How do you calculate stair treads, and how much allowance?
Tread calculation is not an area calculation, it is how many steps come out of one block. Nine out of ten errors sit in two places: the riser-to-tread joint, and the shaped pieces at corners.
Why can treads not be quoted by area?
Because cutting, not area, is what consumes material. Each step needs one long strip, and that strip cannot be pieced from the offcuts of another slab without breaking the veining direction. So you start from how long and wide the piece for one step must be, multiply by the number of steps, then see how many come off a block. When a price is quoted per square metre, the waste is hidden inside the rate.
How does the riser-to-tread joint change the piece count?
Three options: a monolithic piece where tread and riser are one folded or mitred unit; a split two-piece; and tread overhanging the riser. Monolithic is strongest and wastes most. Split saves material but adds joints that collect dirt. Overhang saves material and installs fast, but the projecting edge chips more easily. Which one you want must be on the drawing, not left for the factory to guess.
Why do corners always need separate calculation?
Because landings, corner steps and fan-shaped steps are not rectangular, so cutting them from standard strips leaves a large offcut. The more corners, the higher the loss. So list corner pieces separately by quantity and shape and have the factory lay them out as shaped work — that is a different calculation from rectangular layout.
How much allowance should you add?
There is no universal number, only a calculated one. Work out the theoretical piece count from the layout, then add for saw kerf, edge trimming and corner scrap, giving the number to actually cut. The ratio between the two is your allowance. Ask the factory to list those three separately in the quotation and you can see which one is eating material.
Which processing items get missed on treads?
Anti-slip grooves or inserts, the rounded or chamfered nosing, the skirting joint where the tread meets the wall, and the bonding treatment on the underside. Anti-slip grooves are the usual omission — they are neither in the area nor in polishing the edge, so if the quotation does not list them you may receive plain edges.
Which drawings should go to the factory?
Three: the plan with step count and corners; the section showing tread width, riser height and overhang; and the detail showing where the anti-slip groove sits, how the edge is finished and how it closes to the wall. With all three the layout and price have a basis. With only 36 steps total, the factory can only guess the common practice.