A window sill looks like a simple slab of stone, but on the outside of a building it has an important job: it takes rainwater running down the window and throws it clear of the wall. A well-made sill keeps the facade clean and dry for decades. A poorly detailed one leads to dirty streaks, damp plaster and, in cold winters, frost damage. Inside, the sill is mostly about looks and practicality, but there are still a few things worth getting right.
External sills: what they need to do
An external sill should do three things:
- Lead water away from the window frame.
- Carry the water past the face of the wall.
- Stop water from running back under the sill and into the wall.
Each of these is handled by a specific detail, and all three are needed.
Slope, overhang and the drip groove
Slope: the top surface of an external sill should fall away from the window so water never stands on it. The slope does not need to be dramatic, but it needs to be there along the whole length.
Overhang: the sill should project beyond the finished face of the wall, including any insulation and render. If it ends flush with the facade, water runs straight down the wall below and leaves streaks. When an old building is insulated, sills often have to be replaced with deeper ones for this reason.
Drip groove: on the underside of the projecting part, a small groove is cut along the length of the sill, a short distance back from the front edge. Water that curls around the edge and tries to run back underneath reaches the groove and drops off instead. Without a drip, water often travels back to the wall and soaks in.
Many sills also have upstands or end details at the sides so water does not run off the ends into the reveals.
Choosing the stone
External sills are exposed to rain, sun and frost, and they often stay wet longer than the wall around them. The stone should have low water absorption and good frost resistance. Granite and dense limestones are common choices. Softer or more porous stones can be used on sheltered facades, but they are at greater risk where sills catch driving rain.
A slightly textured finish, such as honed, brushed or fine bush-hammered, usually works well outside. It weathers evenly and does not show water marks as much as polished stone.
Inside, almost any stone can be used. Marble, limestone and granite are all popular. The main practical points are resistance to staining from plant pots and cleaning products, and enough thickness and support that the sill does not crack if someone leans or sits on it.
Fixing and joints
Even the best sill fails if it is badly fixed. Good practice includes:
- Full, even bedding so the sill does not rock or crack under load.
- Correct sealing of the joint between sill and window frame, usually with a flexible sealant suited to stone.
- Allowing for movement where long sills are made of several pieces, with properly designed joints.
- Making sure the window frame's own drainage leads onto the sill and not behind it.
Sealants that are not designed for natural stone can sometimes leave greasy dark marks along the edges, so the product choice matters.
Common problems
When a stone sill causes trouble, the reason is usually one of these:
- No drip groove, or a drip that has been filled with render or paint.
- Too little overhang after facade insulation was added.
- A slope towards the window rather than away from it.
- Cracks from uneven bedding, letting water into the wall below.
- Porous stone that holds water and flakes after frost.
Some of these can be corrected by recutting, cleaning or resealing. Others, especially insufficient overhang, usually mean replacing the sill.
Whether you are building new, insulating an older house or replacing tired sills, BIHACIT can make stone window sills to measure, with the right slope, overhang and drip for your facade. Get in touch and we will help you choose a stone that suits the building.
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