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Securing large boulders on a flatbed: 49 CFR 393.136

Helm PartnersUpdated Oct 30, 2026Sources checked Oct 30, 2026

This guide is general information, not advice on any policy.

Short answer

Under 49 CFR 393.136, a boulder over 11,000 pounds or 2 cubic meters on an open trailer sits flattest side down on at least two hardwood blocks of 4 by 4 inches or more. A boulder that could roll goes in a hardwood crib. Only chain may be used. The chain pattern depends on shape, and aggregate working load limit is at least half the boulder's weight.

In this guide
  • What the rule covers
  • Positioning
  • Chain only
  • Patterns by shape
  • FAQ

What the rule covers

Large natural rock over 11,000 pounds or 2 cubic meters on an open vehicle (49 CFR 393.136(a)). Cut rock with a stable base may use this rule or the general rules.

Positioning

  • Flattest or largest side down.
  • At least two hardwood blocks, 4 by 4 inches or larger, across the full width, supporting about three-fourths of the length.
  • A rounded base goes in a hardwood crib fixed to the deck, with three or more contact points.
  • Tapered boulders: narrow end forward.

Chain only

  • Only chain may be used.
  • Chains sit in valleys or notches where possible and are arranged so they do not slide.

Patterns by shape

  • Cubic: two or more transverse chains near the blocking, aggregate working load limit at least half the weight.
  • Non-cubic, stable base: two chains in an X over the center, joined by a shackle, aggregate at least half the weight.
  • Non-cubic, unstable base: a surrounding chain at half to two-thirds height plus four chains to the vehicle.

Related: flatbed cargo securement and steel coils, concrete pipe securement on a flatbed, heavy equipment securement on a flatbed, certificates of insurance for mills and plants.

FAQ

Which rocks does this cover?

Natural, irregular rock over 5,000 kg (11,000 lb) or over 2 cubic meters, on an open vehicle or one whose sides are not designed and rated for it.

What about smaller rock?

Pieces from 100 kg (220 lb) to 5,000 kg may follow this rule or the general rules in 393.100 to 393.114.

Can I use straps?

No. Only chain may be used as tiedowns for large boulders.

Which way does a tapered boulder face?

Narrow end toward the front of the vehicle.

What does an unstable base need?

One chain around the top between half and two-thirds of its height, rated at least half the weight, plus four chains to the vehicle each rated at least a quarter of the weight.

This guide is general information, not advice on any policy.

Sources