What is a Window Jamb?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon
Window jambs are common to many window installations.
Window jambs are common to many window installations.

A window jamb is part of the structural support of a window. Jambs are located inside the framing of the window, along the top and sides, and in the case of windows which open, jambs can house tracks or rails used to manipulate the window, along with devices such as latches which are used to lock the window when it is closed. Similar structural supports around a door are also known as “jambs,” and a “jamb” more generally is any type of vertical support around an opening.

Jambs are located inside the framing of the window, along the top and sides.
Jambs are located inside the framing of the window, along the top and sides.

Technically, window jambs are known as side jambs when they are along the sides of the window, and as head jambs when they run along the top of the window. Jambs are not necessarily required, but they are common to many window installations. Numerous companies which produce windows also make matching jambs and jamb extensions which can be used to help their products fit more snugly; the jamb can be a weak point in a window if it is not installed properly, allowing for leaks or security concerns.

The material used to make a window jamb can vary, and may be wood, plastic, vinyl, or metal. Window jambs may be painted to match the trim on a structure, or left unfinished, depending on the material and the aesthetics of the structure. In cases where windows start to move unevenly or gaps appear around the window, replacing the window jamb can sometimes resolve the problem, with less cost than replacing an entire window or reframing the whole window.

A contractor can determine whether or not a window jamb needs to be replaced and do the work relatively quickly. People who are handy with basic construction tasks can also do work on their own window jambs, ranging from fitting new ones to allow a new window to fit into an existing opening to replacing old jambs which are starting to warp, rot, or leak so that an original window can be left in place. It is important to take time when working with window jambs, as a failure to fit them snugly and properly can result in problems ranging from a window which will not open to a window which is so loose that it is easy to pop out of the frame.

People inspecting structures they are thinking about purchasing should also take time to examine the window jambs. If windows are leaky, the jambs will betray signs in the form of staining and rot. In addition to indicating that the jambs need to be replaced, rotting and staining around a window jamb could suggest that there may be other water damage which is not visible, such as rot as the result of water seeping into the structural framing.

The Difference Between a Window Jamb and Sill

The jamb and sill of a window are often confused. The jamb is the vertical part of the window frame that supports the window’s sides. On the other hand, the sill is the bottom portion of the window frame. For windows that open vertically, this portion of the window holds the glass section when it’s closed. If the window slides left or right to open, the sill has a track to support this movement. Both parts are equally important for providing support to the window.

Cleaning Window Jambs

Since windows open to the outside, it doesn’t take much for the window jambs and sills to become dirty. Dust, water and bug debris fill in the small gaps of the tracks. As the window moves, it spreads the dirt and debris within the frame. When residue remains in the jamb, it can lead to mold and bacteria buildup. It might also build up enough to make it challenging to move with the window or even force it out of the frame.

Steps to Cleaning a Window Jamb

  1. Open the window to expose the window jambs. 
  2. Use a narrow vacuum attachment or portable vacuum to suck loose debris out of each jamb. 
  3. Mix a few tablespoons of baking soda with just enough water to make a paste. Apply the paste to the window jamb with a cleaning towel.
  4. Put cleaning vinegar into a spray bottle, and spray the jambs until the baking soda bubbles. 
  5. Let this mixture sit for five to ten minutes to loosen up the debris.
  6. Use a cleaning cloth or paper towel to remove the mixture. 
  7. Scrub the jamb with a toothbrush to loosen the remaining debris and wipe it away with a towel. 
  8. Use bleach spray on areas where there’s mold. 
  9. Apply a silicone or wax lubricant to keep the window moving freely.

Weatherproofing Window Jambs and Sills

A small amount of space is left between the outer edge of the window and the jamb or sill to make it possible to move the window. However, this also lets outside air and water into the home. Not only does this interfere with keeping the house at an ideal temperature, but it can also lead to water damage inside the home. Therefore, it’s necessary to have adequate weatherstripping to prevent this.

The type of product used to weatherize the windows is available in different options.

  • Felt Weatherstrips – This material comes on a roll, so it can be cut to fit all sizes of windows. It is placed in the window jamb to fill in the gap while still allowing the window to slide up and down. While the product is effective against air gaps, it usually needs to be replaced every few years. 
  • Foam Strips – Insulating foam works great for the contact portion of the window. It’s best when placed on the part of the sill or jamb that makes contact with the closed window. The foam compresses to block out any draft when the window is shut. 
  • Silicone Tubing – Like foam strips, this weatherstripping fits within the window sill or jamb. This material protects against air and water leaks. 

Security Measures for Window Jambs

The windows of a home are a prime target for burglars. Keeping them locked with a secure latch is just one way to improve the security of your home. Here are a few additional possibilities.

