Stair geometry
Tread blank depth
- where
- blank depth (mm)
- going (mm)
- nosing projection (mm)
- rear engagement (mm)
Explain this equation
Gives the depth of timber needed for a stair tread from the going, the nosing projection and the rear engagement.
g is the going, the horizontal distance between neighbouring nosings, in millimetres. p is how far the nosing overhangs the riser below. e is how much the tread runs into the riser or housing at the back. D is the blank depth to cut.
The going is measured nosing to nosing, so the actual tread is longer by the overhang at the front and the engagement at the back. Adding the three together gives the board depth.
A 250 mm going with a 25 mm nosing and 10 mm engagement needs a 285 mm blank.
Watch out: the going is not the same as the tread board depth. Use the actual joint and nosing details.
Add a specified nosing projection and rear engagement to a going.
Enter your values to see the result.
Good to know
Use the actual stair joint and nosing design. Going is the horizontal distance between adjacent nosing lines, not necessarily the timber tread depth.
Limits
Stair going is measured horizontally between nosing lines. Building guidance differs across UK nations. Stair geometry must be checked against the applicable nation, building use and project requirements.
Sources
- Geometry / arithmetic
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.