Gaps or holes in top layers: why flat tops won't close

This guide is for anyone whose prints have see-through lines, pinholes or open gaps on a flat or sloped top. It's the neighbour of pillowing, the lumpy, sagging top that comes from bridging over sparse infill. Pillowing is one cause of top-layer gaps, so the fixes overlap. This page covers the wider set of causes and the slicer settings that Simplify3D, Prusa, Bambu Lab, OrcaSlicer and UltiMaker Cura document for each.

What do top-layer gaps look like?

Simplify3D describes the problem as top solid layers that "are not completely solid," with "gaps or holes between the extrusions that make up these solid layers." The pattern of the gaps tells you where to look.

Step 1: Check the slicer preview

Before printing again, look at the top layers in your slicer's layer preview. Polymaker's guide, written for Bambu Studio, says to inspect the top layers in layer view to confirm the roof looks fully closed, and "if you still see infill showing through in preview, keep increasing top layers or infill until the top skin looks continuous." If the preview looks closed but the print doesn't, work through the flow and cooling steps below as well as the layer and infill settings.

Step 2: Add top layers or top thickness

This is Simplify3D's first fix. It explains that a solid layer printed over partly hollow infill "has to span across the hollow air pockets," and that the extrusions "have a tendency to droop or sag down into the air pocket," so you want several solid layers on top. Extra top layers sit inside the part and "do not add size to the exterior."

What each maker suggests:

• Simplify3D: a solid top section "at least 0.5mm thick" as a rule of thumb, so 0.25 mm layers need at least 2 top layers and 0.1 mm layers may need 5. If you see gaps with 3 top layers, try 5. The setting is on the Layer tab of Edit Process Settings.
• PrusaSlicer: Prusa suggests "at least 3 top layers," and more at thin layer heights, for example 3 top layers at 0.3 mm but 9 at 0.1 mm. You can also set a minimum thickness for the top and bottom layers in Layers and perimeters (Prusa's article calls it minimum wall thickness). Prusa notes that setting top solid layers to 0 overrides the minimum thickness.
• OrcaSlicer: Shell layers sets the number of solid top layers, and OrcaSlicer says it's "usually recommended to have at least 3 shell layers for most prints." Shell thickness adds layers when the count alone would be too thin, which "avoids having too thin a shell when layer height is small."
• Cura: Top Thickness "divided by the layer height defines the number of top layers." In Cura's base definition (fdmprinter.def.json), Top Thickness takes its value from Top/Bottom Thickness, which defaults to 0.8 mm, and Cura warns if Top Thickness is set below twice the layer height. Printer profiles may override this.
• Bambu Studio: Polymaker's guide says to open the Strength tab and raise "Top shell layers" under shells.
• Polymaker suggests a total top thickness "at least 4 to 6 times the layer height," for example 5 or 6 top layers at 0.2 mm.

The numbers differ, so treat them as starting points and check the result in preview.

Step 3: Give the top more support

If more top layers don't close it, support the top better from below.

• Raise infill density. Simplify3D says that at 10% infill, "the remaining 90% of the interior of your part would be hollow," leaving large gaps for the top to bridge. If 30% still shows gaps, it suggests trying 50%. Polymaker suggests "typically 15 to 30% for normal parts, higher for very wide, flat roofs." Prusa says the main purpose of infill "is to provide internal support for top layers," and that most models print fine at 10 to 15%.
• Change the pattern. Polymaker says grid or rectilinear infill "gives straight ribs with clear gaps, which can make pillowing worse on big spans," while gyroid "spreads support more evenly in both directions."
• Add density only near the top. Prusa suggests a modifier mesh that increases infill "for the last few layers before the solid infill." Cura's Gradual Infill Steps gives areas "closer to top surfaces" a higher density.
• Fill small areas solid. PrusaSlicer's Solid infill threshold area forces solid infill "for regions having a smaller area than the specified threshold."

Step 4: Rule out under-extrusion and low flow

Simplify3D says that if more top layers and more infill don't fix it, "then you likely have an under-extrusion issue," meaning the nozzle "is not extruding as much plastic as the software expects."

The flow-related signs on a top surface, per the makers:

• Bambu Lab: if flow is too low, "under-extrusion is present on the top layers, and the lines will not be connected leaving a bit of space between them." Its under-extrusion article says that if under-extrusion is visible across the entire model, it may be an insufficient flow ratio, but for genuine Bambu filaments it recommends keeping the default flow ratio.
• Prusa: minor under-extrusion "causes small gaps that appear between layer lines or perimeters." In its visual calibration, if there are "visible gaps between layer lines," increase the extrusion multiplier. Prusa adds that "microscopic gaps near the perimeters are OK."
• OrcaSlicer: too low a flow ratio "causes under-extrusion, which leads to gaps, weak layers, and poor structural integrity." Its flow ratio calibration prints blocks at different flow values so you can pick the one with no visible gaps.

Look at the walls and infill as well as the top. If lines look thin everywhere, work through our under-extrusion guide first. If you lower flow too far while chasing a smooth top, you can swing the other way. Our over-extrusion guide covers flow calibration in Bambu Studio, PrusaSlicer, OrcaSlicer and Cura.

Step 5: Pinholes where the top meets the walls

Small holes just inside the walls, with the rest of the top closed, come from the join between the top skin and the perimeters.

