Granite Step Riser Height and Tread Depth: Fix the Step Count First
Granite step riser height and tread depth are not two independent numbers. Fix the step count, work back to tread depth from the 2R + T ≈ 620mm stride rule, then convert both into board widths. At a 3000mm floor height, 18 steps splits into 12 at 167 plus 6 at 166, which lands exactly on 3000mm.
How do granite step riser height and tread depth fit together?
Granite step riser height and tread depth are set in one order: fix the number of steps, then make the two add up to a comfortable stride. Three moves only: estimate the step count from a target riser, n ≈ H ÷ target riser rounded to a whole number; then fix the riser for real with R = H ÷ n; then work back to tread depth T from 2R + T ≈ 620mm and turn both into board widths. The 620 is a rule of thumb for human stride, sources quote anything from 600 to 640, and it is neither a code value nor a pass mark. Once the step count is fixed, R and T are locked and every other dimension on the drawing comes out of those two numbers, which is why the step count comes first.
What if the total rise does not divide evenly?
It rarely does, and the fix is not to bend the step count but to spread the remainder over most of the steps. A 3000mm floor height over 18 steps gives 3000 ÷ 18 = 166.67mm, and nobody cuts to that in the shop. What we do when setting out a flight is split it into 12 risers at 167mm plus 6 at 166mm: 12 × 167 = 2004 and 6 × 166 = 996, which totals exactly 3000mm. Two riser heights 1mm apart in the same flight are imperceptible underfoot. Compare the same 18 steps done as 17 at 166mm plus one at 178mm — 17 × 166 = 2822, so the last step has to carry 178mm, a 12mm jump that reads as a trip point and is exactly where older visitors and children trip. Put the odd millimetres into the majority of steps, never into one step at the top.
With the riser height set, how wide do I cut the riser board?
When the tread board sits on top of the riser board, riser board width is riser height minus 30mm. The 30mm is the thickness of Maple Red G562 (G4562) and of the tread board, and that band stays hidden under the tread, so there is no reason to cut the riser board to full height. At a 150mm riser the riser board is 150 − 30 = 120mm; at 167mm it is 137mm. The premise has to be stated: this is the tread-over-riser detail. Reverse it — riser board in front of the tread board with the full riser face exposed — and the riser board is cut to the full riser height, so 150mm stays 150mm and the 30mm is not deducted. A narrower riser board means less glued area on its back and more risk of hollow spots and debonding, so a very low riser height means deciding early whether you can give up those 30mm.
How deep should the tread be, and does the nosing count?
Finished tread depth equals tread board width plus the front nosing, and the nosing adds to it rather than being deducted. At a 160mm riser — a 3200mm floor height over 20 steps, 3200 ÷ 20 = 160 — 2R + T = 620 gives T = 300mm, and with a 20mm nosing the slab width is W = 300 − 20 = 280mm. The 20mm nosing gives the toe 20mm more to land on, so what gets stepped on is 300mm, not 280mm. Then comes the counter-intuitive part: ripping 280mm strips from a 1200mm slab gives 1200 ÷ 280 = 4.28, so four strips and 80mm left over, which is scrap once the kerf is taken out. Five strips need W under 240mm — 240 is 1200 ÷ 5 with no kerf allowance — meaning T no deeper than 260mm. Push the nosing from 20 to 40, so W falls to 260 and 1200 ÷ 260 = 4.61, and you still get four strips. So do not enlarge the nosing to save material; a longer nosing leaves a longer unsupported lip at the front edge, and that lip chips first under traffic.
At a 3-metre floor height, what do 16 to 20 steps give me?
Between 16 and 20 steps the riser height lands between 150 and 187.5mm. The T column is back-calculated from 2R + T ≈ 620 and the h column from h = R − 30, all in millimetres. Sixteen steps on a 3000mm floor height give 187.5mm, which is steep; to bring 16 steps back into 150 to 167mm, the floor height can only be 2400 to 2672mm. At 3000mm we set out 18 to 20 steps, and that is our working habit. The 19-step row, 157.9mm riser against a 304mm tread, is the combination we are asked for most in the orders we receive.
| Steps | Riser height R | Riser board width h=R−30 | Tread depth T (from 2R+T≈620) |
|---|---|---|---|
| 16 | 187.5 | 157.5 | 245 |
| 17 | 176.5 | 146.5 | 267 |
| 18 | 166.7 | 136.7 | 287 |
| 19 | 157.9 | 127.9 | 304 |
| 20 | 150 | 120 | 320 |
Which six numbers go on the shop drawing?
Six: total rise, step count, riser height, riser board width, tread board width, front nosing. Write total rise from finished floor to finished floor, not slab to slab, because the finish build-up alone can force a different step count. Write the step count as a whole number, not as about 18. If the riser is split into two heights, give the count and the height of each group, the way 12 at 167 plus 6 at 166 does. Note on the drawing that riser board width assumes the tread-over-riser detail, since the figure does not survive a change of detail. Keep tread board width and nosing as two figures instead of one finished depth, because the shop needs board width to plan the strips. All six belong on one drawing; leave one out and you lose a round of emails. Once the drawing is fixed we schedule production: 100 m² minimum, 5 days.