Building Steps Into a Garden Slope

Ten risers of 144, or nine of 160

With 1.44 m committed on the centreline, the whole design collapses to a single integer. Eight risers gives 180 mm, nine gives 160 mm, ten gives 144 mm, eleven gives 131 mm, thirteen gives 111 mm. Every one of those divides the rise exactly enough to build, and every one produces a completely different object: a steep utility flight, a comfortable garden stair, or a long shallow ascent that reads as a terraced slope rather than as steps.

Outdoors the proportion rule loosens: twice the riser plus the going can run out to about 660 mm and beyond, because people take exterior steps at a slower cadence and often carry things up them. What does not loosen is uniformity. The riser has to be the same all the way up whether the flight is indoors or in a hedge, and outdoor flights fail that test far more often, because the ground under them settles unevenly and because the top and bottom steps are tied into surfaces laid by somebody else.

The other constraint arrives from the merchant rather than the code. A riser height is not free: it has to be assembled from whole units of whatever you are building the riser out of, plus the thickness of the tread and its bed. That interacts with the count directly, and it is the reason to settle the number before ordering anything.

Seven and a half millimetres across the tread

A tread laid dead level is a shallow tray. It holds the film of water that surface tension will not release, that film freezes overnight in the shade at the bottom of the bank where the sun never reaches until March, and the step becomes the most dangerous part of the garden at exactly the time of year people are least likely to look down. Everything horizontal outdoors gets a fall, and on a step the fall is small enough that it has to be set deliberately or it will not happen: on a 450 mm tread slab, a fall of one in sixty is 7.5 mm from back edge to nosing. Nobody puts that in by eye.

Which way it points is a real decision rather than a convention. Falling forward is simplest to build and sends every tread's water onto the tread below, so a ten-step flight delivers ten treads' worth of runoff to the bottom step and the ground in front of it — fine if there is somewhere for it to go, a slow-forming sheet of ice if the bottom lands in a hollow. Falling sideways across the flight, into a border or a channel at one cheek, keeps each tread's water to itself and is the better answer on a long flight, on a shaded flight, or anywhere the bottom step meets a wall. On a landing, fall away from the flight rather than into it.

The fall has one consequence people miss, and it eats an inspection tolerance for nothing. Stand a tape at the back of a tread, hard against the foot of the riser above, and measure up to that step's nosing: on this flight you read 136.5 mm. Move the tape forward to the nosing of the tread you are standing on and measure to the same point: 144 mm. Both are honest readings of the same step, and they differ by exactly the fall you built in — 7.5 mm, which is most of the variation a whole flight is allowed. Pick one method, nosing to nosing, because that is the surface a foot arrives on, and use it for every riser, on every visit, with the same tape.

Where the fall gets lost is at the bedding. A slab on five spots of mortar tips wherever the spots are; a slab on a full bed keeps whatever fall you screeded into that bed. Set it in the bed, prove it with a shim of known thickness under one end of a level rather than trusting a bubble that reads level over 450 mm anyway, and prove it again after the slab is tapped down.

Formation, geotextile and sub-base: the part measured on the rake

A garden step fails from underneath. Excavate to a formation that steps roughly parallel to the finished flight rather than digging one deep rectangular hole, cart the arisings away instead of pushing them downhill onto the ground the bottom step will sit on, and get down to material that has not been disturbed. Where the bank is made ground — and on a bank formed when the house was built, some of it always is — the depth you have to reach is a site question rather than a specification one.

Separate before you fill. A geotextile between the formation and the sub-base stops fine soil pumping up into the stone under cyclic loading, which is the mechanism that quietly halves the thickness of a base over a decade; AASHTO M288, Geotextile Specification for Highway Applications, is the usual reference for choosing a separation grade. Then bring the sub-base up in compacted layers of about 100 to 150 mm with a plate compactor, and compact each layer before the next goes on. A single 300 mm dump compacted from the top is loose in the middle whatever the surface feels like, and the laboratory reference for what constitutes compaction — ASTM D698 or ASTM D1557, depending on the effort specified — exists precisely because surfaces lie.

Order it against the raking area, not the plan area. This is where garden steps differ from a patio and where the tonnage comes out short. The plan footprint of the example flight is 1.2 m wide by 4.5 m long, or 5.4 m². The surface the sub-base actually has to cover is the stepped formation, whose developed length is every going plus every riser: 3.2 m of treads, plus the 1.3 m landing, plus 1.44 m of accumulated riser faces, which is 5.94 m, and at 1.2 m wide that is 7.1 m². Thirty per cent more stone than the plan area suggests, before anyone has allowed for compaction or for the over-dig at the edges.

The concrete that stops the flight walking downhill

Everything on a slope is being pushed down it. A masonry step flight built on compacted stone alone will creep: the bottom riser eases forward a few millimetres a year, the joint behind it opens, water gets into the opening, and the freeze does the rest. The restraint is a concrete toe beam under the bottom riser, cast into undisturbed ground and deep enough that it is below the depth seasonal freezing reaches on your site. On the example flight that is a beam roughly 1.5 m long, 400 mm wide and 350 mm deep — 0.21 m³ — sized so it projects past both cheeks of the 1.2 m flight.

Above it, the flight sits on a stepped core with a haunch behind each riser course. A haunch 100 mm thick and 150 mm high behind ten risers 1.2 m wide is another 0.18 m³, giving 0.39 m³ for the flight, and around 0.45 m³ once the over-dig and the losses in a barrow are allowed for. Slab estimators size a rectangle, so a stepped or wedged pour has to be handed to them as a sum of prisms, or as one prism at the average of its thickest and thinnest sections. Doing that on paper takes a minute and stops the classic error of ordering against the thin end.

