Timber Mats Weight Capacity by Type

“Weight capacity” is the number every buyer searches for. It’s also not a real engineering term.

Engineers work with two separate properties: bending strength, or modulus of rupture (MOR), measured in PSI, and ground bearing capacity, the ground’s ability to hold equipment without failing under it. OSHA’s ground conditions standard, 1926.1402, treats these as distinct factors, and mats are classified as supporting material used to help meet that requirement. OSHA rule for crane: We’ll use “weight capacity” as shorthand for both throughout this piece, since that’s the term many users are searching for, but the rest of this article breaks down what’s actually driving that number.

Thickness gets the attention because it’s easy to see and easy to compare. Two mats at the same thickness can carry very different loads depending on species and grade. If you’re specifying by thickness alone, you’re missing half the picture, and that’s exactly where mats fail on site.

What Weight Capacity Actually Measures

A mat’s capacity isn’t one number stamped on the side. It’s bending strength (MOR, in PSI), thickness, species density, and how the load spreads across the surface, all working together, per the FPL Wood Handbook, Chapter 5. Specifically, it’s the square of the thickness times the material strength. That’s why knowing the exact thickness rather than nominal thickness is important. Think of two mats of the same species and timber grade. But one has a nominal thickness of 8” and is true-to-size, and the other has a nominal thickness of 8” but a real thickness of 7”; then you’ve lost about 25% of its load-bearing capacity. (8”x8” = 64 in2; 7” x 7” = 49 in2. Difference is ~25%)

Load spreading is important. Most mat failures aren’t from exceeding a mat’s total rated capacity. They’re from concentrated point loads, an outrigger pad or a track edge, exceeding what the mat can take at that specific contact point, even when the equipment’s overall weight is well within range.

Bending Strength by Species and Grade

Species and grade move the needle more than most buyers expect.

Standard oak mats run around 550 PSI at #2 grade and closer to 1,000 PSI at #1 grade, using American Wood Council National Design Specifications (p. 46 onward). Select Structural grade exists on paper but rarely shows up in mat manufacturing; the cost doesn’t pencil out.

Eucalyptus is a different animal. Tighter grain, denser wood, and ratings closer to 2,000 PSI, roughly double a #1 oak mat. That’s the gap that makes two mats at the same thickness completely different products once you put heavy equipment on them. An 8-inch oak mat and an 8-inch Eucalyptus mat are not interchangeable.

Weight Capacity by Thickness

• 4-inch mats: light loads, foot traffic, light equipment access. Not rated for heavy equipment in most species, and using them under a crane or rig can be problematic.
• 6-inch mats: moderate loads, standard construction and vehicle traffic. Species still matters here. A 6-inch Eucalyptus mat can outperform an 8-inch or 10-inch oak mat depending on the timber grade under the same load, sometimes enough to let you use a lighter and thinner Eucalyptus mat than you’d need in a lower-rated species.
• 8-inch mats: the standard for ROW work.
• 12-inch mats: For cranes, rigs, and anything with concentrated outrigger or track loads. This is where species selection has the biggest impact. The gap between 550 PSI and 2,000 PSI at the same thickness is not small – it’s about 300%. That means you can almost always substitute a 10” Eucalyptus for a 12” mixed hardwood. Sometimes, you can substitute one 12” Eucalyptus for two 12” mixed hardwoods

Matching Capacity to Real Equipment

Work backward from your actual equipment, not from mat specs alone. OSHA 1926.1402 puts the responsibility for adequate ground conditions, including supporting materials, on whoever controls the site, per the regulatory text on cranes and derricks. That makes this a compliance question, not just a practical one.

An 80- to 150-ton mobile crane can put significant load through its outriggers, depending on boom position and load. Neither is a job for a 6-inch mat, or even a typical 8-inch mat. That’s why 12-inch, high-PSI mats are the standard for this equipment class: not because thicker is always better – it’s the species and grade, not the thickness alone – but also because the point loads genuinely demand it.

For lighter equipment, utility trucks, small excavators, general construction traffic, a 6-inch mat in many species usually has margin to spare. Matching capacity to actual need instead of defaulting to the thickest option saves real money on material and freight.

Ground Conditions Change What a Mat Can Actually Do

A PSI rating assumes reasonably stable ground underneath it. OSHA defines ground conditions separately from the mat itself: slope, compaction, firmness, as outlined in this breakdown of ground conditions under OSHA 1926.1402. Soft, saturated, or uneven ground reduces what a mat can deliver, no matter how it’s rated, because the mat is only as good as what it’s resting on.

This is why wetland or seasonal-flood sites usually need more safety and capacity margin than the equipment weight alone would suggest. Skip that step, and you can end up with a correctly rated mat still sinking or shifting under load.

A Worked Example

Bring in a 200-ton mobile crane for a utility project. Depending on the equipment and how it’s used, the “weight capacity” can far exceed at a few points what the crane’s total weight would suggest if you spread it evenly across the ground.

A #2 oak mat at 550 PSI isn’t sized for that, at any thickness. Even at 8 inches, a lower-rated species is working closer to its limit than you want on equipment this heavy, especially across a multi-season project with repeated moisture cycling. An 8-inch Eucalyptus mat near 2,000 PSI carries real margin under the same load, which means less failure risk and a longer mat life.

Now put a utility truck or small excavator on the same site. Total weight and ground pressure are a fraction of the crane’s bearing on a few points. (Note that a tire patch is actually pretty intense weight. That’s why having multiple tires on trucks helps spread the load and provide driving stability.) A typical 6-inch mat in some species handles that fine. Specifying an 8-inch, high-PSI mat here isn’t wrong; it’s just unnecessary weight and freight cost for a load that never gets close to the mat’s limit.

The exercise worth doing before you order: find your heaviest concentrated load, size for that number with a safety margin, and use lighter mats where the load doesn’t demand more. Buying one capacity level for an entire project, in either direction, usually means overpaying somewhere.

Weight Capacity and What It Costs You

Get capacity wrong low, and you risk mat failure, downtime, and safety incidents, all of which cost more than the mat ever would. Get it wrong high, spec 8-inch, high-PSI mats where a 6-inch would do, and you’re paying for weight you don’t need. At freight of $1,500 a truckload, the difference between 6” and 8” mats is significant, and fitting fewer mats per load because you over-specified adds up fast across a large project.

Getting it right the first time avoids both problems. Our total cost to own calculator factors in weight, freight, and mat life together instead of looking at any one variable alone; if you are looking only at weight and size, look at the freight calculator.

Quick Reference

Figures vary by supplier and drying process, so ask for a documented MOR/PSI rating rather than relying on general ranges, per the USDA Forest Products Wood Handbook and FPL Chapter 5.

Where Buyers Usually Go Wrong

• Assuming total equipment weight is the only number that matters, when concentrated point loads from outriggers and tracks are usually the actual failure point.
• Ignoring ground conditions, since a correctly rated mat on unstable ground still won’t perform as expected under OSHA’s standard.
• Defaulting to the thickest, highest-rated mat for every job, which adds cost and freight weight without adding value on lighter loads.
• Skipping documented MOR/PSI ratings from suppliers. Thickness isn’t a substitute for a verified rating specific to species and grade.

Bottom Line

Weight capacity, once you get past the marketing shorthand, is bending strength and ground bearing capacity working together. Match thickness, species, and grade to your actual equipment and site conditions, and skip the reflex just to buy the cheapest (or thickest) mat available. For the full breakdown across mat types, see our complete comparison guide, or run your numbers through the total cost to own calculator.