Rough surface where supports touched: fixing the underside

This guide is for prints whose supports came off, but left a rough, ridged, saggy or scarred surface behind. It covers the settings that shape that surface in Bambu Studio, PrusaSlicer, OrcaSlicer and Cura, what Bambu Lab and Polymaker say about support types and support materials, and the cooling settings slicers offer for that layer. Make sure you're in the right place:

The settings below are the same ones that control removal, pulled the other way. If you tighten them for a better surface and the supports start fusing, our supports hard to remove guide covers the other side of the trade. If they weld into one solid piece with no seam, see our supports fused to the part guide.

Why does the supported surface come out rough?

Bambu Lab's explanation: supports "need to be removed after printing, so there must be a gap (Top Z Distance) between the model and the support." When the support is the same filament as the model, "you need to balance easy removal and high-quality support surfaces, making it difficult to improve the printing quality of the support surface through parameter adjustments."

Bambu sums up the trade-off in one table:

PrusaSlicer adds another part of the story. Its Top contact Z distance tooltip says that setting the gap to 0 "will also prevent Slic3r from using bridge flow and speed for the first object layer." So with a gap, PrusaSlicer prints the first layer over supports with its bridge settings.

Polymaker says too high a Z distance causes "Sagging or poor surface quality," and too low means "Supports fuse to the model." That's the window you're working in.

Step 1: Narrow the top Z distance

Polymaker's troubleshooting advice for "Dropping Undersides or Poor Surface Quality" starts with "Reduce Z Distance by 0.05–0.1mm." Its general range is 0.5 to 2 times the layer height, and it calls 0.2 mm "a great starting point" for 0.2 mm layers.

Printer and filament profiles can set their own values, so check the number your profile shows before you change it. Lower the gap in small steps and check removal each time. If supports start fusing, go back one step.

Step 2: Give the part a denser, thicker interface

The interface is the dense layer or layers at the top of the support, directly under the part. The docs agree that more of it gives a better surface.

• Bambu Lab: with top interface layers set to 0, "the supported area will lack sufficient underlying support during printing, which can cause lines to sag and affect model quality." With 3, "a stable and flat platform will be generated for the supported area." Bambu adds that the supported area "will only be completely free from sagging when the Top Z Distance is also set to 0."
• OrcaSlicer: top interface layers "determine the finish of the supported surface." More layers "give a flatter, better supported surface but take longer to print and are harder to peel off." With no interface at all, the support is "fastest and easiest to remove but leaves the roughest overhangs."
• Prusa: interface layers "form a more uniform surface between the object and the support material," and are "preferably set to a much denser pattern than the non-interface layers." Prusa also warns that interface layers bridge over the support lines below, and if the support pattern spacing is too large, "these bridges will start to sag and have an imperfect connection with the model."
• Cura: the Support Interface Density description says "A higher value results in better overhangs, but the supports are harder to remove."
• Polymaker: "Interface Density: 75–100% for clean surfaces" and "Interface Layers: 2–4 layers to ensure consistent contact." For poor surfaces, "Increase Support Interface Density (e.g., 80–100%)."

Where to find these and what they default to:

Cura's base definition has the interface switched off. Its description says turning it on will "Generate a dense interface between the model and the support." If your Cura profile has it off and the underside is rough, that's the first setting to try.

Step 3: Choose the interface pattern

The pattern of the interface lines changes the surface they leave.

• Bambu Studio: Rectilinear is "suitable for most cases." Concentric is "stronger on non-planar surfaces and useful with support materials," and Bambu says that for best surface quality you can set a very small interface spacing, such as 0, with concentric and a support material. Rectilinear interlaced "Improves adhesion and smoother undersides but may be harder to remove than standard rectilinear." Grid "Provides excellent support for large flat surfaces but can be more difficult to detach."
• OrcaSlicer: Concentric loops "peel off in one piece and leave no line ends on the surface." Rectilinear interlaced "ties the interface together and makes it stiffer under the overhang."
• PrusaSlicer 2.9.6: the tooltip says the default "for non-soluble support interface is Rectilinear, while default pattern for soluble support interface is Concentric." It also has "Interface loops," which Prusa says covers "the top layer of supports" with loops. It's off by default.
• Cura 5.13.0: Support Interface Pattern defaults to Concentric, with Lines, Grid, Triangles and Zig Zag as options.

Bambu's own sources describe its Default differently. The wiki says the default is rectilinear with support material and concentric with other materials. The Bambu Studio 2.08.02.61 tooltip says "Default pattern for support interface is Rectilinear Interlaced." Check the preview to see which your profile is printing.

Step 4: Match the support type to the overhang

Tree and normal supports touch the part differently, and that shows on the surface.

Both makers point to hybrid as the middle path. Bambu says Tree Hybrid generates normal grid supports "Below the big flat overhang regions," and tree supports elsewhere, so "it's safe to choose hybrid(auto) in general." Polymaker says hybrid supports give "a sturdier, larger contact area for better overhang support and easier removal."

Other style settings that affect the surface:

• Tree branch distance (Bambu Studio). "A smaller value means higher sampling density on the overhang surface and, therefore better surface quality, at the cost of more removal difficulty."
• Organic supports (PrusaSlicer). Prusa says they're "easily removable, do not scar the surface, and are fast and cheap to print."
• Snug style. The Bambu Studio and PrusaSlicer tooltips say snug support towers "save material and reduce object scarring." Prusa says the default Grid style tends "to leak out of regions they support along the walls of the object," while Snug keeps to the shape of the overhang.