The mix matters more here than on a slab inside a garage. Exterior concrete in a freezing climate is saturated for weeks at a time and is being asked to survive cycles rather than loads; ACI 318, Building Code Requirements for Structural Concrete, sets freezing and thawing exposure categories for exactly this condition and requires air entrainment across them, with the most severe category covering concrete that will also meet de-icing chemicals. ACI 332, Residential Code Requirements for Structural Concrete, is the residential companion, and ACI 306R, Guide to Cold Weather Concreting, governs whether you should be pouring at all on the day. Specify air-entrained concrete to a named exposure class when you order; the batcher cannot add air on site.

Then give the pour something to hold — reinforcement in the toe beam, a starter bar into the core, and a rough surface rather than a trowelled one where masonry will bed on it — and leave it alone. Masonry set on green concrete moves with it.

Riser courses, and the brick that decides the riser

Work the riser height backwards from the units and the picture changes. A 144 mm riser built from a 65 mm brick laid flat, a 50 mm tread slab and a 15 mm mortar bed leaves 14 mm for the bed joint under the brick — one course, a slightly fat but perfectly normal joint, and the numbers close. A 160 mm riser cannot be built from the same course: 65 plus 50 plus 15 leaves a 30 mm joint, which is not bricklaying. It can be built beautifully from a brick laid on edge, where the 102.5 mm face plus the 50 mm slab plus a 7.5 mm bed comes to 160 mm almost without adjustment. The riser count and the way the brick is laid are one decision, not two.

Choose the brick for saturation, not for colour. A riser sits in the wettest position in the garden, holding back damp fill on one face and taking splash and standing snow on the other, and it will be frozen while saturated many times a year. In Europe that is the F2 freeze-thaw category of BS EN 771-1, Specification for masonry units, Clay masonry units, and clay pavers to BS EN 1344 are the obvious alternative. In North America it is Grade SW under ASTM C216, Standard Specification for Facing Brick, or Class SX under ASTM C902, Standard Specification for Pedestrian and Light Traffic Paving Brick. A soft facing brick that has been perfectly happy in a wall two metres above ground will spall its face off in a riser inside three winters.

The mortar is the other half of that. Use a designation suited to external work below and around ground level rather than the leanest mix on the bag, keep the joints full and struck so water runs off rather than sitting on a recessed ledge, and resist the urge to point the top joint flush with the tread — that joint is the one carrying the drip line off the nosing. On the take-off, the ten riser faces of a 1.2 m flight are one wall 12 m long and a single course high; that is about 54 bricks in stretcher bond before any cuts, which is why the order is dominated by breakage and cheek returns rather than by the run itself.

Treads, nosings and the joint that has to shed

The tread slab is deeper than the going by whatever the nosing projects: a 400 mm going with a 50 mm overhang wants a 450 mm slab. The overhang throws water clear of the riser face below and gives the eye a shadow line that tells a descending walker where the edge is. Too little and the water runs down the riser and stains it; too much and the unsupported lip snaps, which is why the projection is measured rather than eyeballed and why the tread is bedded solid to the front of its support.

Sizing is not patio sizing. A patio is a field with cuts around the edge; a flight is nothing but edges. Every tread is cut to the flight width at both cheeks, the landing is a different module from the treads, and the waste factor that would be generous on an open area is thin here. On the example flight the eight treads at 1.2 m by 0.45 m come to 4.32 m², the landing at 1.2 m by 1.3 m adds 1.56 m², and 5.9 m² in 600 by 450 units is 22 whole slabs before a single cut — which is why the order goes in at 26.

Then check what you are standing on when it is wet. Slip resistance is measured, not asserted: the pendulum method of BS 7976 is the reference test in the UK and the Slip Resistance Group guidelines are what the numbers get compared against, with wet values and a higher expectation on stairs than on level ground. Manufacturers of concrete flags to BS EN 1339 and of natural stone publish tested values, and a honed or polished finish that is entirely sensible on an interior floor is the wrong product on an exterior tread. Sawn sandstone, riven stone, textured concrete and clay pavers all behave differently under a February film of algae, and the algae is not optional in shade.

Measure every riser before the mortar goes off

The check is worth doing at the one moment it can still be acted on: after the treads are bedded and levelled, before the joints are filled and while the beds are green enough that a slab can be lifted and re-laid. Run a tape nosing to nosing on every riser in the flight — not the ones that look wrong, all of them — and write the readings down in order. The two that will be out are the first and the last, because those are the ones tied into a lawn and a terrace built by somebody else and finished to a level that moved after you took it.

What the readings have to satisfy is a flight-wide rule, not a neighbour-to-neighbour one. The International Residential Code and the International Building Code both limit the difference between the greatest and the smallest riser in a flight to 3/8 in, or 9.5 mm, with the adopted edition governing; Approved Document K and BS 5395-1 express riser uniformity in their own terms for work in England and Wales, and the National Construction Code does the same in Australia. A flight can pass every adjacent-pair comparison you make with a spirit level and still fail the spread between its two extremes by a wide margin, which is how uneven flights get signed off and then trip people for twenty years.

1. Measure nosing to nosing on the flight centreline, with the same tape, at the same point on every tread.
2. Record all ten readings in order rather than only the ones that feel wrong.
3. Subtract the smallest from the largest and compare the difference against the variation your jurisdiction allows.
4. If the spread is over, lift and re-bed the offending tread while the mortar is green — packing a slab later moves the fall as well as the riser.
5. Re-measure either side of any tread you adjust, since a correction at one step moves both risers that touch it, and keep the list — it is the only evidence of what the flight measured on the day it was built.