well known that wider and more spacious stairs are more comfortable to climb. Staircase width also ensures faster evacuation in case of emergencies [13; 44; 46]. Narrow staircases make it difficult for several people to use stairs at the same time, but it is easier to grab the handrail in the event of a fall. From a sustainability point of view, narrow staircases also mean lower energy consumption. While it is not obvious whether narrower or wider staircases are safer and more sustainable, the recommended width of staircases in single-family houses is considered to be 90-100 cm. Therefore, the staircase with the smallest deviation from the most comfortable (ergonomic) width can be considered as the safest and most sustainable option among the alternatives. Riser height and tread widthMany scientific studies [13; 14; 15; 44; 47] proved that the dimensions of stairs have a significant influence on the speed, style, and behaviour of pedestrian movement [48]. For example, the tread depth affects the occupant's foot stability when stepping onto the stairs, while a narrow tread can lead to slipping off the edge [49]. Conversely, if the riser is too high, the foot may be overstretched when walking downstairs, and if the riser is too low, the foot may land too heavily on the step. Tread nosing that projects too far might cause individuals to catch their feet on the nosing and fall. However, artificial lightening or other contrast-enhancing decisions regarding tread nosing may benefit stair safety by increasing the visibility of steps. This allows conclusions to be drawn that a ratio of step width and height is crucial for stair safety [46]. This ratio helps to ensure comfortable stride, reduces fatigue, maintains stability and balance, and improves the overall safety on stairs. [PERSON]'s theorem checks the coherence between the step height and going, and states that the sum of the height of two steps (risers) plus one tread is equal to the variation of one step [50]. A study presented by the architect [51] specified that dwellings with no more than two floors have an affective step variation of 63 cm [52]. This ratio may vary according to local building codes, but many professionals use [PERSON]'s formula, expressed as 2C + 1H = 63 cm, where H is the riser height and C is the tread width. Thus, steps where the 2C+1H measure has the smallest deviation from 63 cm can be considered as the safest rise-to-tread ratio among the alternatives. Step coatingThe proper material and texture of the step surface may help reduce slippage and increase the safety and sustainability of stairs. The most popular step coatings are carpet, wood, and ceramic. Carpet has the advantage of softness and warmth, which is a comfort factor and a contributor to injury prevention. It is also a good sound insulator. However, carpets are prone to wear, tear, and peeling; they also collect dust and allergens. Therefore, they require more frequent maintenance than other floor coverings.