I work as a Design Engineer for an industrial ventilation company. In order to complete our outlet for our ventilaion system, we need to install a 54" by 40" steel panel (16 gauge mild steel) onto the roof (think of a skylight on a ceiling for example). My question is: could a 400lb man safely stand atop this panel without falling through? What would be the proper sizing of bolts installed around the perimeter of the panel to accomplish this? Thank you!

  • $\begingroup$ Many small bolts or a few large ones. But also what are they going into? You should know what difference that makes. $\endgroup$
    – Solar Mike
    Commented Oct 8, 2021 at 20:02
  • $\begingroup$ It doesn't matter what any of us think. It only matters what your codes inspector thinks. $\endgroup$
    – Phil Sweet
    Commented Oct 9, 2021 at 1:46
  • $\begingroup$ Without even doing any math, HELL NO that is not safe!! Are you kidding, 1/16" is thinner than the hood of your car. Would you have him stand on that? $\endgroup$
    – RC_23
    Commented Oct 9, 2021 at 4:43
  • $\begingroup$ i would sign it. roof top AC installations require seasonal inspection and maintenance. even new roofs have maintenance as part of warranty. $\endgroup$
    – kamran
    Commented Oct 9, 2021 at 18:26

2 Answers 2


If your framing is lumber, it offers a bit of elasticity for your sheet metal to deflect a little, say 1/8" which is a relaxing deformation.

Then by checking Roark's formulas for stress and strain 7th ed. on flat plates restrained on 4 sides pp 514

$$\sigma_{center of long side}=\frac{\beta_1q*b^2}{t^2}$$



flat plate

Basically, your stresses are nominal. I would use 1-1/2 in. Wood Screw #10 Galvanized Hex-Head Roof Accessory @6" O.C. staggered by half an inch. minimum rafter width 3 by.


enter image description here

  • $\begingroup$ Not where I am you wouldn't. Can't use galvanized anything on PT wood. Have to use ceramic coating. Must use PT wood if using wood. How's that panel going to stand up after 70 years or so? The engineer is personally liable for any injuries or loss unless the inspector signs off on it or some other party indemnifies the engineer. $\endgroup$
    – Phil Sweet
    Commented Oct 9, 2021 at 16:58
  • $\begingroup$ Has anyone managed to personally hold an engineer liable 70 years after they a signed drawing? Muttering a few curses is about as far as I got; sins of the father aren't inherited with the estate in most places. Be sure to follow code where you're at. $\endgroup$
    – Abel
    Commented Oct 9, 2021 at 21:05
  • $\begingroup$ Thank you for pointing me in Roark's direction! I have acquired the text because it has great formulas for practical applications. Can't believe I never heard of it in school. $\endgroup$
    – Jacob S
    Commented Oct 12, 2021 at 17:19

If this is a roof hatch, it shall have its own frame to support the steel panel. You shouldn't have the problem figuring out the required bolts and bolt size by modeling a flexible beam between the hinge supports. The important thing is not only knowing the person will not fall through, but the deflection of the plate also counts heavily on human perception of safety.

Are you a drafting/design technician? If so, this task (bolting and framing) is the engineer in charge's, or your supervisors', job.

Typical Roof Opening/Hatch Support Detail:

enter image description here


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