Getting the Most from Window Sill Trim

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Exterior window sill trim plays a more important role in the way that the facade of a home looks and functions than many people realize. Trim is the final finishing touch on any exterior that helps complete the home’s appearance and bring the entire style and design together. Trim emphasizes architecture, highlights the doors, windows, and siding, and can help cover cut edges and gaps in the siding.

Exterior window sills help bring added dimension to the windows, showing off their size and shape, and helping them stand out from the rest of the facade. They also help prevent moisture and insects from penetrating into the window’s frame and damaging it. This means they’re also one of the first places on the exterior to come in prolonged contact with moisture from rain and snow, making them one of the more vulnerable parts of the facade . As important as they are to the appearance of the home, they’re also the first place that will begin showing signs of age and damage that can be detrimental to the whole facade.

Materials for Window Sill Trim

Window sill trim needs to be strong and durable enough to help protect the window opening in which the window itself sits from moisture, but it also needs to coordinate with the rest of the trim in order to help create a cohesive appearance. 

External window sills are available in a few different materials, but the choice is more limited than with other exterior trim options. Vinyl trim can often be found on many areas of the home, but the nature of an external sill often requires the use of a stronger material.

Most external sills are made from wood and, in some cases, aluminum. Since aluminum sills don’t typically match well with other types of trim, these are usually reserved for masonry and commercial applications, leaving wood as the most commonly used material. 

Wood sills can definitely match the trim used on the rest of the exterior, but standard wood sills have several drawbacks that need to be considered.


Cedar is one of the most commonly used wood species for exterior trim. It naturally resists moisture-related rot as well as insect activity, which means it can hold up well to most exterior uses. Cedar does have a few drawbacks, however, when used on the exterior of a home. When painting the trim, cedar has a tendency to bleed natural extractives  through to the surface, which ends up staining and discoloring the sill surface. This can be prevented by priming the cedar before installation, but it means added expense and time for the installation. Cedar has also become increasingly expensive in recent years, which has led many builders and installers to search for better alternatives.

Pressure-Treated Wood

Pressure-treated wood is less commonly used on trim, but is sometimes used for window sill applications. This type of wood has been soaked in chemicals, then placed in a pressure vacuum to pull the chemicals down into the wood fiber. This preserves the wood and makes it more resistant to wood rot. However, pressure- treated wood has a tendency to display underlying characteristics of the wood species that include splitting, warp, twist, and other fiber movement which can detract from its appearance. It also may leach chemicals into nearby groundwater, which can make it less attractive to many homeowners, particularly those who are looking for greener construction materials. 

Preservative-Treated Wood

A better alternative to both cedar and pressure-treated wood window sills is to use preservative-treated wood sills instead. Preservative-treated sills like those made by Belco Forest Products are given a non-toxic topical treatment that helps the wood resist rot and insect activity, but that doesn’t impact groundwater. 

The trim comes already primed and ready for installation, which means that it will reduce both installation time and expense. Since it is a real wood trim, it also matches up beautifully with the rest of the exterior, without the fake appearance of plastic or aluminum. 

Get a Better Window Sill Material

Window sills are an important part of every property’s trim. Make sure that you choose a material that will complement the aesthetics while holding up well to the elements. Visit Belco Forest Products to learn more about their preservative-treated trims and how you can use them to help create better exteriors.

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