Granite Flower Bed Edging: How to Install It, Flat Border or Retaining Edge?
Granite flower bed edging is installed one of two ways, and the choice depends on whether it has to hold back soil. A plain division is laid flat, flush or 10~20mm above the lawn. A bed that retains soil needs the 300mm side stood up, but a 30mm board on edge cannot hold soil by its own weight: earth pressure gives about 2.7 kg·m of overturning moment per linear metre against 0.36 kg·m of resistance from the board's own weight, about 7.4 times short, so a concrete backing and weep holes are mandatory.
How is granite flower bed edging installed? Flat border or standing edge?
Start with the answer: how granite flower bed edging is installed depends on whether it has to retain soil, not on which granite you buy. If it only divides — keeping grass and soil apart, locking the edge of paving — 30mm boards are laid flat in a ring, the top flush with the lawn or 10~20mm above it, within one board thickness. That is the flat-lay method. If the bed stands above ground and has to hold its soil, the 300mm side is stood up as the height and the 30mm becomes the wall thickness: the edge-set method. Boards set on edge are a facing only; behind them there must be a concrete strip footing and a concrete backing. We stock this item in 30mm only, so there are just these two routes. Choosing wrong means redoing the work: boards laid flat and asked to retain soil lean after the first heavy rain, and boards set on edge without a backing fall once the soil settles.
How do you lay it when it only divides two surfaces?
Flat-lay does one job: it separates two surfaces and carries no earth pressure. Cut a shallow trench along the bed line, compact the bottom, spread a bed of stiff mortar or sand, seat the boards and tap them down with a rubber mallet, the top flush with the lawn or paving. Where a mower may catch the edge, keep it 10~20mm high — still within one board thickness, so nobody trips. Width follows how much each side has to be held down: lawn on one side and paving on the other takes the full 300mm board; lawn only can use 150mm, one 300mm board ripped in two. Joint width comes from board length and temperature swing: on a 600mm edge with ±1mm length tolerance and 2mm joint sand, our article on paving joint width adds the three items to 5.19mm and takes 6mm, and an exposed bed rounds up, never down. The bed under the board must be fully laid and compacted, never spot-bonded with voids — a hollow spot under the bed means it is not fully bonded, it sounds dull under a hammer, and water that enters the joint stays there.
| Board width | Where it is used | Material per linear metre |
|---|---|---|
| 300mm (full board) | Bed-to-paving edge where the line to hold down is wide | 0.30 m² / 24.3 kg (1.67 pieces) |
| 150mm (300 board ripped in two) | Narrow division between lawn and paving | 0.15 m² / 12.2 kg (0.83 board) |
How do you install it to retain soil, and why does 30mm on edge fail?
Earth pressure and the board's own weight are almost the same order, about 27 kg against 24.3 kg per linear metre; what decides the outcome is the lever arm — the pressure acts 100mm above the base and the weight 15mm from the front toe, so 27 kg becomes 2.7 kg·m of overturning against 0.36 kg·m of resistance, about 7.4 times short. Those figures are an estimate at a sandy soil unit weight of 18 kN/m³ and a lateral pressure coefficient of about 0.33 — an empirical figure, not a code value, and the actual work follows the design drawing. That gap is not something more pressure by hand can make up: the board goes over forwards under the soil. So a board on edge is a facing only, and earth pressure is taken by the concrete backing behind it.
| Item | Value |
|---|---|
| Lateral force at 300mm retained height | about 27 kg per linear metre (estimate at 18 kN/m³, lateral coefficient about 0.33) |
| Point of action | one third of the retained height, about 100mm above the base |
| Overturning moment | about 2.7 kg·m per linear metre |
| Resistance from the board's own weight | about 0.36 kg·m per linear metre (24.3 kg × 15mm) |
| Difference | about 7.4 times |
How are the backing and the footing built? Three steps
Three steps, in this order; skip one and you leave a fault line. Step one, the strip footing: excavate along the bed line to the level on the drawing, compact the bottom, and pour a concrete strip footing sized from the backing, not from the board — board 30mm + backing 120mm + 100mm out front and back gives a footing 350mm wide, an example, not a standard size. The board sits on the front 30mm of it and the backing on the rear, both poured in one go, so the boards never sit on backfill that settles. Step two, the backing: the concrete or brickwork behind the boards is the wall that actually holds the soil; it follows the bed height and is tied into the footing, and the boards are fixed to its face with mortar as a facing. Step three, weep holes: a bed is a place where water collects, so leave a weep hole in the base of the backing at intervals to lead water out to the drainage layer under the paving. Bed the back face and the bottom edge fully with no voids — a board on edge is held by its back face against the backing, so a hollow back face means it is not bonded at all; water expands about 9% when it freezes, and a void that holds water lifts the board year after year.
What happens at corners and ends? Mitre, butt joint, or stop against a wall
External corners are mitred: at a corner that projects, both boards are cut at 45° and joined into a right angle, with the joint running down the point of the corner so it reads as one line from either side. Which dimension the mitre consumes depends on the method: laid flat it takes the 300mm width and the 30mm thickness stays full; set on edge it takes the 30mm thickness and the 300mm height stays full, leaving a vertical joint — fix the method before you cut. Internal corners are butt-jointed: at a corner that turns inwards, each board runs up to the corner and stops, with no 45° cut; a mitre there puts two diagonal joints across each other inside the corner and is harder to close. At an end, the board stops against a wall, a step or a kerb and is butt-jointed to it, with the joint width taken from the method in our paving joint article — never jammed tight, because a tight end has nowhere to move and chips first. Cut corner boards to size; do not force a straight length around a corner on site, it will chip.
How do you check the finished edge? Four checks
Four checks, then sign off. One, pull a line: run a taut line along the bed edge and see whether the top arris of every board touches it; pull it in shorter runs around curves. Two, check levels: the top should be flush with the lawn or paving, or 10~20mm above it, checked with a straightedge laid across both sides — the same all along, not high at one end and low at the other. Three, tap for hollow spots: tap each board with a small hammer; a clear ring means it is fully bedded, a dull sound means a void under the bed, so mark it and lift it to re-bed — spot-bonded work rings hollow board after board. Four, water it: run a hose along the inside of the bed and watch whether water leaves through the weep holes or rises out of the joints; if it rises through the joints, the weep holes are blocked or were never left. Pass all four before you sign off the work.