Block the Door Jamb

A window’s latch can be broken or loosened to enter a home. However, it’s simple to reinforce the windows by blocking the tracks to prevent the window from moving. The easiest way to do this is by using a wooden dowel cut to the size of the window and placing it in the jamb. If someone tries to move the window, the wooden dowl prevents the window from opening. This simple and inexpensive option works for both vertical and horizontal windows.

Sensors Secured to the Window Jamb

Small sensors are available from various security companies. These devices come in two parts. One part attaches to the face of the window, while the other piece mounts to the outside of the jamb. When the two pieces align, the device is off. If someone opens the window, the connection between the two sides breaks, and the alarm will sound.

Keyed or Screw Locks Along the Window Jamb

A screw or key lock looks like a clamp that attaches to the track in the window jamb. With the screw tightened or the lock engaged, the device secures against the track, and the window cannot open past this device.

How to Shim a Window Jamb 

Shimming a window jamb isn't as difficult as it might sound. It's important because it provides evenness to the installation of the window and ensures that the window will open and close properly, and shimming correctly will ensure that changes in pressure and temperature won't cause parts of the window to warp. Shims can be made of wood or plastic (PVC), and sometimes they are made of rubber.

Also, keep in mind that you can also check with the manufacturer of the window you are handling to see if they have instructions for properly shimming the window.

Placement 

Shims go on the bottom and sides of a window. Typically, the top of a window is not shimmed.

On the bottom, be sure to insert the shims underneath the sides of the window frame so as to support their weight. If there is a seam where two windows meet, that also means you should install a shim in the middle.

On the sides, install the shims several inches off of the horizontal window frames, because if you line the shims up with those pieces it could cause the frame to warp later on.

Installation 

When putting the shims in, you'll need to use a hammer to very carefully tap the shims into place. You should use two shims in each spot that you are shimming, with the thin ends of the shims facing each other and fitting together like puzzle pieces. It may help you to use a little bit of sealant to help the shims stay in place, but just keep in mind that you may need to adjust their placement later on, so it's best not to secure them too permanently at first.

How to Build a Window Jamb 

Building a window jamb involves several important steps, but first, make sure you have the tools and supplies you need, which include the following:

  • Wood planks of your choice
  • Saw or electrical saw setup 
  • Wood glue 
  • Clamps 

Once you have the necessary tools, proceed with the following steps.

1. Measure for the Frame 

First, place the window you are working with into the space it will go in to measure what the dimensions of the window frame need to be to make it fit. Then measure the wood you are working with accordingly in order to cut the four sides of the frame.

2. Cut and Sand the Frame Pieces 

Second, cut the wood according to the measurements you have made. After the wood is cut, make additional cuts on each end of each piece at a 45-degree angle so that all four pieces can be glued and held together. After the cutting is finished, you might want to sand each piece with fine sandpaper to give the frame a cleaner feel and look. However, be sure not to over-sand the wood, because you want to compromise how well they will fit together as one frame.

3. Glue the Frame Together 

Next, glue the four sides of the frame together with wood glue, being sure to apply enough glue at each seam. Use the clamps to hold the frame together while it dries. If there is any excess glue at any of the seams, carefully wipe it off. (If the excess glue accidentally dries, you can also try gently sanding it off.)

4. Coat and Paint the Frame 

Finally, apply any coating and paint to the frame that you want to use. Polyurethane can help seal and strengthen the frame without altering the color and appearance greatly, and stains and paint can be a great way to add a particular style to your window.

How to Clean Window Jambs 

There are several ways to clean window jambs and the whole frame of a window.

Buy a Cleaning Kit 

You can buy cleaning kits made specifically for windows. These kits usually have brushes and other tools designed to reach every part of the window. However, you might not need a kit like this, so see if you can piece together the cleaning tools you need at home before spending money on one.

Baking Soda and Vinegar 

For especially tough dirt, it might take a stronger solution to clean. Put some baking soda on the stubborn areas and follow that by spraying vinegar and letting it sit for several minutes. The strength of this solution will likely be enough to get the dirt out.

Use a Knife 

If part of the window you are trying to clean is hard to reach, try using a knife or similar tool with a light cloth and some cleaning solution on it to get at those areas. You can always follow everything up with an aerosol duster, too, to get dust and little pieces out.

How To Extend Window Jambs

Precisely as it sounds, a window jamb extension is a frame that fills the depth of the wall space from the window's jamb to the face of the plaster. Typically, a jamb extension is a 3/4" board or MDF that is optimal for nailing the casing trim to. Regardless of what your window jamb looks like, the extensions follow the same basic principles.

