Modular Joist-Mounted Filament Rack

I designed this rack to make use of otherwise wasted space underneath exposed floor/ceiling joists while keeping a large number of filament spools organized and easily accessible.

The system consists of two bracket types:

Top Bracket
Mounts directly to a standard dimensional-lumber joist and supports the first pair of spool shafts.

Lower / Extension Bracket
Suspends from the shafts of the level above and provides another pair of shaft holes below, allowing additional filament-storage levels to be added without making additional attachments to the joist.

My current installation uses three brackets across a 43" / 1092 mm span, which accommodates approximately 16 standard 1 kg filament spools per rack level.

The brackets are designed around 19 mm shafts. Depending on the actual diameter and tolerance of the pipe, tubing, rod or shaft material you use, you may need to adjust the shaft holes slightly before printing.

Main dimensions

Both parts are 12 mm thick.

The supplied STL files are approximately:

• Top bracket: 179 × 294 × 12 mm

• Lower bracket: 179 × 267 × 12 mm

• Shaft holes: 19 mm

• Joist opening: designed for 1.5" / 38.1 mm actual joist thickness

The upper bracket is intended for standard 2-by dimensional lumber, where the nominal 2" dimension is approximately 1.5" actual thickness.

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Mounting

The top bracket uses a fork that fits over the joist.

Each side of the fork has:

• 2 countersunk #8 screw holes

• 4 screws total per bracket

• designed around 1" #8 countersunk wood screws

Slide the bracket over the joist so that the joist is fully seated inside the 1.5" fork, then fasten both sides of the bracket to the joist.

I use three top brackets over a 43" span.

For a three-bracket installation:

Joist
────────────────────────────────────
   │             │             │
 Bracket       Bracket       Bracket
   O=============O=============O
   O=============O=============O
          spool shafts

The two shafts run through all three brackets.

Spools are then loaded onto the shafts between the brackets.

Adding another level

The lower bracket has two pairs of 19 mm holes.

The upper pair shares the existing spool shafts from the level above. The bracket hangs from those shafts and its lower pair of holes supports another pair of shafts:

TOP BRACKET
       │
     O===O       Level 1 shafts
       │
 LOWER BRACKET
       │
     O===O       Level 2 shafts

Multiple lower brackets can therefore potentially be used to create additional levels.

Important: I have printed and tested the top joist bracket. The lower extension bracket is currently a functional design but has not yet been physically load-tested, so additional suspended levels should be treated as experimental until independently tested.

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Capacity / expected loads

The rack was designed around standard 1 kg filament spools.

My current configuration is:

• 43" / 1092 mm shaft span

• 3 brackets

• approximately 16 × 1 kg spools per level

• approximately 16 kg / 35 lb of filament per level, excluding the shafts themselves

With three evenly spaced brackets, a single 16 kg level represents an average static load of approximately:

 16/3 ~ 5.3kg per bracket

or approximately 52 N per bracket, before accounting for the shaft weight and unequal load distribution.

The 12 mm PLA+ bracket geometry was intentionally designed with continuous structural paths between the joist mount and shaft holes, while the internal cutouts reduce filament usage in relatively low-stress areas.

The top brackets in my installation have been tested with the intended spool-rack application.

This is not a certified load rating

The 16 kg / 35 lb per level figure describes my intended and tested use case, not a formally engineered or certified maximum capacity.

Actual capacity depends on factors including:

• filament material and print quality;

• number of walls and infill;

• layer adhesion;

• shaft material and diameter;

• shaft span;

• joist material and condition;

• screw type and embedment;

• bracket spacing;

• uneven loading;

• number of suspended levels;

• ambient temperature.

Do not use this design for supporting people or other safety-critical loads.

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Recommended print settings

These are the settings I am currently using:

• Material: PLA+ 2.0

• Nozzle: 0.6 mm

• Layer height: 0.28 mm

• Walls/perimeters: 4

• Infill: 10%

• Infill pattern: Gyroid

• Top shell: 4 layers

• Bottom shell: 4 layers

I specifically designed the brackets so that the structural outer members carry most of the load rather than relying on extremely high infill percentages.

Print orientation

Print the brackets flat, with one of the large 12 mm-thick faces on the build plate.

This is important.

The primary load path then runs predominantly in the XY plane of the printed filament, rather than relying on Z-axis layer adhesion to support the rack.

Correct:

  bracket profile
┌───────────────┐
│               │

════╧═══════════════╧════ build plate

Do not print the bracket standing on its 12 mm edge.

No supports should be required when printed in the intended orientation, although slicer behavior around the countersinks should be checked before printing.

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Material choice

I am currently using PLA+.

For an indoor filament rack at normal room temperature, PLA+ is a good fit because it is stiff and resists flexing well under this type of static load.

PETG should also be printable, particularly if the rack will be exposed to somewhat higher temperatures, but PETG is generally more flexible and may exhibit more noticeable long-term deformation under continuous load.

For my application, PLA+ is preferred.

Avoid installing PLA brackets in areas that can become unusually hot, such as immediately above heated enclosures or in direct sunlight.

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Shaft selection

The supplied models use 19 mm shaft holes.

I intentionally consider this dimension something users may need to customize because tubing, conduit, pipe and rod marketed at similar nominal sizes can have significantly different actual outside diameters. 
I personally used dollar store bamboo broomsticks that were 19mm in diameter.

Before printing multiple brackets:

1. Measure the actual shaft with calipers.

2. Print or test-fit one bracket.

3. Allow appropriate clearance for your printer and material.

4. Resize the hole in CAD/Orca/Slicer if necessary.

The shaft itself can become the limiting factor on a long rack. For a 43" span carrying roughly 16 kg, choose tubing/rod with adequate bending stiffness rather than assuming any 19 mm material will work.

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Hardware

For one three-bracket rack level using the top brackets:

• 3× Top Brackets

• 12× #8 × 1" countersunk wood screws

• 2× 19 mm shafts, approximately 43" long or sized for your installation

For an additional suspended level:

• 3× Lower Brackets

• 2× additional 19 mm shafts

The upper holes of the lower brackets use the previous level's shafts, so no additional joist screws are required for the extension brackets.

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Installation notes

For best results:

• Space the three brackets evenly along the shafts.

• Make sure all brackets are aligned before tightening the mounting screws.

• Fully seat the top bracket on the joist.

• Fasten both sides of the fork.

• Load spools reasonably evenly across the rack rather than concentrating everything in one section.

• Inspect the brackets periodically for cracking, deformation or movement.

• Test your installation progressively before fully loading it.

If you change the span significantly beyond my 43" setup, shaft deflection becomes increasingly important even if the printed brackets remain strong enough.

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Current testing status

Top bracket: Printed and in use/tested with the intended rack configuration.

Lower bracket: Designed and included, but not yet physically tested under a loaded second level.

I will update the model description once I have had enough time to test the multi-level configuration.

If you print the lower brackets before then, please treat them as experimental and load-test them gradually.