There is so many different ways that professional siding installers cut vinyl siding trims.  If you are just starting out, it can be overwhelming trying to learn it for the first time. But, it doesn’t have to be hard with this easy 1-2-3 method for cutting J-Channel.

When I was first learning I thought I would lose my mind trying to remember how to cut just one piece of J-trim.  The boys would show me over and over again. But, I just couldn’t seem to follow along. That was until; someone showed me this super simple method of cutting J-trims.

I have taught this simple 1-2-3 method to many people just learning to install vinyl siding.  And they have all master it in 20 minutes or less.

Even this guy!


The 1-2-3 Method gets its name from the three trim pieces you need to cut to go around an opening, as well as the number of cuts you need to make per piece. 1-2-3! If the picture tutorial doesn’t make sense to you, I have attached a video tutorial at the bottom of this page,  that you will be able to follow along with.

There are only a few ways you need to cut a piece of J-trim in order to go around doors and windows.

How to “J” up a Window or other wall penetrations.

First you need to install the proper drip cap (if needed) under the window.  Here in our area, we need to install a reverse aluminum drip cap to the bottom of the window in order to meet the building codes in our area.  Inspectors like to see the reverse drips have folded tabs that wrap around the bottom of the window like this. 

If you need to learn how to cut Drip Caps for Openings, just pop over here.

Next we need to cut the bottom J-trim of the window.  I call this the Goaltender cut. (maybe, because I live in Canada, and love Hockey) But most likely, because it reminds me of a goaltender with his arms out to the side… saying “No Goal.”

Cut a manageable piece of J-trim about 3 inches longer then width of the window.  Next you are going to Cut &remove the back section of the J-trim, about 1- 1.5 inches from the end of your piece of J.   (We call this “gutting out the back” or “cutting the guts out”.) Leaving the front face intact and un-marred.

cutting guts out of vinyl j

Now, hold your J-trim up to the bottom of the window, making sure the side you just cut is flush with the side of the window frame.  Then on the other side of the window, mark where the window frame touches the J-trim. Give yourself about 1 to 1.5 inches extra beyond that mark and cut your piece.  Go back to the pencil mark you just made and cut the guts out or cutback section of the J-trim as you did on the other side.

siding j cut for below windows

You should now have a J-trim piece that looks like this.  (A Goal tender, with his arms out) Nail that to the bottom of your window opening, and let’s keep moving.

How to cut J-Trim pieces for the sides of a window or other opening.

Rough cut two pieces of J-trim for the sides of you window or other opening, ensure your pieces of vinyl J-trim are 5 inches longer then the window.  (longer if in cold weather, in case you get breakage.) 

You can figure out how deep into the J-trim you need to make your first cut by tucking the J-trim behind the bottom window J you just cut.  Hang it, ever so slightly past the front face of the bottom J-trim, so the side trim sits a little proud. Mark it with a pencil where the two trims meet.

After doing this for so many years, I just know that about 1” should be enough to give me enough length to cover the face of the bottom J-trim once I am done cutting.   You can eyeball it, and trim the piece after it is nailed on.

first cut on vinyl j trim for upright

Cut your piece on a 45 degree angle. (1)   You can see in the picture below that I do not cut right from the leading edge of the trim piece.  Rather about ¼ ‘ from the rolled edge. I do this, because over time the vinyl trim will shrink. If I cut it at too steep of an angle, over time you will end up with a gap in your trims. 

However, lots of professional siding installers will cut the J’s to a sharp point.  It’s not wrong, just different. I like to go back years later, to a project I have completed and see that my J’trims are still neat and tidy without any gaps.  (each to his own)

 Next, roll the J-channel one quarter turn, so you are looking at the side of the trim and make another 1” cut, on a 45 degree angle in the same direction as the first cut. (2)  The tips of your siding scissors or snips should be pointing at the same direction & angle as your first cut.

second cut on vinyl j for upright

Finally,  Roll the J- Channel another ¼ turn,  so you are looking at the back side of the trim piece.  You should be looking at the side with the nailing flange on it.  You will cut about 1” on a 45 degree angle, with your siding scissors /snips pointing in the opposite direction of your first two cuts. (3) 

Third cut on vinyl j for upright

This third cut, nips out a triangle piece from the back edge.  And will leave you with a free moving, foldable tab.

final look for vinyl j trim

 This moveable tab is used to lock the two pieces together. 

Slip the newly cut side J-trim into the bottom window trim once again, and marking just shy of where the two pieces meet on the bottom J-channel & trim it so the bottom J will definitely tuck inside the J-channel for the side of the window.  Now slide the “side window “trim over top of the bottom J-trim, and fold the tab under the window until you hear or feel it click into the rolled face of the bottom J-channel. Nail it on & repeat for the other side of the window.

marking the vinyl j trim for final fit

How to Cut Vinyl J-Trim to go over the top of an opening – Like a window, door, vent or electrical penetration.

Make sure you have the proper drip cap installed over top of your opening.  Again in our area the inspectors like to see folded aluminum drip caps installed over opens with at least 1’ gap.  This gap is for water management. The theory behind this style of drip cap is that water cannot fight gravity. The one inch space allows water to free fall off a vinyl trim piece, rather than climb/walk up behind a trim piece; potentially forcing water under the building paper, or into window frame openings.

Cut a manageable piece of J-channel about 3 – 4 inches longer then the window width.  (longer if you are siding in cold weather, as you may get breakage once you start cutting.)

Tuck the piece into the two upright side pieces that you nailed to the side of the window.   Slide the top trim piece so that it is just a smidge longer than the side window J-trim. Make sure it is not flush, you want it sits a bit proud and hang over the side trim. 

Mark where upright “side” j-channel meets the bottom face of the “top” j-trim.  Pull the piece out and make your 1-2-3 cut, remembering to roll each time ¼ inch after each 1” cut. 

Nip the J-trim on the side of the window,  so the top channel will fit over top of it neatly.   Fold the moveable tab down until it locks into place.  

Next, moving to the other side of the window, repeat the marking steps of where the j-channel meets the bottom face of your top trim.  Then mark piece to length or where you want the trim to stop. (this cut should be just proud of the side J-trim. The 1/8 over hang will allowing the vinyl to shrink over time and not show any gaps.)

top cut for a window with vinyl j trim

Finally, click or seat the top J-Channel piece into place by folding the movable tabs downward into the side window j-trims until you hear them click.   Nail her on.

And there you have it, a fully  Jay-ed up window or opening.

Pictures not making a ton of sense !?? Simply follow along with this easy step-by-step video tutorial. 

Please note: You can easily purchase a fancy tool that will cut the j-trims for you.  But, then someone needs to teach you how to use that tool. But, do you really need another silly tool in your pouch?? (Seriously, mine is heavy enough!)  Or, what if you have plans of tackling only ONE siding project. You are not looking to quit your day job and becoming a professional siding installer. Soooo….Why would you need to purchase another tool?

Don’t waste money on a tool you quiet possible will never use again in your future? Just simply follow along with this easy step-by-step video tutorial.  

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