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Window Sill-A Vulnerable Location Needing Excellent Materials

window sill

Window Sill Purposes

Window sill, or sill stock, is one of the best additions you can make for a custom design look. It can elevate a simple facade for great curb appeal. 

Sills protrude from the window trim and siding with a slight slope, typically between 12 and 15 degrees to help rain runoff.


Heavy Exposure

Sill materials will encounter heavy weather exposure from sun and rain.  Therefore you need a product that can withstand a beating and still look great for the maximum length of time possible. 

Whatever material is used, it needs to be strong enough to withstand the elements. 

Window Sill Important Features

A relief cut is a narrow cut in the bottom of the sill stock.  However, not all manufacturers include this on their sill products.  

It may look small and insignificant, but most importantly, a relief cut is designed to prevent water from wicking back underneath the sill and traveling up behind the window trim and siding.  

Material Options For Window Sills


There are several options for window sill materials in the wood category.  

Cedar is a great choice if budget is not a factor and you don’t care about warranties. Certainly, the price alone can be a deterrent. 

Primed white wood, or PWW, is a versatile and cost-effective option. However, not all PWW sill stock performs well. 

It is imperative to use a preservative treated PWW to prevent rot.   

XT Trim sill is manufactured using a unique process.  Each board is kiln dried then directly treated with Wolman preservative on the raw wood. 

Once dry, each board is painted with an industry leading primer then baked in an oven.  This results in a board with such great durability that it comes with a 20-year warranty.

Belco XT® Trim window sill is manufactured with a 12-degree slope.  Each board has a relief cut on the bottom.

As a result, window sill by XT Trim is cost-effective, long-lasting, accepts paint well, and is a beautiful material that will enhance the finish of any window. 

Our article about Belco’s Wood Preservative Process will answer all your questions

OSB or Fiber Cement

Neither OSB nor fiber cement trim manufacturers currently offer window sill material.  

Most likely, those products are too heavy and brittle to be appropriate for sill materials.

In addition,  the porous ends of these products make for less than ideal finished looks for paint application. 


PVCs and plastics are another option for window sill material.  As plastic is not porous, water can’t penetrate the surface, making it a good choice for sills.

It is important the PVC and plastic sills are properly caulked, to prevent rainwater from traveling to the siding.

There are a few downsides to consider, however.
If the plastic is too smooth, it is difficult to get good paint adherence.

Adding a texture to the surface makes it difficult to match with the siding.

Probably the most important issue is the cost.  PVC is expensive. Therefore, it can be a budget-buster.

If you price out cedar and you’re not happy with the price, plastic may give you a heart attack.


In conclusion the best sill material by  XT Trim by Belco is the clear winner for the cost-conscious builder that is not willing to compromise on quality. 

Our Happy Customers

Preservative treated, primed, and available with a combed, brushed, resawn, or smooth face. The real beauty of your home comes to life through a well made trim package.

Preservative treated, primed, and available with a combed or smooth face. These glulam columns are strong, beautiful, and enhance any exterior design. 

Belco XT®-T&G is graded smooth face with edge vees on all four edges and a textured ungraded back.