Bridging is what happens when the printer lays filament across an open gap with no support underneath. Done right, the filament stretches tight and flat. Done wrong, it sags into a droop or a curtain of strings. The fix is mostly cooling, speed, and flow, in that order.
Per Prusa’s poor-bridging guide, a clean bridge comes from enough part cooling, a slow bridge speed, and a slightly lower flow so the filament does not droop under its own weight.
Your material sets the difficulty. PLA bridges cleanest because it takes high cooling and stiffens fast. PETG and nylon stay runnier longer and sag more, so they want more cooling and slower speeds.
What causes it
| Likely cause | Fix | Severity |
|---|---|---|
| Part cooling too low | Raise cooling for bridges toward 100 percent so the filament stiffens before it sags. | hi |
| Bridge speed too high | Slow bridge speed to 20 to 30 mm/s so the filament has time to set. | hi |
| Bridge flow too high | Drop the bridge flow ratio to about 0.95 so less filament droops. | md |
| Bridge span too wide | Orient the part to shorten the span, or add a support under it. | md |
| Wet filament | Dry the spool, then reprint. Wet filament sags and strings more. | lo |
Fast fixes to try first
Print a bridge test, a flat span with open air below it, then work top to bottom and reprint after each step.
Raise the cooling
Set part cooling to 80 to 100 percent for the bridge layers and reprint.
Slow the bridge
Drop bridge speed to 20 to 30 mm/s and reprint.
Cut the bridge flow
Lower the bridge flow ratio to about 0.95 and reprint.
Dry the filament
If the bridge strings or sags with popping, dry the spool and reprint.
If the quick fixes stall
When cooling and speed do not pull the bridge flat, the part setup and the hardware are the next place to look.
Orient the part
Rotate the model so the bridge spans the shortest distance, or runs along an existing wall.
Add a support
For a span over about 15 mm, add a support or a sacrificial pillar under the bridge.
Check the fan ducts
If one side of the bridge sags, make sure the part-cooling fan blows evenly on the nozzle from both sides.
What your material changes
PLA bridges the cleanest, because it takes high cooling and stiffens fast after it leaves the nozzle. PETG and nylon stay soft longer, so they sag more and want slower bridge speeds and full cooling. TPU bridges poorly and is best avoided for long spans.
Habits that backfire
A few common habits hide the real cause or let the bridge sag again.
Avoiddo not
- Running the bridge at full print speed. Fast bridges sag.
- Dropping flow so far the bridge goes thin and tears.
- Leaving a long span unsupported and hoping cooling saves it.
- Changing cooling, speed, and flow in one pass, so the real cause stays hidden.
Key takeaways
- A clean bridge needs high cooling, a slow speed, and slightly lower flow.
- PLA bridges best; PETG and nylon need more cooling and slower speeds.
- Shorten or support any span over about 15 mm.
- Change one setting and reprint the same bridge test.
For related topics, the overhangs guide covers printing angled surfaces, and the filament storage guide covers keeping spools dry.
Related guides
Related
More in this area
Cross-reference
Sources & methodology
2 citations · reviewed 2026-07-10- 01Prusa Knowledge Base: Poor bridging (flow ratio, speed, orientation)accessed 2026-07-09Tier 1
- 02All3DP: Bridging (cooling, speed, and flow tips)accessed 2026-07-09Tier 2