Procurement teams sourcing fiberglass grating panels for the first time often approach the specification the same way they’d specify a commodity product: pick the size that fits the opening, confirm the material, place the order. The problem with this approach is that FRP grating panels have several interdependent variables — mesh opening size, bar depth, bar spacing, and panel thickness — that collectively determine structural performance. Getting the size right without getting these other parameters right produces a panel that fits the opening but may deflect excessively under load, have surface openings too large for the intended foot traffic, or cost more than necessary because the specification exceeded what the application actually required.
The following covers the main specification variables and what drives each decision.
Mesh opening size
The mesh opening — the clear space between adjacent bearing bars and cross bars — is the most visible characteristic of a grating panel and the one that most directly affects what the grating can and can’t be used for.
Large mesh openings (38mm × 38mm and above) allow debris, liquids, and small objects to pass through freely. This is appropriate for drainage gratings in industrial areas where wash-down flow needs to be unobstructed, and for elevated platforms where visual inspection of the area below matters. It’s not appropriate for areas where small tools, fasteners, or personal protective equipment items might be dropped — a wrench through a 38mm opening in an elevated platform becomes a falling object hazard.
Smaller mesh openings (19mm × 19mm or 25mm × 25mm) reduce fall-through risk and are required in applications where personnel wear safety footwear with spike heels or narrow-soled boots, where the grating is adjacent to areas where dropped object hazards are a concern, or where regulatory requirements specify maximum opening dimensions for walking surfaces. Some occupational safety standards specify maximum opening sizes for grating used in personnel walkways — this requirement should be identified at the specification stage and confirmed with the applicable standard for the jurisdiction and application.
For wheel loads — trolleys, carts, pallet jacks — the mesh opening must accommodate the wheel width with sufficient bearing surface on each side. A wheel that spans only one bar in a large-mesh grating concentrates load in a way that the panel wasn’t designed for. The manufacturer’s load tables typically specify the minimum panel dimensions that apply to uniformly distributed loads; point loads from wheels need to be evaluated separately.
Bar depth and load capacity
Bar depth is the primary structural variable in a grating panel. Deeper bars have greater moment of inertia and therefore greater resistance to bending under load. For fiberglass grating panels, standard bar depths in molded grating typically run from 25mm to 50mm, with deeper options available from some manufacturers. Pultruded grating uses a different construction — the bearing bars are pultruded members rather than integrally molded — and can achieve greater depths for high-load applications.
The relationship between bar depth, span, and allowable load is captured in the manufacturer’s load tables. These tables typically show uniformly distributed load capacity (in kPa or psf) for a given panel specification across a range of support spans, at a specified maximum deflection. Standard deflection limits are expressed as a fraction of the span — L/100 and L/150 are the most common, meaning maximum deflection of span/100 or span/150 at the rated load.
The practical implication: a panel that meets load capacity at L/100 deflection may still feel noticeably springy under foot at that deflection limit. For personnel walkways where rigidity underfoot is a comfort or perception-of-safety issue, specifying to L/150 or tighter gives a stiffer result. For drainage covers or structural platforms where personnel don’t walk directly on the grating, L/100 may be entirely acceptable.
Support span
The support span — the unsupported distance between structural members that bear the grating panel — directly determines which panel specifications are adequate. The same panel that meets load requirements at a 600mm span may fail at 900mm. This seems obvious stated plainly, but projects regularly produce grating specifications that were developed for one span and then installed at a different span when field conditions changed.
The correct process is to confirm the actual support spacing in the installed condition before finalizing the panel specification, not after. For retrofit applications where grating is replacing an existing material, the support structure may have been designed around the specific stiffness of that material. Substituting FRP grating with different deflection characteristics at the same span may change how the support structure behaves under load in ways that aren’t immediately visible.
For new construction where the support structure is being designed alongside the grating specification, the support spacing can be optimized for the grating. Reducing span by adding an intermediate support beam often costs less than upgrading to a thicker panel specification — this tradeoff is worth evaluating at the design stage.
Panel dimensions and cutting allowance
Standard molded FRP grating panels are manufactured in nominal sizes — 1000mm × 2000mm and 1220mm × 3660mm are common, though available sizes vary by manufacturer and mesh configuration. Projects rarely have plan dimensions that align perfectly with standard panel sizes, which means cutting is almost always required.
The cutting allowance matters for two reasons. First, FRP grating cut edges expose the internal fiber structure and should be sealed — manufacturers typically recommend applying resin or a sealing compound to cut edges in corrosive environments to prevent moisture ingress along the exposed fibers. Second, the structural capacity of a cut panel must be recalculated based on its actual dimensions, not the standard panel dimensions from the load table. A panel cut to 60% of its original width has different spanning capacity than the full-width panel.
For panels with critical structural requirements that will be cut significantly, the reduced dimensions should be run through the load calculation before cutting, not assumed to be adequate because the full-size panel was adequate.
Resin system and mesh specification interaction
One variable that isn’t always obvious from catalog data: the available mesh configurations are not always the same across resin systems. A manufacturer’s standard product line in isophthalic polyester may include mesh configurations that are not available in vinyl ester, because the more expensive resin is typically offered in a narrower range of standard configurations. For applications requiring vinyl ester corrosion resistance in a specific mesh size, confirming availability before the specification is finalized avoids the situation where the design is locked to a configuration the manufacturer doesn’t stock and the lead time extends to accommodate a custom production run.
The same applies to bar depth. Deep-bar configurations in small mesh sizes are less common than the mid-range combinations, and some configurations are only available as special orders. For projects with both a corrosive environment and a small mesh requirement, the specification needs to be confirmed against what the manufacturer actually produces in that combination.
