Question 1

What is the ideal stair angle?

Accepted Answer

30-37° for residential stairs, 20-30° for public stairs (gentler for high traffic). Above 42° is classified as a ladder in most codes. The angle is determined by the riser-tread combination — steeper stairs save floor space but are less comfortable.

Question 2

What does the 2R+T rule mean?

Accepted Answer

Blondel's formula (2×Riser + Tread = 600-650mm) approximates the average human stride length on stairs. A value within this range feels natural. Below 600mm feels too cramped, above 650mm feels like you're overstepping. Some codes use the range 610-640mm for a stricter comfort zone.