Install Extension Jamb Window

I am going to go over the tools required for installing trim and the process for installing window buildouts, casing windows and doors, and baseboard installation in a series of articles. Before I go into detail it’s not as intimidating as some may think. There is a lot of steps to a properly installing interior window trim but they are just that a series of steps mostly simple ones that build up to a grand finale if you will. Tools required for installing interior window trimYou will need:. or full size. compressor. 16 gauge and 18 gauge finish nailers (23 gauge micro pinner is optional).

quick grip clamps. chisels.

angle finder. framing square. caulking gun. coping saw or. corded or. 12″, 24″, 48″ levels (a 72″ level is optional). japanese pull style hand sawInstalling Interior Window Trim.

The first step to trimming a window is the installation of Jamb extensions or “the window box”. Depending on the type of window be it a new construction vinyl window, fiberglass or aluminum the method can vary, the easiest to install is on vinyl windows. The frame of the window unit itself will have either a slot in the frame or a track mounted to the inside face which will accept drywall for drywall returns and 3/4″ wood for extensions which will hide the rough framing and spray foam insulation. Now it’s time to start building the box. 2) Determine the depth of the extensions by sticking your tape into the slot and measure to the face of the drywall or plaster, when I do this I check at all 4 corners because there can be variations in the measurement at each corner. If all 4 corners are within 1/8″ I will use the smallest measurement, this is so that the jamb won’t be proud of the face of the wall, when this happens the casing won’t lie flat.

Now if the measurements vary by more than 1/4″ each piece will have to be cut separately for it to work, some may have to be tapered if their measurements vary quite a bit. I’ve trimmed basement windows where there was upwards of 1″ variation from one corner to the other on a window. 3) Assemble and install the box.

Some carpenters will install each piece of the box individually, this can be a sloppy method as the jamb components won’t have tight joints. A better method is to pre assemble the box first on a work table if you have one or just on the ground. The preferred method of fastening this is with a 18 gauge brad nailer or with trim screws, if you use trim screws be sure to pre-drill pilot holes as screws going into 3/4 stock end grain will split it. For windows getting picture framed I will assemble the entire box and install it as one with a traditional trim detail with a stool the sides and top of the box are assembled then installed, trying to install it with the stool assembled is much harder and can get damaged if the extension slot on the window is a snug fit. The first step is determining the depth of the stool, a proper stool should extend past the face of the drywall 1 1/2″- 2″ depending on the thickness of the casing.

If it protrudes too much it can break off over time. For example, if your jamb extension box is 4″ deep I would use stool material that is 6″ wide to start.

Next is determining the length of the stool. First take the measurement of the window jamb extensions header piece, now you have to add on the length of the required horn length on each end. The most efficient and reliable method I’ve found is to add 1/2″ to the width of the casing you will be using as this will create a 1″ reveal on the stool past the edge of the casing as shown in the photo, for stools getting mitre returns allow a little extra for waste so you can cut the mitre on the ends. So if the final math for this stool measure goes as this if the window jamb is 36″ wide and using 3 1/2″ wide casing the stools total length will be 44″ long finished.

The stool is now ready to install, simply slip the stool into place into the slot on the window. Check to make sure it’s in the right position so you have the same measurement from the tip of the horn to the jamb extension legs. Next shim underneath the stool between it and the stool so it’s fitting snug to the bottom of the jamb legs, and snug in the window slot. I will shim every 16″ or so that it doesn’t sag.

Before you nail it off though, stand back and eyeball it to make sure its straight. If it’s good, nail it in place using 2 1 /2 ” 16 gauge or 15 gauge nails, if you don’t have a nail gun hand nails will work too. The last part to the stool is to install mitre returns for the horns. Simply measure the distance between the face of the drywall on both sides of the stool to the tip of the stools horn. This will be your long point measurement for the returns. The cut is a simple 45 degree mitre.

To install the mitre returns you can use regular wood glue in conjunction with painters tape to hold them until the glue sets up. Cross nail the mitres with a 23 gauge micro pinner or use 2 part adhesive such as mitre pro or mitre bond which are comprised of the adhesive and an activator spray. The first thing to do is to mark the reveal on the jamb extensions. A reveal will create a shadow line on the jamb which can trick the eye if it’s not perfectly even. A standard reveal is 3/16″.

A combination square is the handiest tool for this as it can be adjusted to different measurements, simply set the square at 3/16″. Next hold the square in the top corners of the jambs inside edges. Mark a pencil line along the tip of the squares blade both on the leg and header of the jamb creating an intersecting line on both sides of the window. From there slide the square along the legs of the jamb and make pencil marks along the leg, and also at the middle of the jambs header. Measure and cutting the casing. The first piece of casing I measure up for on mitred window trim is the header.

Simply measure the distance between the marked reveals where they intersect. Just note that trying to hold the end of the tape on the reveal lines on one side and reading the tape on the opposite is tricky, even more so on a larger window. So a simpler way is to just butt the tape into the jamb extension on one side then measure to the opposite sides jamb extension then add the amount of the reveals for both sides i.e 3/8″ this will be the short point to short point measurement of the top casings mitre cuts. For example, if you measure 45 1/4″ between jamb extensions the casing measurement will be 45 5/8″.

A little trick that comes in handy for keeping the face of the mitre tight on casing is to back cut the mitre. Essentially this is when you are intentionally cutting the mitre out of square on the bevel.

You will make a bevel cut to the back side of the casing. By doing this when the casing is set in place, if the drywall isn’t flat, the casing doesn’t always lay flat which in turn causes the mitre to be pushed open on the face. You can do it two ways. 1) If you have a dual compound mitre saw simply set the bevel on 1 degree. 2) If your saw only bevels one way the next best thing is to place a 1/8″ shim or small piece of cardboard under the piece of casing which tips it up. Then make the mitre cut.

How To Install Extension Jambs On Windows

Now it’s time to nail the casing together. First smear some wood glue on the mitres of the casing legs. Take the first piece and hold it in place, it should be a snug fit between the stool and header casing but not so much to cause either to push away. Check the mitre joint, it should be nice and tight if everything was cut correctly. If it is, tack it in place at the short point of the leg, nailing into the jamb extension. Next, tack it in place at the long point of both the leg and the header.

From there nail off the casing along the jamb and then the drywall doing so every 8-12″. NOTE: You can add an extra nail in any spots that feel it is loose. Repeat the process for the opposite side.

Install Extension Jamb Window Installation

Now if the mitres were open when you checked the fit you can allow for this by adjusting the position of the leg casing. If the casing is open where the short points of the mitres are, simply pull the bottom of the leg in towards the window until tight and tack it to the jamb extension.

Install Extension Jamb Window Screen

If the mitre is open where the long points meet, push out on the bottom of the leg when the mitre is tight, then tack it and nail off. Just note that if you were to place the casing legs first and you cut the header last, and if the mitres aren’t tight, you will have to re-cut the mitre angle to get it tight. If open at the long points, the angle needs to be increased to more than 45 degrees and if it’s open at the short point the mitre will have to be less than 45 degrees. All the cutting and nailing is now done, it’s time to prep it for paint. Flush sand all mitres with 100 grit sandpaper so that all matching mitres are flush.

If you don’t it will create a small shadow line, and once painted it will become even more noticeable. Fill and small gaps and nail holes with spackle or wood putty and then sand off the excess. Finally caulk the trim where the casing meets the jamb extensions and where the casing meets drywall, use latex acrylic painters caulk such as the kind Dap makes. Even if the joints are tight over time the paint can crack at these junctions leaving unsightly black lines.

The long horizontal piece of trim is called a lentil.