I am building a vehicle roof platform. The roof has 2 40mm T slot bars running along the length spaced 48" apart. They can handle static load of 700lbs

I am wondering what is the max safe load capacity of a 48" span of stainless steel 304 bar 1/8" thick?

Each end will be supported on the vehicle T slot, and bolted through As seen in image, the red is the sleet flat bar 1/8"x1" resting over a span of 48"

The length of the vehicle cross bars is 60" and I intend to put 4 evenly spaced 1" steel bars such as they can support a PVC plastic sheet 1/2" thick which can take around 600lbs weight

enter image description here

I was originaly going with 8020, but from the deflection calculator and other online sources, a 8020 1"x1" can safely go 20-30kg/bar which is too less. And if I go with 1515 or higher its very expensive and also increases the roof height, affecting clearance

  • $\begingroup$ Have 3 or 4 bars instead of 2. Why do you think flooring is on 16" or 20" inch spaced joists? $\endgroup$
    – Solar Mike
    Nov 12, 2023 at 10:25
  • 1
    $\begingroup$ See engineering.stackexchange.com/q/47801/10902 $\endgroup$
    – Solar Mike
    Nov 13, 2023 at 1:19
  • $\begingroup$ Thanks. Yes. I plan to use 4 bars spaced 14" apart. That should do the trick, right? $\endgroup$
    – user61237
    Nov 13, 2023 at 8:39

1 Answer 1


As you found out from the deflection calculator, the 1/8 inch thick stainless steel bars are deflecting too much. This is because the l/d ratio of 48" to 1/8" is 384, a very shallow beam in bending. One solution is to increase the number of bars, to reduce the load per bar, but a more effective solution is to increase the depth, of the bars, or of a different shape profile like a square tube section.

On a related matter, PVC is generally not UV resistant and will become brittle due to UV exposure. Since you mentioned that this is for a vehicle, you should consider the durability of the material and its performance in the ambient environment. PVC also will be affected by heat, and PVC tubes and sheets tend to sag between supports when exposed to high heat, as occurs on the roof of a vehicle in a hot climate.


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