Granite Bullnose Edge Radius: Why It Is Half the Slab Thickness
A granite bullnose edge radius is not a design choice. Full bullnose is slab thickness ÷ 2, so a 30mm slab takes R15 and a 20mm slab takes R10. The same thickness decides how much vertical face a half, double or eased edge leaves. This guide gives the arithmetic for all four, what a small radius ruins, and the edges you must mark square.
What actually sets the radius on a granite bullnose edge?
Slab thickness sets it. On a full bullnose the radius is thickness ÷ 2: R15 on a 30mm slab, R10 on a 20mm slab. The reason is geometric. The half circle has to span exactly one slab thickness so the arc lands on the top face and on the bottom arris. Every extra millimetre of radius takes 2mm off the face left (face = thickness − 2R). On a 30mm slab, R16 gives 30 − 32 = −2: the arc is larger than the thickness and has to be sunk 2mm into the face. R14 gives 30 − 28 = 2, leaving 2mm of straight face above the bottom arris. Only at R15 does 30 − 30 = 0, with the arc running from face to bottom. A 20mm slab and a 30mm slab both end up with a complete half circle, only at a different scale: R10 is a half circle 20mm across, exactly the 20mm thickness, and R15 is 30mm across, exactly the 30mm thickness, with zero face left in both cases. Change the thickness and the R has to be recalculated; R cannot be carried over from the last slab. Set the thickness first, then the radius.
Full, half and double bullnose: is the radius the same thing as the face left standing?
No. They are three fixed arithmetic relations. Full bullnose: R = thickness ÷ 2, no vertical face left, the whole edge becomes a half circle. Half bullnose: only the top arris is rounded, R is still thickness ÷ 2, the bottom arris stays square, and the face left = thickness − R. Double bullnose: both arrises are rounded, each side takes its own R, and the straight face between them = thickness − 2R, e.g. R8 top and bottom on a 30mm slab. Eased: a few millimetres only, to kill the sharp edge and stop chipping, not a decorative profile, and it barely reduces the face. Four edge types, four different tool paths: a full bullnose takes one arc across the whole thickness, a half bullnose the top arris only, a double bullnose one pass on each arris, and an eased edge a single light pass. Change the R on a double bullnose and the straight face between the arcs moves with it (thickness − 2R), so the two numbers are checked together.
| Edge type | How the radius is set | Face left | Where it is used |
|---|---|---|---|
| Full bullnose | Thickness ÷ 2 | 0, the whole edge is a half circle | Pool coping outer edge, exposed countertop edge |
| Half bullnose | Thickness ÷ 2, top arris only | Thickness − R, on a 30mm slab 15mm remains | Stair nosing, window sill outer edge |
| Double bullnose | R on each arris, e.g. 30mm slab R8 | Thickness − 2R, on a 30mm slab 14mm remains | Freestanding walls, island edges seen from both sides |
| Eased | A few millimetres, not taken from thickness | Thickness minus a few millimetres, barely visible | Paving, kerbstones, edges handled by hand |
What goes wrong when the radius is too small?
Three things, all found on site. First, a very narrow straight face is left along the arris. It has no arc to shed impact, so it chips first during stacking, transport and tamping, and it chips on the exposed line of the piece. Second, the arc closes too fast: run a hand along it and you feel a distinct edge, which defeats the purpose on pool coping and countertops. Polishing does not hide that edge; it is a shape problem, not a gloss problem. Third, when two pieces are joined the start of the arc does not line up, one begins at the face and the other behind the straight face, so the joint steps and no amount of pointing hides it. A radius that is slightly small is not a small error; it is the wrong edge type. And the cut only goes one way: grinding the residual face away means taking another strip off the width, and width taken off cannot be put back.
Pool coping and stair treads: which arris does the round start from?
Separate the visible edge from the bearing edge, and round only the visible one. Pool coping: full bullnose on the outer long edge over the water; the side that beds against the pool wall stays square, otherwise it will not sit tight, the joint cannot be filled, and water finds the gap. Stair treads: round the top of the front nosing only; the two ends that meet the wall stay square. Window sills: round the outer edge, keep the wall side square, or the sealant and trim have nothing to sit on. When one piece carries both, call the edges out by letter: edge A full bullnose, edge B square. Mark the square edges as well; writing only "all edges visible" leaves the workshop to guess by habit. On a medium-grain granite such as Sesame White G603 the pattern has to run continuously around the arc; round the wrong edge and it shows once the piece is fixed.
The slab thickness has a tolerance. Which thickness do I set the radius from?
Setting the radius from nominal thickness pushes the whole tolerance onto the straight face above the bottom arris. Face left on a full bullnose = thickness − 2R. With R locked at 15 for a nominal 30mm, a 29mm slab gives 29 − 30 = −1: the arc is already larger than the thickness, so it has to be sunk 1mm into the face and leaves a dished band along the edge. A 31mm slab gives 31 − 30 = 1, leaving 1mm of straight face above the bottom arris. Thin slabs eat into the face, thick slabs leave a straight face, and one drawing produces two different edges. Two workable methods: grade by measured thickness and give each group its own R, or keep R on nominal and state the permitted face width on the drawing, so the workshop knows when to correct. Put the method on the order; saying close enough is not an instruction. Measure ten pieces from the batch, note the high and the low, and subtract one from the other: that difference is how far the straight face will drift. How to negotiate thickness tolerance itself is a separate subject, covered in the tolerance guide.
What four numbers do I put on the order for the factory?
① Thickness: nominal plus the tolerance you will inspect against, not about 30mm. ② Edge type by its proper name: full bullnose, half bullnose, double bullnose or eased, not rounded edge treatment. ③ The radius itself: R15 means R15, not match the slab. ④ Which edges are visible and rounded and which stay square, listed edge by edge as A, B, C, D. Miss any one of the four and the workshop follows its own habit; a piece that comes back off drawing is an order problem, not a machining problem.