How to Install Window Jamb Extensions

A step-by-step DIY tutorial on how to install window jamb extensions, the perfect details to easily update your home.

It’s funny how tackling the smallest, most tedious things can make a big impression when you’re updating your home. 

Take our window jambs, for instance. We are upgrading our windows and finally got all the tile removed from the window sill. 

But the next part of the project is what we have been worrying about from day one. 

The problem is our windows are framed around brick and plaster and it’s difficult to get any type of material to hold or stay. 

For example, when we took down all the blinds that were installed in the house, the screws that were holding them up quickly fell out. We thought it was going to be impossible to get a wood frame to stay in place. 

But then, we figured it out!  

And here is a guide on installing window jambs extension, even if the wood is not surrounding your windows.

Here are our windows before:

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After removing the tile from our windows, we had a big gap where the mortar was; notice how we have brick surrounding the window.

To fix this problem, we added a 2 x 6 board.

NOTE about CUTTING BOARDS: To cut the boards, we measured first. We measured right next to the window, then the length next to the wall, because sometimes they are different sizes. Then, we cut and tested them to make sure they were going to fit.

One of our best friends during the whole process was liquid nails. To say the least, this stuff is amazing… We applied the liquid nails to the brick to hold the board in place. We also added shims to make sure the board was level.

We also added a 1/2″ x 4″ board to the top of the window to fill in the gap at the top (one of many gaps we found). To do this, we added Liquid Nails to each piece of wood, then taped the wood to the wall. We then let it all sit for 24 hours to dry. 

Do you love the old sheet as our curtains? With tape holding it to the wall, we are high class in this house…

Once we had a base, we started on the jamb extensions. We used 1 x 6 boards cut to the width of the window (which was cut down to 5 1/4″ to 5″ x the length of the window). 

After making sure the window sill (bottom board) was going to fit, we applied Liquid Nails to the base of the window.  

TIP: Make sure all the boards fit properly before adding the Liquid Nails. (It dries very fast so you won’t be able to make any adjustments afterwards)

Next, we added the new window sill.

Make sure the sill is flush with the wall.

You can have a little gap between the window and the board; you will fill this in with caulk later.

Then, we nailed the board into place with 2″ nails.

There are no pictures of the next few steps because they required both of us to complete them. 

I will explain what we did but it is the same process as the window sill step (last step above).

First, we added the board that would be placed at the top of the window jamb, then moved to the sides. Again, make sure you test your boards first to make sure they will fit.

To add the top board/jamb, we applied Liquid Nails to the little 1/2″ board, then nailed the 1 x 6 board to the 1/2″ boards. Next, we added Liquid Nails to the side boards and nailed them into place. 

The finished window looked like this:

Before and after installing the window jambs, I applied window foam to the gaps to insulate around the windows.

It is amazing how quiet our windows are now that we have added the jambs (we could hear cars driving up and down the road before).

I cannot wait to get the trim added to the windows. And once that’s done, I have lots of painting ahead of me.

The whole process wasn’t as difficult as we thought. The only problem we encountered during the step was the quality of the boards. 

Problem 1: We bought the cheap pine board and had them for about 3 weeks before getting to this step and during that time the boards warped. Next time, we will buy the boards and complete the project within a couple of days, instead of delaying.

Problem 2: We had the width cut at the hardware store but some were not at the right width, so we have some bigger gaps. The good thing is the caulk will fix that. We wished we owned a table saw instead of having to find someone with one.

SIDE NOTE: If you are wondering how to apply caulk like a pro, be sure to see my tips. 

Anyone thinking about giving their windows a makeover? Have you thought about the window jamb extension?