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Being far more than just a simple architectural feature, windowsills are a necessary part of a window. The sill is a crucial part of the window trim, the head casing and side jambs and together they help to keep out the cold and keep heat in. Without these essential components, windows would be structurally unsound and emit noise to an irritating level. Contrastly, Interior sills are very different from exterior sills due to the different roles they play. Exterior window sills are usually integrated into the window itself, whereas interior sills are a separate entity entirely. As a predominantly decorative feature, interior sills have the aesthetic advantage of ranging in styles.

Exterior Sills

The window sills on the outside of a property are exposed to nature and therefore must be able to withstand extreme weather elements. The windowsills main job is to protect the masonry from rainwater that runs down the exterior wall. It’s best fitted with a drip edge, metal sills or a drip groove on stone sills. Both of these halt the accumulation of rainwater collecting underneath the sill and drip feeding water onto the joint between the windowsill and the brickwork. Exterior windowsills are required to be installed at a downward gradient of at least 8 % and the recommended distance for the drip groove from the wall is a minimum 30mm. One of the most popular materials for exterior windowsills is metal; aluminium, zinc or copper, but these don’t come without a downside. During the summer months they can become very hot and in the cold they can produce ice formations which can be highly dangerous when protruding from upstairs windows. Another annoying trait of metal windowsills is the noise that falling rain produces when bouncing off the metal. To avoid this it’s recommended to apply an anti-noise film.

In addition to metal windowsills there are also natural, artificial or concrete stone, roof or quarry tiles, again positioned when fitting at a downward angle.

Interior window sills

Interior sills are mostly made from wood, concrete and artificial stone, polished marble or granite. If you choose wooden sills for bathroom and kitchen though it’s important to either tile them or finish the wood off with varnish. The most highly recommended is yacht varnish. DIY Doctor recommends a minimum of three thin coats, rubbed down between applications. Although it’s not necessary on a structural level to install an interior window sill, they do add a lot of character to a room, and provide an extra spot to make a house feel more like a home.


An intriguing feature of window sills are the superstitions surrounding them –

  • Placing a penny, tails side up on the window sill is supposed to bring the house luck, and ward off evil spirits. Also, birds have long been symbolic of a link to the spirit world. The myth says that if a bird lands on your window sill and taps the glass with its beak, it’s not good news!

In a later post we will look at some of the great ways to decorate your window sill.