What rise and tread should outdoor garden steps have?

A garden step wants 6 in (152 mm) of rise on a 14 in (356 mm) tread, or 150 mm on 360 mm in metric. The two numbers are tied: on an outdoor flight the tread plus twice the rise should come to 26 to 27 in (660 to 686 mm), so each riser has one tread that walks properly with it. The figures in circulation conflict, 15 cm against 17 cm against 20 cm and more, because they come from rulebooks written for different stairs, and the indoor maximum most people reach for, 7 3/4 in (196 mm), is a house stair limit the code itself does not apply to a freestanding garden flight. Path widths, which the rest of outdoor room dimensions assumes, close the page.

Measure the rise at the nosing, and the tread without the overhang#

The IRC measures the riser "vertically between leading edges of adjacent treads". That matters outdoors because garden treads are laid with a fall for drainage: measure at the front edge, not at the back where the slab has dropped.

The tread, or going in British usage, runs horizontally from nosing to nosing. A slab that oversails the riser below adds nothing to it: a 450 mm slab with a 25 mm overhang and 65 mm tucked under the step above gives a going of 360 mm. Decide the going first, then add overhang and bearing to get the slab depth.

The top tread is the landing or terrace, so six risers have five treads, as stair dimensions: rise, run, headroom and handrails counts them indoors.

Where 26 in comes from: tread plus twice the rise#

The rule ties each riser to one tread so every step stays the same length of stride: an inch more rise costs two inches of tread. Indoors this site uses 24 to 25 in (610 to 635 mm). Garden Gate's outdoor chart is built on "step tread + (2 x riser) = 26 or 27", 1 to 2 in longer, and Approved Document K's band for the same relationship, 550 to 700 mm (about 21 1/2 to 27 1/2 in), contains both.

For the headline pair, 2 x 6 + 14 = 26 in, and in metric 2 x 150 + 360 = 660 mm. The claim that a 15 cm riser wants a 33 cm tread is the indoor figure: 2 x 150 + 330 = 630 mm. Outdoors, 15 cm takes 36 cm.

These are Garden Gate's 26 in pairs plus its steepest; each tread also has a 27 in partner with half an inch more rise.

150, 170 or 220 mm: why the published maximums disagree#

150 mm (6 in) tops Approved Document M paragraph 1.8, the external stepped approach to a Category 1 visitable dwelling: "a rise of 75-150mm and a minimum going of 280mm". One secondary summary gives that going as 200 mm; the document says 280 mm.

170 mm (6 3/4 in) tops paragraphs 2.11 and 3.11, the stepped route on Category 2 and 3 dwellings: "a rise of between 150mm and 170mm and a going of between 280mm and 425mm". Approved Document K uses 150 to 170 mm for a general access stair.

220 mm (8 5/8 in) is Approved Document K's maximum for a private stair, and 7 3/4 in (196 mm) is the IRC maximum for a dwelling stair. Neither was written for a garden.

The UK rules are narrower than they look. Approved Document M applies to new dwellings and material alterations, not extensions (paragraph 0.2), and Section 1A covers only the approach route to the entrance. Categories 2 and 3 apply only under a planning condition (paragraph 0.3). Approved Document K covers external steps "when they are part of the building". Steps from the lawn to the vegetable beds sit inside none of these (standards and codes indexes the rest).

The IRC's 7 3/4 in stops at the garden#

The 2021 IRC opens its stairway section with "Where required by this code or provided, stairways shall comply with this section", and its first exception is "Stairways not within or serving a building, porch or deck." California's 2025 Residential Code, numbering the section R318.7, prints the same exception above the same 7 3/4 in (196 mm) riser and 10 in (254 mm) tread.

So the line is serving against not serving, not indoor against outdoor. Steps from the back door to the patio serve the building, and steps from a deck to the lawn serve the deck, which is why deck sizes, railing heights and baluster spacing holds deck stairs to the full IRC geometry. A flight cut into a bank between two lawns serves nothing the code names.

