How to Weld an Open Root

Figuring out how to weld an open root is a big step in your welding journey—it can seem intimidating—but you don’t need anything fancy or to overcomplicate things. It’s so simple, you can even learn right at home.

In this guide, we'll teach you the easiest and fastest way to learn this important (and fun) welding technique, using a simple outside corner joint. This guide focuses on TIG welding, but the same setup can be used for stick or MIG.

welder tig welding an open root

For an overview of different methods, check out our guide on the 3 ways to TIG an open root.

Contents

  1. Materials – what you need 
  2. Prep – spend your time welding, not prepping
  3. Jig – make your first welding Jig 
  4. Fitting up
  5. Tacking 
  6. Amps and Rod placement
  7. Starting the open root weld – How to launch off the tack 
  8. Welding the open root – What to watch for 
  9. What the back and front should look like

Materials

All you need is a couple pieces of scrap plate. Anything from ¼”-½" will work. These will be tacked into an outside corner with a nice gap between them.

two pieces of 3/8 inch scrap mild steel plate covered in mill scale

This will allow you to spend more of your time welding open roots, not meticulously prepping and beveling metal. The basic techniques and skills learned will transfer over to plate and pipe.

Prep

Clean the metal up with an angle grinder. A flapper disc or hard stone is fine.

The metal plates have been prepped. The mill scale has been removed revealing shiny clean base metal.

Remove all the mill-scale and get down to shiny silver metal. Clean one flat edge on each plate, as well as at least an inch back on both sides. Keep those corner edges sharp.  

Make a jig

A piece of angle iron with a foot on the bottom. Any metal will work. The example below happens to be aluminum.A small piece of aluminum flat bar is welded on the bottom of a length of aluminum angle iron.

You want the angle iron to be 6-12" long. Flip it upside down and tack one or two pieces of flat bar, scrap, or strips of plate onto it, so it’s able to stand. This jig will come in handy for other welding drills and projects in the future.

Fitting up

Now it’s time to fit up your outside corner. First, bend a piece of 1/8” filler metal (this will form the gap between the plates) into a horseshoe shape like shown below.

The two plates are fit up into an outside corner on the jig. A piece of 1/8 inch filler rod is being used as a spacer to form a gap.

Put both plates into the jig with the filler rod spacing them apart. You want to get the edges of each plate lined up with one another. It’s helpful to get on the end of the plate and look down them like a sight. Another trick is to look at the corner of each plate on the outside. The corner of each plate should be pointing right across at the other. If you don’t get the plates lined up here, your gap will end up uneven, with one plate higher than the other, which will make your initial practice harder. You want everything in your favor when first starting out.  

Tacking

After the corner is lined up on one edge, it’s time to tack it up. The other end doesn’t have to be perfect for the first tack, because you can bend it later before the second tack. It’s helpful to face the edge off your table or work bench. Push the Jig around to where you can rest your tig torch hand comfortably on the table for the tacks.

 A fresh tack has been welded on one end of the outside corner joint. It is glowing hot red.

There are two ways you can tack this. Both methods you want to first start the puddle on the corner edge of a plate. The first method, you’ll place a 1/8” filler rod (the same width as the gap) between the two corners, before pulling the puddle across the rod and onto the other plate’s corner edge. With the other method, you use a smaller filler rod, let’s say a 1/16" and add it to the puddle that’s formed on the one plate, which starts to build up material, as you pull the puddle across the gap while adding more filler until it connects to the other side.  

After tacking the first side, turn your jig around and use your hand to make sure the other end is lined up. Bend it so the gap is tight against the filler rod in the gap. As you put in this second tack, you should feel the gap squeeze against the filler rod. Pull out the filler spacer rod right after doing this tack. The gap should be just a HAIR smaller than the 1/8” filler rod, so you can push the rod against it and it doesn’t go through the gap.

Amps and Rod Placement

Set your machine anywhere from 90-150+ amps. I recommend starting around the 100-110 amps range when first learning. Remember, you can get the same effect from lower amps just by moving slower and letting the heat soak in. So not only will running slightly lower give you more time to react, you’ll also have a more wiggle room with your arc length (the higher the amps, the more precise your arc length needs to be, because a long arc at higher amps is more powerful and thus more likely to melt back your filler rod or cause undercut).

A filler rod is placed in the open root gap, demonstrating the size of the gap is slightly smaller than the filler rod diameter.The first open root method to learn is called the lay wire method. Move your jig around so you are comfortably able to slide your torch and hand along the full length of the outside corner. It’s best to PULL the torch towards you. Place the filler rod on the gap (you’ll want to keep slight pressure against the rod as you weld) and slide it up against one tack. Now it’s time to fire up.

Starting Your Weld

Gently start your puddle on the tack. Keep your filler rod pressed firmly against the gap on the bottom while pushing it up against the tack. Pull the puddle off the tack and into the filler rod. Gently wiggle the puddle slightly back and forth to fuse to each bevel edge.

A time-lapse of a weld off a tack, starting with the filler rod pressed against the tack, to a weld puddle starting on the tack, to that puddle being spread around and expanding.

You want your torch to be almost straight up and down, with a tiny forward angle pointing in the direction you’re going to travel.

Welding the Open Root

After gently wiggling the puddle off your tack/rod ever so slightly onto each bevel side, raise your amps up to your full setting (100-110amps to start) and put your arc right in the middle of where your gap is. Let the puddle grow and expand into a balloon shape that fills the entire gap span. Briefly let the heat soak in as the puddle expands before you start traveling forward. Keep a tight arc length (around 1/16”).

What a welder sees welding an open root.

After the puddle expands to slightly larger than the filler rod, start to pull it forward. You can pull your torch in a straight line, gently wiggle it side to side, or gently wiggle it forward and back as you move forward. You’ll develop your own style and feel. That’s one of the benefits of using an outside corner joint to learn open roots: you’ll spend a lot of time welding vs prepping and will start to get a feel for your own style.

Red arrow pointing out exactly what a welder should look for while welding an open root.

The key is to not let the puddle go too far up the bevel edges. You want it to just barely fuse on either side. The arrow above shows where you can watch the puddle fusing your inside edge. The puddle sort of scoops down into it. But if the puddle starts going up the sides too far, then you aren’t getting enough penetration on the inside (all the metal is going onto the bevels on the top). If you are seeing this, then your arc length is too long, or you have overheated the spot you’re in and need to move forward faster.  

Final Result

What the top and bottom of a finished open root should look like.

*Weld puddle pictures were taken through Grumpy Vision, our auto darkening welding lens.

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