Granite door threshold size and price: measure the opening, work back from the door gap
Granite door threshold size and price rest on three measurements — clear opening, jamb depth, height above the floor — and four cost items. The finished length runs past the opening and tucks under the jamb, thickness is worked back from the door gap, and anything longer than one piece needs a joint.
How are granite door threshold size and price set, and why is the finished length not the clear opening?
A granite door threshold — a door sill, in the older term — is sized by three numbers, plus one insertion rule; the price then follows from four machining items, so start with the size. The clear opening is measured between the inner faces of the two jambs. It is a base figure, not the length of the stone. Once the jamb is fixed its lining has thickness, so a threshold cut to the clear opening is held by a bead of sealant alone; two wet seasons and it works loose. The finished length runs past the opening at both ends and sits under the lining: finished length = clear opening + the two insertions. A 900 mm opening with 30 mm into the lining at each end gives 960 mm. The second number is jamb depth, usually the wall thickness (240 mm in this example), and it sets the width — too wide and the stone projects past the wall and catches feet, too narrow and it leaves the wall opening uncovered. The third is the height above the finished floor, which sets the slab thickness.
| Which number | Where it comes from | What it fixes |
|---|---|---|
| Clear opening | The clear distance between the inner faces of the two jambs | The base figure for finished length |
| Jamb depth | How far the jamb lining crosses the wall, usually the wall thickness | Finished width |
| Height above floor | Rise from the adjacent finished floor to the threshold face | Slab thickness, and whether the door gap still works |
| Insertion at each end | Set by the jamb lining thickness — 30 mm here — not measured on site | Finished length = opening + both insertions |
Should the threshold be 20 mm or 30 mm thick? Read the gap under the leaf first
Slab thickness = height above the floor + depth bedded in the mortar. The bedded depth is what stops the piece rocking, normally not less than 15 mm (an estimate from usual practice); the height above the floor is capped by the gap under the leaf, so the rise must be smaller than the gap. Worked example: one door measured at a 12 mm gap — a size above the usual range — with an 8 mm rise gives 8 + 15 = 23 mm, so a 20 mm slab is 3 mm short of a full bed and a 30 mm slab has room to spare. Fix the floor level first, read the gap second, work out the thickness third — not the other way round. The gap between the bottom of the leaf and the finished floor is commonly 5~10 mm (an estimate from usual practice; the door hardware supplier has the final say), and whatever the threshold raises comes out of that gap. A 20 mm slab is almost fully buried in the screed, leaving a thin arris held by adhesive that a trolley takes off first; 30 mm is our standard cut-to-size thickness, still showing a face above the bed and lining up with the underside of the jamb. Where a bathroom floor has to be raised to fall to the drain, the thicker slab absorbs it; past 30 mm of level change, a plinth is still needed.
How long can one threshold run, and where does the joint go when it cannot?
There is no universal maximum. Three things cap it: the long side of the block, the bed of the cutting machine, and the route from the yard to the fixing position. So do not ask how many metres you can do; ask for the maximum single-piece length in the granite and thickness you have chosen, then hold that figure against your clear opening. Past it the piece has to be jointed, and a joint costs two things: an extra machining step, because both halves are re-squared, matched and re-polished, and a seam, which is where a step, a water path and a colour mismatch all begin. Put the seam where the closed leaf covers it. Both halves must come from the same block and run in the same direction, or the two sides of one threshold read as two different greys.
What does one granite threshold cost, and which items make up the price?
Four items: slab area, edge profile, jointing to length, and packing. Only the first is about the stone; the rest is machining and logistics. Agree the unit before you argue the rate. Thresholds are quoted three ways: per piece, per square metre, or per linear metre of finished length — the familiar "how much a metre" figure. On the same 960×240 slab, a per-piece price usually carries one long edge, a per-square-metre price usually does not, and a per-metre price covers 0.96 m of length with the edge listed separately; compare them as they stand and you are weighing machining against no machining. Multiply the metre rate by 0.96 to get the stone in one piece. A rate per metre of visible edge is a machining unit, not a way to quote the whole threshold; put it on the same line as the slab price and the edge work is counted twice. Ask for three numbers on one line — area per piece (0.23 m² here), machining per piece, total per piece — and the gap shows up as either edge work or stone. Edge profile is charged by the metre of visible edge: if one long edge shows, one is ground; the two ends that tuck under the jamb are not seen and need no profile. A quote that grinds all four edges is charging for work nobody will see. Jointing is charged per joint, packing per piece or per pallet; a long strip fails in the middle, so anything longer than the pallet needs a bearer or a wooden case.
| Cost item | Charged on | On a 960×240×30 mm piece |
|---|---|---|
| Slab area | Finished length × finished width in m², priced at your slab thickness | 0.23 m², about 18.7 kg at 2.7 t/m³ |
| Edge profile | Per metre of visible edge, one pass per edge | One long edge, 0.96 m |
| Jointing | Per joint: one seam and two re-cut edges | Not needed on this piece |
| Packing | Per piece or per pallet; long strips need support in the middle | 18.7 kg per piece, packed flat |
One edge, two edges or a bullnose — and how do the ends finish?
Ask who can see the edge and whether a foot or a wheel will meet it. One side against a wall or a cabinet: single edge. Both sides open to a corridor: double edge. A doorway with children running through or trolleys passing: bullnose. A threshold has one edge worth rounding: the top one people walk on. Rounding the top edge takes the blow from a foot or a trolley; the bottom stays square, because rounding it neither adds area nor counts as a visible edge — it is one extra pass and nothing else. The radius is set by how much of the edge stands proud of the floor, not by slab thickness: with an 8 mm rise only 8 mm shows, so keep the radius inside 5 mm (R5, or a 5×5 chamfer) and the arc still lands on the face instead of running under the tile; R15 and R10 belong to counters and stair nosings where the whole edge is exposed, where the exposed height is the slab thickness. Put it on the order with the edge letter: edge A, the raised side, R5 round. How full, half and double bullnose are cut differently is set out in the bullnose radius article; what is left here is which one a threshold needs — the more edges, the more forming and polishing. The edge flush with the floor where it butts tile or timber takes a 2 mm chamfer only, because a round there leaves nothing for a trim strip to press on; the rest square.
What has to be on the enquiry sheet before you order?
Eight items: clear opening, insertion at each end, wall thickness (jamb depth), height above the finished floor, slab thickness, which edges are visible, whether the length needs a joint, and packing; give the granite and surface finish by their full product name. Without the insertion the factory lengthens by its own default and the finished length stops being your decision. Without the opening it quotes a stock length and you cut on site; without the wall thickness the width is wrong; without the height the thickness is wrong and the door will not close. Mark the visible edges A, B, C, D one by one; all edges polished tells the workshop nothing. A threshold is a small area: twelve pieces come to about 2.8 m² (0.23 m² × 12), far short of our 100 m² minimum, so the usual route is to place them with the floor tiles and stair treads in the same granite, cut from the same block, for a matching colour; the 5-day lead time is scheduled per order. Sesame Black G654 (GB code G3554) is the usual choice: dark enough to read clearly against the jamb.