How to trim a window in a custom home

How to trim a window

In the world of a million DYI and builder shows you have probable seen your share of people trimming a window. I will not explain to you every way to trim a window; I will explain to you how custom home builders trim out a window properly;

The material

Here is what you are going to need to trim a window out the way we do it;

• Build out material (if the window did not come with the proper depth of window build outs on it already.

• Enough casing and back band to go around they entire window plus 1 extra foot per side.

• You will need a piece of stool for the bottom of the window, this should also be 1 foot longer then the window is wide.

The first thing that you have to do is install the build out in the window. That is the wood that goes from the actual window out to the edge of the drywall. Some windows come with this already attached but most window build outs end up to short to make it flush to the drywall and need to be extended.

Take flat stock material that matches the trim that you are using and rip it on a table saw to the desired thickness. Cut all four sides to the length required and nail and glue them together on the floor. When you measure this out you do not want your build outs to be flush with the existing build outs that are already in the window. You want to leave a review of a 1/8th of an inch. This allows the trim to move a little between the different seasons and humidity levels that a home will go through. When measuring the depth of the build out material you allow the build out to stick slightly past the finished drywall. This is because drywall is not perfectly flat, drywall is skim coated with drywall mud making it an uneven surface to work on, by leaving the build out proud of the drywall you will be able to install the trim without any bumps or bulges in your trim. Install the build out with shims and trim nails making sure that it fits tight against the window frame.

Now that your build out is in its time to install the stool. What is a stool you ask? It is the piece that is installed on the bottom of the window that extends out into the room like a little shelf.

We do not picture frame windows when we trim them, it is a cheap way to trim a window and in a custom home for a little extra money and effort you can make the window look like it was installed by a professional and not a handy man. Picture framing is when you install four pieces of the same trim around the window with a 45 degree miter on each corner.

When measuring for the stool measure the bottom of the window between the new build outs that you just installed. You must now add the width of the trim and backband for each side of the window. Then add one extra inch to the measurement. You want the stool to be extended past the backband on the trim when you are done.

Cut the stool at the required length at a 45 degree angle on each end. Then cut end pieces at a 45 degree angle so that the ends of the stool will have a return on it. Glue and nail the returns on the end before installing the stool. Set the stool at the bottom of the window and leave a 1/8th of an inch reveal from the edge of your window build out. To install the stool you will need to pre-drill holes through the face of the stool into something solid like the window build out or the framing around the window. Use long trim nails and glue to attach the stool to the bottom of the window through the pre-drilled holes.

Now that the stool is installed you need to install the header of the window. You will be using your casing material for this.

Measure from the inside edge of the build out across the window to the other inside edge then add ¼ inch for you reveal. Cut the casing at a 45 degree angle from the measurements that you just took. Once you have both ends cut at 45 degrees then install it with trim nails and remember that you want to leave a 1/8th reveal from the build out.

Once the top piece of trim is installed then you need to install the legs. Measure from the top piece of trim down to the stool at the bottom of the window. You are going to want the trim to sit right on top of the stool, cut and install them so that there is 45 degree angle at the top so that it fits perfectly with the top piece of trim. Nail the trim and remember to leave the 1/8th of an inch reveal from the build out.

Now that the trim is installed around the window you need to install the apron. The apron goes under the stool; it must be installed upside down so that the thick end of the casing is against the stool. You will want to cut the apron so that it has the same returns on the end as the stool. Cut and install the apron so that it is smaller the width of the window with the casing added. This will mean that the stool will extend past the apron on each end.

The only thing that you have left to do is to install the backband on the casing. The backband is installed by cutting the top piece first and installing it with the backband cut at a 45 degree angle at each end. Then install the side backbands on the casing so that they join at 45 degrees with the top piece.

You have now just trimmed a window the proper way and if you did it right it won’t look like some hack handy man did it.

Rob Abbott
Operations Manager
Village Builders Inc.