What is a drip detail and why does every sill need one?

The drip detail is the single most important functional feature on a window sill. It is also the feature most commonly omitted on cheap sills ordered from builders merchants.

What it is

A drip detail is a groove or undercut profile formed into the underside of the sill near the front edge. Its job is simple: when water runs along the underside of the sill, the groove interrupts the flow and causes a drip to fall clear of the wall face.

Without it, water tracks along the underside and runs straight down the render or blockwork. Over time this causes staining, render damage, and eventually water ingress at the base of the wall.

Why it matters more on Irish builds

Ireland has one of the highest annual rainfall totals in Europe. A window sill without a proper drip detail on an Irish house is not a theoretical problem. It is a guarantee of damage within a few seasons.

On retrofit and EWI projects the problem is worse. The wall is thicker, the sill projection is greater, and the area of underside exposed to weather is larger. If the drip detail is not formed correctly, water has a longer path to travel and more opportunity to track back to the wall.

What a proper drip detail looks like

On a formed aluminium sill, the drip detail is a continuous groove pressed into the underside of the profile, typically 15 to 20mm back from the front edge. It runs the full width of the sill. Water cannot pass it.

On a poorly made or off-the-shelf sill, you may get a partial groove, a shallow scratch, or nothing at all. These are not equivalent to a properly formed drip.

How to check your sill before ordering

Look at the cross-section drawing for the profile. The drip groove should be clearly shown on the underside. If it is not on the drawing, it is not on the sill.

At Fabricator.ie, every aluminium sill includes a fully formed drip detail as standard. It is not an optional extra. It is part of the profile because a sill without one is not doing its job.