About Window Molding

Types

• A window will have wood trim that is installed on both the inside and outside of the window. Sometimes the same wood stock can be used for both sides of the window, but just as often the woodworker will want to use different types of trim for the two areas. It is imperative that the trim on the exterior side of the building be watertight as well as waterproof. Appearance is important here, but the practical side comes first. Trim must be chosen that will properly shed moisture and then it must be prepared (paint is still the best option) so the trim will not rot or mildew.

Preparation

• Before any trim is added, the window must be correctly installed. Windows usually come as pre-assembled units and so the window opening is created to take a certain size of window. However, there is one area of concern that relates directly to the installation of the window trim. And that is the width of the window jamb. When cutting and fitting the jamb, it is very important to calculate the thickness of both the exterior and interior thickness of the wall. For instance, if one uses ½-inch thick sheetrock on the inside of the room, then the jamb should extend inwards, so that the edge of the jamb is flush with the finished surface of the sheetrock.

  Unfortunately, for the outside things are not as simple for two possible situations exist. One is that a stucco finish (or something similar) will be installed, so in that case the exterior trim is installed exactly as just described for the inside. However, a very common scenario exists, where a sub-layer of exterior sheathing (plywood ) is installed with clapboard or weatherboard going on top of the sheathing. In this case the plywood is installed, flush with the jamb and then the trim is applied so that it covers both the jamb and sheathing. Finally, the clapboard or weatherboard goes up and it is butted tight against the side of the trim. This means that the trim must be nailed in place first.

Weatherproofing

• Weatherproofing on the outside involves several areas of concern. First it is imperative that the wood is treated, so that it can withstand the elements. House paint works well here, especially if you prime each piece before it is nailed in place. You can apply the top coat after the wood is put up. Next, it is important to caulk all cracks and nail holes. Nails should be set and covered with some caulk. All miter joints can be caulked as should all boundaries between the trim and the jamb or sheathing.
  Many carpenters add a drip cap at the top of window. It goes right on top of the molding and it is shaped and designed, so it will shed water off the side of the building and prevent moisture from collecting on top of the window. Sometimes it is the little things that make the most difference.

Interior Considerations

• A tight fit is important, when installing interior trim, but what matters most in this case is a neat and clean job that adds to the interior design of the building. For the inside window, often there is a sill that juts out into the room for a few inches and thus changes the way in which the molding is fitted. The four corner method that you used on the outside, often becomes the three corner method for the interior. A special piece of trim goes underneath the sill and for the rest of the window only three pieces are needed and two of them will have butt joints (90-degree cuts) that rest right on top of the sill.

Expert Insight: Cutting and Fitting

• Theoretically, all miter joints on a window in a new building will be at a forty-five degree angle. Sometimes this is true and sometimes it is not. If you are remodeling, this is seldom the case. One method of installation is to install the top and bottom piece with both ends mitered to exactly 45-degrees. Then you can cut and fit the two vertical pieces to be placed in the remaining space. In most cases these cuts will only differ slightly from the preferred 45-degrees, but if the building is really out of whack, you may have to readjust the preliminary cut at the top and bottom of the window.