Granite window sill horn length: set the lower limit, then check the pier
Granite window sill horn length is not a figure to guess. With a stone frame the lower limit is the frame width; the upper limit is the pier width minus the thermal gap. On a 1.80m opening and 0.30m depth, a 60mm ear gives a 1.92m board at 46.7kg, and a 100mm ear costs 7 pieces out of every 100 m².
What is granite window sill horn length, and what are the ears actually doing?
Start with the answer: the length is not a number you pick. You set a lower limit from the edge the ear must cover, then check an upper limit against the pier. The ears, also called horns, are the lengths of board that run past the clear opening at each end. They do two jobs at once. First, they cover the reveal of the opening: with a stone frame they cover the bottom of the frame legs, and without a frame they cover the plaster reveal. Second, they sit the board ends on the piers at both sides, so the board is really supported; some drawings call this burying the end in the wall. One job sets the lower limit, the other sets the upper limit, and the ears can take any value between them. Get the frame width and the pier width first; with only one of them the length cannot be fixed. Keep the direction clear as well: the ears appear only at the two ends and they set the board length, while the depth edge follows the wall thickness and takes no part in this subtraction. A drip groove under the board is a separate item and does not change this line.
How long is long enough? The lower limit comes from the reveal, the upper from the pier.
The lower limit is the width of what the ear covers. With a frame, take the frame width B and keep the ear end face flush with the outer face of the frame; which saves stone and needs no extra allowance. To run past the frame end, add a cover allowance to B, and the allowance comes from the reveal detail. It is tied to how the frame end is built, and a change there means checking the ear again. With plaster only, take the plaster reveal width C and measure it on site: plaster thickness plus the width of the stop bead. Below the lower limit the reveal stands proud of the board end and the joint faces the wall. The upper limit is the pier width D minus the end gap; go past it and the board runs into the corner or the next opening. One example: a 100mm frame and a 120mm pier leave E between 100 and 119mm; 100 sits flush with the frame and saves stone, and at 119 the board end still has 1mm to the corner. If B already exceeds D minus that gap the range is empty, and the only ways out are a narrower frame, a single ear, or a mitred corner piece. That gap is thermal movement, not habit. Granite expands at 8×10⁻⁶/°C, and at a 40°C range that is 0.32mm per metre. On a 1.96m finished board the two ends add up to about 0.6mm, roughly 0.3mm at each end. So the gap is set in whole millimetres, and neither end may be packed tight.
The two piers are different widths, so can the ears still be equal?
Yes, and they have to be, so that the two end gaps match. Take the value from the narrower side and let the wider pier keep its extra width; do not stretch an ear to use it up. Where one side has no pier, as on a corner window, two windows meeting with nothing between them, or a wall that ends at an internal or external corner, make a single ear and keep it at one end only. Say on the drawing what supports the other end instead of assuming it will stand on its own. At a corner you can also cut a 45° mitre. That piece is corner stock, its length comes from the mitre joint, and it is no longer an ear, so the subtraction above does not apply to it. A run of windows is a different case: the ears of two frames go into one shared pier, and together they must not exceed the pier width minus two end gaps, or the two boards meet inside the pier and push against each other.
Is the sill horn, the ear, cut from the same piece, or glued on afterwards?
Cut it from the same piece. A glued-on strip puts the joint exactly at the root, where the bending stress is highest, and the grain never lines up; that is where a broken ear starts. Three real causes, one action each. A glued joint: cut the ear as part of the board. Hollow bedding under the board end: bed the full length in mortar and press it down, then tap every piece after installation, and a dull sound means a void. Both ends packed tight: leave the thermal gap from the previous section. Finished length equals clear opening W plus 2 × ear E, and the board width is the depth into the wall, including whatever stands proud of the wall face. Round the internal corner at the root instead of leaving it sharp, because a sharp corner is where stress concentrates. Cutting it in one piece uses a little more stone than a glued strip, and that stone buys an ear that does not crack.
How much stone does one window take? Run the length times the depth.
Three lines do the whole calculation: finished length = W + 2E, area = finished length × depth, weight = area × 81kg. The 81kg is the weight of one square metre of 30mm granite, from 0.03 × 2.7 t/m³. Below, the clear opening is 1.80m and the depth is 0.30m. Sesame White G603 is kept in 30mm polished slabs, with a 100 m² minimum and 5 days for regular production, and a 20ft container takes 346 m² flat packed. The three ear values only show the arithmetic; they are not recommendations. The lengths in the table are finished lengths; cutting length adds a trim allowance at each end. Read the table in one sentence: going from a 60mm ear to a 100mm one adds 1.9kg to a piece and costs 7 pieces out of every 100 m², because the ear is bought as extra board length, not as a narrow strip.
| Ear E | Finished length | Area per piece | Weight per piece | Pieces per 100 m² |
|---|---|---|---|---|
| 60mm | 1.92 m | 0.576 m² | 46.7 kg | 173 pcs |
| 80mm | 1.96 m | 0.588 m² | 47.6 kg | 170 pcs |
| 100mm | 2.00 m | 0.600 m² | 48.6 kg | 166 pcs |
What has to be on the shop drawing so the ear comes out right?
Six figures, and a missing one means a wrong ear. Clear opening W. Pier width D on each side, written separately rather than as one figure. Frame width B. Depth into the wall, including the projection past the wall face. Reveal type, whether framed, plaster only, or run past the frame end, with the cover allowance if there is one. And a note that the ear is cut from the same piece, with no glued strip allowed. Fix B first, then E. Change B and E has to change with it; leave E alone and the board comes up short at one end or jams at the corner. Where there is external insulation or a curtain wall finish, lengthen the ear by the finish thickness, or the board end stops inside the finish and the end becomes a pocket that holds water. A 50 or 60 copied from another drawing is someone else's value; run the subtraction above and get your own. Write all six in millimetres.