What causes marks on upper surfaces?

Supports that start on the part, rather than the bed, can mark the surface they stand on. Prusa says that with supports everywhere, "small imperfections may appear on the surface where supports started growing from." Two ways around it:

• Build plate only. PrusaSlicer and OrcaSlicer can keep supports off the model. Prusa says to check the preview so every part is still supported or within a manageable overhang.
• Bottom interface layers. OrcaSlicer says these are printed "where a support lands on top of the model, protecting that surface from the support body." Cura's Support Interface covers both the top of the support and the bottom, "where it rests on the model."

Step 5: Print the interface in a support material

A support material at the interface is how makers get the smoothest underside. Bambu says that with a dedicated support filament, "you can ensure there is no gap between the model and the support (Top Z Distance = 0) while still enabling easy removal of the support. The support surface of the model can achieve a smooth printing effect."

Bambu's pairings, from its Bambu Studio support guide:

• PLA Basic or PLA-CF: Bambu Support for PLA, Bambu Support for PLA/PETG, or PVA.
• PETG Basic, PETG HF or PETG-CF: Bambu Support for PLA/PETG.
• PA, PA-CF/GF, PA6-CF/GF, PAHT-CF/GF, PET-CF/GF: Bambu Support for PA/PET. For several of these high-temperature filaments, Bambu also recommends Bambu ABS or ASA as the support filament.

Bambu recommends using the support filament "only for the support interface" in most cases, to save time. It warns against mismatched pairs, such as Support for PLA under ABS, ASA, PC or PAHT-CF, because this "may cause malfunctions such as extruder or nozzle clogging."

PLA and PETG as supports for each other

Bambu says PLA and PETG "don't bond well to each other — and that's exactly what makes them a great support pair." In its comparison of a PETG model, PETG supports bonded more strongly and "may leave marks or defects on the model surface," while with PLA supports "The surface finish is improved." Bambu's guide covers only Bambu PLA Basic with PETG HF or PETG Basic. It excludes PLA Matte, PLA Silk, PLA-CF, PETG Translucent, PETG-CF and any third-party PLA or PETG, and it requires a Smooth PEI or Textured PEI plate.

Our supports hard to remove guide covers the full Bambu pairing table, Polymaker's support filaments and the PrusaSlicer soluble and breakaway profiles.

Step 6: Check cooling and bridge settings over the supports

Polymaker's poor-surface fix includes "Increase cooling if possible with material being used." For overhangs, Polymaker says to maximize the fan for PLA and minimize it for ABS to prevent warping. Our overhangs and bridging guide covers fan speed by material.

Cura and OrcaSlicer have a fan setting just for this area. Both describe it as making supports easier to remove, not as a surface-quality setting:

• Cura 5.13.0: "Fan Speed Override" (off by default) changes the fan for "the skin regions immediately above the support." "Supported Skin Fan Speed" (default 100%) says "Using a high fan speed can make the support easier to remove."
• OrcaSlicer: "Support interface fan speed" is applied "when printing support interfaces." The wiki says a higher than regular speed "reduces the layer binding strength between supports and the supported part, making them easier to separate."

Bridge settings matter too, because PrusaSlicer prints the first layer over a gapped support with bridge flow and speed. In Bambu Studio, "Thick bridges" makes bridges more reliable, but Bambu's support guide says "the overhang surface quality may be worse because of possible overflow." Its source default in 2.08.02.61 is off. Bambu Studio's "Bridge flow" tooltip suggests a value slightly below 1, "for example 0.9," to "improve sag."

A sensible order

1. Confirm it's the surface that's the problem, not removal.
2. Reduce the top Z distance by 0.05 to 0.1 mm, and check removal is still possible.
3. Turn on or add top interface layers, and make the interface denser.
4. Try a different interface pattern, checking the preview.
5. Use normal or hybrid supports under large flat overhangs, and tree or organic for small scattered ones.
6. Keep supports off finished surfaces with build-plate-only or bottom interface layers.
7. If your printer can, print the interface in a matched support material with no gap.
8. Check cooling and bridge settings for the material.

Change one thing per print and note the result.

Checklist

• Supports come off, but the surface under them is rough or droopy.
• Top Z distance lowered in small steps, within the 0.5 to 2 times layer height range Polymaker gives.
• Top interface layers on, with the 2 to 4 layers Polymaker suggests.
• Interface dense, or solid where your slicer allows.
• Interface pattern checked in the preview.
• Normal or hybrid supports under big flat overhangs.
• Supports kept off finished upper surfaces, or a bottom interface added.
• Support material at the interface, if your printer can use one, matched to the model filament.
• Fan and bridge settings suit the material.

How this guide was checked

This is a research-based guide. We haven't printed these tests for it. Every cause, setting name, default and recommendation comes from the Bambu Lab, Prusa Research, OrcaSlicer, UltiMaker Cura and Polymaker sources listed above, retrieved on 2026-10-12. Defaults are from Bambu Studio 2.08.02.61, PrusaSlicer 2.9.6 and Cura 5.13.0 source files. Read more in our editorial standards.

Print problems: what to check before you buy ↗