First, the jamb's extensions for the sides and top must have the same depth as the opening so they'll sit flush with the face of the wall. This is true of all types of jamb extensions except for "picture frame" casings. The sill or stool of the window jamb will also need to be deep enough for the casings to be on top. Typically, you'll need about 1/2" in the front and about 1" on the casing's wall side.

The sill will also need to be notched at each end. This helps to ensure that it will fit into the opening properly. The depth should be nearly equal to the widths of the top and side extensions. The best idea is to cut shallow and then trim them as needed. This prevents you from overcutting and needing to restart the project.

Once you've done the measuring and cutting, it's time to build and install the frame. If your jamb will butt to a flat window jamb, you'll have an easier time if you assemble it on a flat surface first before installing it into the opening. Keep in mind, though, that it will be smaller than the rough opening, so you'll have to shim it to keep it square and centered.

If you want the job you're doing to look professional, keep your focus on the reveals. Most reveals for window casings are 1/8-1/4". You'll need a reveal block or trim gauge, which you can either make or purchase on Amazon, to ensure the reveal trim is precise.

How To Measure Window Jamb Depth

Measuring the window jamb's depth requires a few tools, so first you'll need to gather a pen, paper, and tape measure. Before measuring, keep in mind that most window jambs fit into 2-by-4 wall studs that secure them into place. The opening is typically 4 and 9/16" or 6 and 9/16". Measuring is quite simple.

First, open the window shade or shutter so that you can see its interior jamb. Open it so that you can see the entire side o the jamb. Measure the depth. This means measuring from where the exterior mold attaches to the window and across to where the interior trim attaches to the window's jamb. Typically, this is where you'll get the above-mentioned measurements. It's a good idea to measure more than once to ensure you get the right measurement. It also helps to measure all of the jamb's dimensions in case you find that you need another measurement for your project.

How To Clean Window Jambs

Cleaning your window jambs, sills, and tracks is important for maintaining the integrity of the windows, preventing mold from building up, and keeping your home or office building safe, clean, and aesthetically pleasing. Before you start cleaning them, you'll need to gather your supplies. Grab a spray bottle, some dishwashing liquid, and a few microfiber cloths.

First, wipe down your windowsills and jambs with one of the microfiber cloths to remove any dust or debris buildup. Next, mix a few drops of the dish liquid with some warm water in the spray bottle. Spritz the cleaning solution onto a microfiber cloth. Avoid spritzing it directly onto the windowsills and jambs, especially if they are wood. Oversaturating these areas with water or cleaners can damage the finishes or even warp wood.

Once you've created your solution and sprayed it onto a microfiber cloth, use the cloth to gently wipe the windowsill, frame, and jambs. Don't be afraid to apply a bit of pressure in areas where smudges don't easily come off. Next, wipe everything down with another dry cloth to remove excess moisture. Finally, maintain the cleanliness by dusting once every week or two.

Cleaning Window Tracks

Cleaning the window tracks will take a few different tools because the crevices are harder to get into. You'll need a vacuum cleaner with small handheld attachments, a plastic knife, disposable wipes, and a microfiber cloth.

First, use the vacuum cleaner's handheld tool to suck up any loose dirt and debris in the window tracks. Next, use a disposable wipe to get rid of dirt buildup in the center of the track. To clean the crevices, wrap one of the wipes around the plastic knife. Once you've cleaned the crevices and removed all the dirt and debris, wipe everything with a clean, dry microfiber cloth to remove moisture.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a HomeQuestionsAnswered researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a HomeQuestionsAnswered researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments

rallenwriter

When installing a basement window, how do I get the jamb right? I've been trying to install a replacement basement window, and I can't for the life of my get the jamb to look right.

Does anybody out there have any good basement window installing tips?

Thanks!

EarlyForest

How exactly can I clean a window jamb? I have window jamb liners, but they always seem to get so dusty and gross, so how can I clean these out? Is there a way that I can actually keep these clean, or do I just have to keep trying to clean them whenever they get dirty?

Is there some kind of window protection to install on a jamb to keep it from getting dirty?

It's an aluminum window jamb, by the way.

Planch

When choosing a window jamb replacement, how do I know what size to buy? All I can find is information on replacement window sizes, I can't find any advice specifically on jambs. Does anybody have any tips about getting the right kind of replacement window jambs?

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    • Window jambs are common to many window installations.
      By: Richard J Thompson
      Window jambs are common to many window installations.
    • Jambs are located inside the framing of the window, along the top and sides.
      By: Iriana Shiyan
      Jambs are located inside the framing of the window, along the top and sides.