How to Install J-Trim

  • 2-6 hours
  • Intermediate
  • 100-500
What You'll Need
1-inch Roofing Nails
Window Wrap Tape
Tape Measure
Tin Shears
Safety Goggles

J-Trim is a material used to perfect the edge on aluminum or vinyl siding on a home or small building. You will see this trim around windows and doors and at the bottom and top of the siding so that it may be held in place. Installing J-Trim is not a particularly difficult task and can be done in a moderate time with the proper materials and instructions. Installing it correctly will make the difference in whether your window is subject to leaks or not. Follow the below steps to add J-Trim to your siding.

Step 1 - Prepare Windows

Before beginning installation of your J-Trim, ensure that the top of the window is equipped with drip flashing overlapped by felt paper. This will help keep water from being able to reach underneath the flashing. Ensure that the sides of the window is taped with the wrap tape to keep water out from the sides of the window. Leave the bottom of the window free to allow for drainage should water penetrate the window.

Step 2 - Measurements for Sides

Measure your windows and on the sides and cut J-Trim to specifications with the tin sheers.

Step 3 - Attach J-Trim

After you have cut the J-Trim to measurements, secure the trim to the sides of the windows first with the 1-inch roofing nails. Because vinyl expands and contracts, do not nail the trim too secure. Leave room for the expansion.

Step 4 - Measurements for Top

To get the proper measurements for the top of the windows, you will need to measure across the outside edges of the trim you just installed on the sides of the window. Cut the trim to the specifications of your measurements.

Step 5 - Cut Inside the J-Trim

With the edge of your shears, cut the inside of the trim on both ends to create a tab. This will allow the tab to easily fold down and allow any water to run along the outside of the J-Trim when rain comes.

Step 6 - Attach J-Trim

After you have created the tabs, attached the J-Trim to the top of the window. As with the side pieces of the J-Trim, do not nail the trim too securely in order to allow room for expansion of the siding. Ensure your tabs are bent downward for proper water flow.

Step 7 - Measurements for Bottom

Use same instructions from Step 4 to get the measurements for the bottom of the window. You will need to follow Step 5 when making the cuts for the tabs. The fold of the tabs for the bottom piece of the J-Trim will fold upward and fit snugly into the inside of the side J-Trim moldings. This will prevent water from flowing into the siding from the bottom.

Step 8 - Finishing Touches

Add the clear caulking along the window trim and the J-trim for a professional finish to your project. Use these same steps when doing the doors to your home.