• OrcaSlicer: its Infill/Wall Overlap setting slightly enlarges the top solid infill "to overlap with walls for better bonding and to minimize pinholes where the infill meets the walls." OrcaSlicer calls 25 to 30% "a good starting point," relative to the sparse infill line width.
• Cura: Skin Overlap Percentage sets the overlap between the walls and the skin lines. Cura says "a slight overlap allows the walls to connect firmly to the skin," and notes that, with equal skin and wall line widths, anything over 50% may already push the skin past the wall. The base definition's default is 5%.

Raise the overlap in small steps. Too much can push the top skin into the walls. If the gaps run between the wall lines themselves, see our gaps between walls guide.

Step 6: Holes on sloped or rounded tops

Holes on a slanted surface are a different problem from a flat top. Prusa says older slicers often left "holes between the perimeters on a slanted surface," typical on busts, which "had a few holes on top of the head." PrusaSlicer's Ensure vertical shell thickness setting adds solid infill near sloping surfaces "to guarantee the vertical shell thickness." Its Enabled option is the standard setting. Prusa notes the option isn't available in versions 2.6.0 to 2.8.1.

Also check a small or sloped top isn't being dropped by a skin removal setting. Cura's Top Skin Removal Width removes "every skin area smaller than this value," which Cura says saves time on slanted surfaces.

Step 7: Check top speed and cooling

The first top layers bridge over infill, so cooling and speed affect how well they close.

• Cooling helps bridges set. Prusa says when printing bridges, the plastic needs to "freeze and solidify" quickly, "otherwise, the filament will sag down." PrusaSlicer's Bridges fan speed, which applies to bridges and overhangs, is typically set higher than normal printing. Prusa warns that with some materials, more fan "could make things actually worse and cause warping or layer separation."
• Slow the top when you can't use fan. Polymaker says ABS, ASA and some nylons are printed with reduced or no fan, "which makes bridging and top surfaces more difficult," and that "slowing down the top solid layers can give the filament more time to stick." Cura has Top/Bottom Speed and Top Surface Skin Speed for this.
• Small tops need time to cool. Polymaker says a small area, such as the tip of a pyramid, gets a melted look because "there is not enough time for a layer to cool before the next hot layer is laid on top of it." Its fixes are printing more than one object, more fan where the material allows, and a longer minimum layer time. It says most materials are fine with a 3 to 5 second minimum layer time.

Stay within your filament maker's cooling and temperature recommendations.

Step 8: Check the surface settings themselves

A couple of settings can leave a top open on purpose:

• Surface density. OrcaSlicer's Surface Density controls the density of top and bottom surfaces. OrcaSlicer says "a value of 100% means a solid surface, while lower values create a sparse surface," which can be used for looks or grip. If your top is a regular open mesh, check this hasn't been lowered.
• Top surface pattern. OrcaSlicer lists Monotonic Line as usually recommended for top surfaces. Cura's Monotonic Top Surface Order "makes flat surfaces look more consistent." Prusa says Original Prusa profiles already use monotonic infill for top layers.

Can ironing close fine gaps on a flat top?

Ironing runs the nozzle over the finished top surface a second time with very little flow. The makers describe it as smoothing a flat top and filling small gaps between its lines. None of the sources we reviewed present it as a fix for too few top layers or too little infill, so close the top with those settings first. If ironing itself leaves grooves, edge ridges or rough patches, see our ironing problems guide.

• What it does. Bambu Lab says a small amount of extruded material can "penetrate between the lines and fill the gaps." Prusa says the nozzle "extrudes a small amount of filament to fill any holes in the top surface." Cura says it keeps nozzle pressure high "so that the creases in the surface are filled with material."
• Where to find it. Bambu Studio: Ironing Type, which is "disabled by default (No ironing)," with options for top surfaces, the topmost surface, or all solid layers. PrusaSlicer: Print settings, Infill, Ironing (Advanced or Expert mode). OrcaSlicer: Ironing type, with the same three choices. Cura: Enable Ironing and Iron Only Highest Layer.
• Flat tops only. Bambu says ironing "is only effective when the top surface is flat." Prusa says it isn't useful for "round objects, figures, and organic shapes."
• Flow matters. Prusa says ironing is "very sensitive to accurate extruder calibration": too little leaves "shiny groves," too much drags plastic to the edges. OrcaSlicer says a lower ironing flow "may not cover the surface completely." Bambu says too high a value "will result in over-extrusion on the surface." Cura's base Ironing Flow default is 10%.
• It costs time and can pull a part loose. Bambu says the nozzle rubs the top harder than in normal printing, so a part that isn't firmly stuck down "is likely to fall off the build plate."

A sensible order

1. Look at where the gaps are: across a flat top, between every line, along the walls, on slopes, or on small tops.
2. Check the slicer preview. If infill shows through, add top layers or top thickness.
3. Raise infill density, try gyroid, or add denser infill just under the top.
4. If gaps remain, check for under-extrusion and run your slicer's flow calibration.
5. For pinholes at the walls, raise the infill/wall or skin overlap a little.
6. For holes on slopes, check shell thickness settings on slanted surfaces.
7. Slow the top surface and check cooling, within the filament's limits.
8. Once the top is closed, turn on ironing if you want it smoother.

Change one setting per print and keep the same test file, so you know what fixed it.

Print problems: what to check before you buy ↗