Why does a window sill need a drip groove?
A wall that goes dark and blistered six months after a sill is fitted is usually not a stone problem — water has tracked back from the underside of the slab into the wall. The drip groove exists to stop exactly that.
How does water travel back into the wall?
By surface tension. Rain runs to the outer edge of the sill; if the underside is flat and sits close to the wall, water clings and runs inward along it, landing on the wall right below the window. Kept damp, that produces water marks, efflorescence and peeling paint. A drip groove is a small channel cut into the underside near the outer edge, which breaks the path so the water drips off instead of running back.
Should the groove be cut at the factory or on site?
At the factory, always. A site-cut groove with an angle grinder varies in depth and has a rough edge that actually holds water; a factory machine cuts it in one pass, smooth and continuous. So the sill order must list the drip groove as a separate item alongside edge polishing and chamfering, stating its position, width and depth, or simply referring to the detail drawing.
What two other things must be handled with it?
The fall on the outer edge, because the sill must slope outward for water to leave willingly, and the joint to the wall, sealed or rebated so water cannot get in through the gap. The groove only handles the underside; the fall directs the water; the joint stops ingress. Miss one and the other two stop mattering.
Why is this item so often dropped?
Because it is neither in the area nor in standard edge polishing, and whether it was done is almost invisible at handover. By the time water marks appear, six months have passed and responsibility is unclear. The only defence is to write it into the order and, on arrival, hold one slab up to the light and look for the channel on the underside.
How should it be written to leave no ambiguity?
Four lines: the position measured from the outer edge, the groove width and depth, that it is machined at the factory, and that it follows the detail drawing. Do not write the two words drip groove — wrong position is the same as none, too shallow is the same as none. Those four lines are under twenty words and save an entire wall of rework.
Does an interior sill need one?
Interior sills see no rain, so no drip groove, but they need the same thinking about slope — not for drainage, but to stop water on the surface, from watering plants or cleaning windows, seeping in at the wall side. What matters indoors is the upstand or seal at the wall, not a groove. Do not copy exterior practice indoors; the extra machining buys nothing.