Local adoption changes the numbers, not the logic: Connecticut allows an 8 1/4 in (209.5 mm) riser and 9 in (229 mm) tread, with the same exception. Where the code applies, the hard part outdoors is uniformity: risers may differ by no more than 3/8 in (9.5 mm), and so may treads.

By the outdoor rule, a 7 3/4 in riser wants a tread of only 10 1/2 in (267 mm). The shallow garden step is the same stride with a lower lift.

Long treads: space them in whole paces#

On gentle slopes the answer is a long tread: one riser, several paces, the next riser. Garden Gate stops at 18 in (457 mm), so these spacings are our own arithmetic.

With no riser, the stride rule reduces to a level pace of 26 in (660 mm). A tread taking n paces, the last of which climbs the next riser, is therefore 26n - 2R in inches, or 660n - 2R in millimetres. With a 6 in riser and n = 2 that is 52 - 12 = 40 in (1,020 mm); with n = 3, 66 in (1,680 mm). Anything between puts a half pace on every tread.

The slope, rise over tread, is the bank each spacing suits. With an odd number of paces the feet alternate at each riser, as on a stair; with two, the same foot takes every riser. Long treads cannot be used on a UK Category 2 or 3 approach, which caps the going at 425 mm (16 3/4 in); Category 1 sets only a minimum.

At the shallow end the table meets ramps. Three paces on 4 in risers averages 1:17.5, gentler than the ADA's 1:12 ramp limit and the 1:15 Approved Document M allows for a 10 m flight on a Category 1 approach, which it says should "be step-free where possible". Ramps belong with accessibility and accessible clearances.

Landings every 1,800 mm, and a rail from three risers#

Flight height. Approved Document M limits any external approach flight to 1,800 mm (71 in) of rise between landings: 12 risers at 150 mm.

Landings. At least 900 mm (35 in) long under Approved Document M, top, bottom and between flights. Where the IRC applies, a straight flight needs a landing 36 in (914 mm) deep and as wide as the flight.

Width. At least 900 mm (35 in) under Approved Document M, 36 in (914 mm) under the IRC, and ideally the width of the arriving path.

Handrails. Approved Document M asks for a grippable rail on one side of any flight with three or more risers, 850 to 1,000 mm (33 to 39 in) above the pitch line, running 300 mm (12 in) past the top and bottom nosings, and on both sides of Category 2 and 3 flights wider than 1,000 mm. The IRC threshold is four risers, at 34 to 38 in (864 to 965 mm).

Single steps. Approved Document M paragraph 3.11 says single steps are avoided, as does entry door and approach clearances. Make a single riser two, or ramp it; the door step most houses already have is covered in garage to house door and step.

Water. The ADA requires wet treads to prevent the accumulation of water. We suggest a forward fall of up to 1 in 40, Approved Document M's approach cross fall limit: 9 mm on a 360 mm going.

Path widths: 48 in for the main route#

The main path from the house wants 48 in (1,220 mm), which lets two people walk side by side.

The bed paths come from planter and raised bed dimensions, routes across paving from patio sizes and layouts, and the indoor equivalents from walkway and circulation widths. Approved Document M caps cross fall at 1 in 40 and the ADA at 1:48; the ADA's steepest walking surface is 1:20, where Approved Document M's ramp tables begin, and past that the long-tread table takes over.

Where these numbers stop applying#

The 26 to 27 in rule is garden practice, not code. Where a flight serves the house, a porch or a deck, the IRC as adopted locally takes priority. Where a new English dwelling's approach includes steps, Approved Document M's bands apply. Sites covered by the ADA follow its 4 to 7 in band and may not use open risers.

For children, people with a stick, or use after dark, our advice is the shallow end of every band, with lighting set as in entryway lighting heights. Going and nosing are defined in the glossary, and the unit converter checks any figure in the other unit.