Three garden steps: riser height, tread depth and a frosty night’s worry
The slope from the back door down to the vegetable beds had been a mud slide every winter, so last spring I built three stone steps into it. Steps are more arithmetic than walling. The comfortable rule is that twice the riser plus the tread should come to somewhere around 630 to 650 millimetres, so I settled on risers of 150 and treads of 350, which gives 650 and matches the stride of everyone in the house. Three risers of 150 took up the 450 millimetres of drop almost exactly.
The tread stones were the hard part to find. Each one needed to be at least 400 millimetres front to back so it could overhang the riser below by a few centimetres, 60 to 80 millimetres thick, and reasonably flat on top. I found three in the sorted piles from the field wall, the heaviest somewhere around 80 kilograms, which is more than I can lift cleanly. They were moved on a sack barrow and then walked into place on their edges with a bar.
Underneath each step I dug out to firm ground, laid 100 millimetres of broken stone and compacted it with the end of a fence post, then a bed of sharp sand to take the shape of the stone’s underside. Each riser is a small dry-stone wall two courses high, built on the hardcore, with the tread resting on it and on the sand behind. Every tread was set with a fall of about 10 millimetres from back to front so that water runs off and does not sit in a puddle to freeze. I checked each one with a 1.2 metre level in both directions, lifting and re-bedding until it did not rock when I stood on each corner in turn.
That was the state of things in May. The worry came in the first hard frost of the following winter, when I heard the middle tread knock under my boot. The ground beneath had heaved and the stone was rocking on two high points. In the first dry spell I lifted it, which took two of us and a lever, scraped the sand bed back, replaced it with a stiffer mix of sand and a little lime, and re-bedded the stone with a few taps from a rubber mallet. It has not moved since, through a second winter.
The lesson is that a sand bed is fine on well-drained ground and not fine on clay that holds water. The other two treads, on the better-drained part of the slope, never budged.