The first layer is thin, semi-transparent and noticeably wider than the lines above it. Look along it at a low angle and there is a raised ridge at every boundary between two lines, where displaced plastic had nowhere to go but up.
Almost nobody arrives here by accident
This fault is usually the result of a decision. Prints were not sticking, the offset went down, the prints stuck, and the offset stayed there. It works — a heavily squashed first layer does adhere — so it persists on a lot of machines as a permanent state rather than a temporary fix.
It is worth knowing what that decision costs, because the costs land somewhere other than the first layer.
What the over-squash actually costs
Dimensional error at the base. Displaced plastic spreads sideways, so the bottom of the part is oversized. Combined with a hot bed it becomes elephant's foot, and it makes any measurement taken near the plate meaningless.
Ridges that the nozzle then catches. The raised boundaries between lines stand above the intended layer height. The second layer is printed into them, which compacts material unevenly and can start the surface artefacts that persist for many layers.
Parts that are difficult or impossible to remove. An over-squashed PETG first layer on smooth PEI is how plates get chunks taken out of them — see the print will not come off the plate.
Extruder strain. If the gap is small enough, the extruder is pushing plastic into a space it does not fit. The motor skips, which sounds like clicking and is often misdiagnosed as a clog when the machine is simply being asked to do something impossible.
Plate damage. At the extreme, the nozzle is dragging on the sheet rather than printing onto it. On textured powder-coated plates this wears the finish away in exactly the region you print most; on smooth PEI it scores lines that print into every subsequent first layer. Plates are not cheap and this damage does not recover.
How far to back off
Raise the offset in 0.02 mm steps, printing a single-layer patch each time, and stop at the first setting where the lines are still merged with no gaps between them but the ridges have gone. The surface should be uniformly matte and flat, and a fingernail run across it should feel no corrugation.
You are aiming for the least squash that still produces a continuous sheet. That position gives full adhesion, no dimensional penalty and no plate wear, and it is a couple of hundredths of a millimetre away from where most machines are sitting.
If backing off costs you adhesion, the offset was compensating for something else — nearly always a plate that needs washing, or a bed temperature at the bottom of the material's range.
The other cause: flow, not height
If the ridges persist at a correct-looking nozzle height, check the first-layer flow multiplier. Many profiles ship with it above 100% to help adhesion on the first pass, and stacked on a low offset that produces exactly this appearance. Bring it back to 100% and re-judge the height afterwards.
An over-set extrusion multiplier for the whole print does the same thing and shows up everywhere else too — the wall thickness test on the over-extrusion page settles it in one print.
Check the mesh is not lying to you
A machine that probes cold and prints hot generates a mesh describing a plate that is no longer that shape, and the error usually appears as over-squash in the middle where the plate has bowed upward. Re-probe at printing temperature after a proper soak. That single change fixes a surprising number of machines whose offset has been nudged downward repeatedly to compensate for a mesh taken under the wrong conditions.
Which materials tolerate it least
TPU and other elastomers, because the excess spreads under the nozzle and welds into a sheet that is genuinely hard to remove. PETG, for the plate damage described above. Filled and fibre-loaded grades, because the abrasive is being dragged across the build surface under pressure. PLA is the most forgiving and will happily print over-squashed for a year, which is precisely how a machine ends up with a scarred plate and a base flare on every part before anyone investigates.