How to weld vertically: uphill and downhill
Vertical is where gravity stops helping. The whole technique comes down to keeping the pool small enough that surface tension can hold it in place while it freezes.
Two different jobs
Vertical up (PF) is the structural position. Welding upwards, each bead freezes into a shelf that supports the pool above it, so the arc can stay in the joint and penetrate. It is slower and it is what a drawing means when it specifies vertical welding without qualification.
Vertical down (PG) is fast and shallow. The arc runs ahead of the pool, which limits penetration — useful on thin sheet, on cellulosic root runs in pipe, and nowhere that a full-strength joint is required. Codes are explicit about this because the temptation is obvious: down is easier and looks neat.
Settings for vertical up
About 10 % below the flat-position current, from the current calculator. Electrode no larger than 4 mm in MMA — 5 and 6 mm electrodes produce a pool that no technique will hold. In MIG, short arc mode only: spray transfer has far too much heat and the pool simply leaves.
Basic electrodes are easier than rutile here, which surprises people. The slag from a basic coating freezes faster and does a better job of holding the pool on the wall.
The weave
The instinct is to sweep quickly across the edges and pause in the middle. That is exactly backwards, and it is what produces undercut down both sides of a vertical weld.
Move briskly through the centre and pause at each edge — half a second is enough. The pause lets the pool wash out to the toe and fill it. On a fillet, a triangular weave with the point into the corner puts the extra time where the root needs it.
Root and fill
The root run is usually made without weaving at all: a straight stringer, slow and with a short arc. Weaving belongs to the fill and cap runs, once there is a shelf to build on. Clean the slag between every run — vertical welds trap slag at the toes more readily than flat ones, and trapped slag is an inclusion.
Practising it
Take a 10 mm offcut, clamp it upright, and run beads on the plate surface without any joint. The point is to learn what the pool does when it is about to run — the moment it goes shiny and starts to sag — and to react before it does. That reaction is the whole skill, and it is faster to learn on a plate than on a job.
Frequently asked questions
Up or down?
Up for anything that has to carry load: the arc runs into metal that is already molten behind it, so penetration is good. Down for thin sheet and cellulosic pipeline root runs, where speed matters and deep penetration would blow through. Choosing down on 10 mm structural steel is a reliable way to produce lack of fusion.
The pool keeps running out of the joint.
Too much heat for the position. Drop the current, shorten the arc, and narrow the weave. If it still runs, the electrode is too large — go down a diameter rather than trying to out-manoeuvre a pool that is too big.
Which weave pattern?
Triangle for a fillet, Christmas tree or a shallow zig-zag for a butt. The pattern matters less than the pause at the edges: whichever shape you use, the time spent at the toes is what prevents undercut.
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