3 Ways to Improve Bridging in Flat Roof Structures

[MakerWorld]

In some designs, a long bridge span exists with no support structure beneath it. This causes steep overhang angles during printing, resulting in filament sagging. Filament sagging can make the printed object look bad or cause issues with its function. vandragon_de ‘s Wind-up Motor Boat and Bambu Lab’s A1 Mini Wireless Charger illustrate the bridging issues.

To ensure print quality, using support structures while printing bridges is usually required. If a bridge is too long and no support structure is underneath it, the filament tends to sag. Taking vandragon_de’s Wind-up Motor Boat as an example, you can see that the cabin’s interior roof has a long bridge span. If no support structure is used, this may lead to filament sagging. Filament sagging can impact its appearance and height accuracy. A similar situation exists at the base of the A1 Mini Wireless Charger.

If you encounter similar problems, the following methods will help you resolve them. Next, we will go over how to handle the issues of short bridging and long bridging.

• Short bridge span: Adjusting settings in Bambu Studio.

• Long bridge span: Designing a chamfer and manual supports via CAD fixes

Short bridge span - Do I need a support structure?

Not all bridge spans can lead to issues. If bridge spans are 12 mm to 25 mm, not supporting them can maintain the appearance quality and size accuracy. This method saves time and materials for printing support structures. It also avoids the hassle of removing supports in tight spaces. The image below shows the difference in printing time with and without the support structure. Using support, the total printing time is 182 minutes. Without supporting the bridges, the total printing time is 143 minutes. This results in a saving of 39 minutes.

Long bridge span

Chamfering method

If bridge spans exceed 25 mm and the design permits chamfering, adding a chamfer can reduce or eliminate part of the bridge span. This helps control filament sagging effectively. The bridge span drops from 46 mm to 26mm. This change happens by adding a chamfer at the spot where the cabin’s inner wall meets the interior roof, as seen in the Wind-up Motor Boat model.

We recommend setting a chamfer angle in a range of 45° to 60°. A chamfer angle can be set to a maximum of 70° to follow specific design criteria. The image below demonstrates the details of a chamfer application.

Manual support design methods

If bridge spans exceed 25 mm and chamfering isn’t allowed, you can manually draw a support structure. This helps control filament sagging effectively. This constraint can happen when the area requires face-to-face assembly with another part. This constraint can also occur if chamfers can’t reduce the bridge span enough to an acceptable range. For the A1 Mini Wireless Charger, the image below shows where to draw a support structure by hand.

Draw a support structure manually by following these steps. In Bambu Studio, first, activate ‘Enable Support’. Then, set the Support Type to ‘Normal (Manual)’.

Next, choose the flat part of the base. Then, draw a support structure in the middle area. The two support structures help to divide a long bridge span into shorter bridge spans. That helps to avoid filament sagging.

Slice your file and navigate to the layer that contains bridging. We recommend setting the angle between the bridge direction and the support structure to 90°. This vertical cross-layout can provide uniform support for all bridging lines.

The left image shows the result of using manually drawn supports. The right image displays the effect without them.

This article shares practical ways to improve bridging in flat roof structures. By effectively adjusting settings, using chamfer design and manual support design, it can address the different challenges of short and long bridging and improve print quality.

Supplementary reading:
[1] Bambu Lab Wiki, "Bridge settings,“ 2024
[2] All3DP, “3D Printing Troubleshooting Guide,” 2025
[3] Bambu Lab Wiki, “Bambu Studio Modifier Operation Guide,” 2025

These articles might help you as well — take a look!
Ways to Fix Filament Sagging in Supportless Holes
Ways to Reduce Line Detachment in Dome Structures

If this guide sparked ideas or felt familiar, share your thoughts in the comments — let’s chat! Like and save if it helped.

[DWdesigns]

@MakerWorld

We recommend setting a chamfer angle in a range of 45° to 60°. A chamfer angle can be set to a maximum of 70° to follow specific design criteria

I used chamfers on this kind of situation, but I mostly did 45 degree and occasionally a little over. Will 70 degree result in a hit-or-miss, i.e. sometimes it works sometimes it fails?

[MakerWorld]

Thank you for your feedback. For PLA, 60° is relatively safe to print and 70° is the approximate upper limit. We may not be able to obtain an ideal print effect with a sharp corner. When using countersunk screws, we recommend a 45° angle to match the screw head.

[DWdesigns]

Thanks for the info.

The degree of the chamfers in this example also applies to other overhangs. I have wondered when using a high degree such as one over 60, would it be helpful to increase the layer width. For example, for a 0.4mm nozzle, does increasing the layer width from the default 0.42mm to 0.50mm help to increase the chance of a successful printing of the overhang?

[MakerWorld]

The setting method you mentioned is similar to the “thick bridge” approach. It aims to increase the bridging flow and reduce line detachment for more reliable bridging. However, it can cause insufficient cooling and insufficient pulling tension, leading to more sagging.

Bridging width cannot be set independently. We recommend adjusting the bridging flow and enabling the ‘thick bridge’ option.

Similarly, we sometimes reduce the flow to optimise cooling and improve sagging performance. But the bridging may severely separate or even break.

Bridging is an operation that seeks a balance between cooling and pulling tension. Non-default settings require extensive testing depending on the individual situation. And, it is hard to come up with a universal solution. We recommend keeping the default settings in these areas. Meanwhile, consider manually adding supports or modifying the model to fix the problem.

For more related information and tests, please refer to the related articles at the end of the article!

[DWdesigns]

Thanks for the info about bridging. It’s helpful.

On my follow-up though, I was referring to the chamfers that are created to make the bridging distance shorter.

When the chamfers have a higher degree, such as one that exceeds 65, can increasing the layer width from the default of 0.42mm to a value like 0.5mm help to ensure the chamfers printed successfully?

Or maybe you are saying that increasing the layer width when printing the chamfers is similar to the “thick bridge” approach and is subjected to the same potential risks such as sagging?

[MakerWorld]

Thick bridges or increasing the flow rate of bridge lines help with bridging. The reason is that they enable the suspended part to withstand greater tension and more stretching without breaking.

The chamfer is maintained not by this tension of the bridging, but by the connection with the material underneath the overhang and inner walls. Excessive flow causes slower cooling, worsening the sagging.

From the perspective of the cooling’s influence, increasing the layer width and flow rate for overhangs can not improve the printing quality of these chamfers.
A smaller layer height can improve overhang performance. Bambu Studio offers a “Slow Down for Overhangs” setting to optimise cooling in overhang areas. Meanwhile, the layer width of the overhang part cannot be adjusted separately.

For more details about Slow Down for Overhangs, please refer to this source: Slow Down for Overhangs | Bambu Lab Wiki.

[DWdesigns]

@MakerWorld

From the perspective of the cooling’s influence, increasing the layer width and flow rate for overhangs can not improve the printing quality of these chamfers.

This is the info I was looking for. Thanks.

[MakerWorld]

Bridge flow settings and thick bridge functions can be configured simultaneously. The difference between them is that the bridge flow setting does not change the distance between the bridge lines. In comparison, the thick bridge function increases the distance between the bridge lines while increasing the flow rate.

To achieve the best surface quality at the bottom, we need to adjust three settings: bridge flow, thick bridge, and bridging speed.