Top rail height
29 CFR 1926.502(b)(1) sets the top edge of a guardrail at 42 in plus or minus 3 in above the walking/working level. That is a range, 39 to 45 in, not a single number.
Trap: "42 in exactly" drops the ±3 in tolerance. Two other ranges look similar and belong to other rules:
- 36 in is the minimum stairrail height (Subpart X), not a guardrail figure.
- 38 to 45 in is the toprail range for supported scaffolds placed in service after January 1, 2000 (1926.451), not the Subpart M guardrail.
Strength of the top edge and midrail
Under 1926.502(b)(3) the top edge must withstand at least 200 lb applied within 2 in of the top edge, in any outward or downward direction.
Under 1926.502(b)(5) midrails, screens, mesh and other intermediate members must withstand at least 150 lb.
Trap: swapping the two. 200 lb is the top edge; 150 lb is the midrail. The other big numbers belong elsewhere: 5,000 lb is personal fall arrest anchorage strength, 1,800 lb is the arresting force on a harnessed worker, and 250 lb is the load a fixed ladder rung must hold.
Deflection under load
1926.502(b)(4) says that when the 200 lb load is applied, the top edge may not deflect to a height less than 39 in. That is simply the bottom of the 42 ± 3 in range, so the rail may drop below the nominal 42 in, but never below 39 in. The 36 in stairrail minimum and the 30 in handrail minimum are not deflection limits.
Openings
1926.502(b)(2) limits openings in the guardrail system, including the gaps between intermediate members used instead of a single midrail, to no wider than 19 in.
Trap: 3.5 in is the minimum toeboard height, not an opening. And 19 in shows up twice in OSHA: here as the maximum guardrail opening, and in 1926.1051(a) as the break in elevation that requires a stairway or ladder.