FRP grating is not one product. The term covers at least three distinct manufacturing processes — molded, pultruded, and phenolic — each with different structural properties, chemical resistance profiles, and cost points. The engineer specifying grating for an industrial walkway, a chemical plant platform, or an offshore escape route needs a clear mental map of which type does what, and more importantly, where each one creates avoidable problems. This guide draws on decades of installed grating performance to clarify the selection logic.
Molded FRP Grating
Molded grating is produced by laying continuous glass fiber rovings into a open mold in two perpendicular directions and saturating them with resin. The result is a monolithic panel with integral cross-bars, available in standard sizes (typically 1 m × 1 m to 1 m × 4 m) and thicknesses of 25 mm to 50 mm. Because the glass fibers are oriented bi-directionally within the plane of the panel, molded grating has roughly equal strength and stiffness in both directions. This bi-directional character means the panel can be cut to fit irregular shapes in the field without losing structural integrity — you can notch a molded panel around a pipe support without creating a weak line, something that is not advisable with unidirectional pultruded grating.
The open-mesh construction (typically 38 mm × 38 mm or 50 mm × 50 mm openings) provides natural drainage and ventilation. The resin choices — isophthalic polyester for general chemical service, vinyl ester for strong acid or combined acid-alkali exposure, and fire-retardant formulations to meet ASTM E84 Class 1 — allow the grating to be matched to the specific chemical environment. A single 38 mm thick isophthalic panel with a 1 m span will typically support a uniform load of 4.8 kN/m² at a deflection of approximately span/180, which is the common serviceability limit for pedestrian access.
Pultruded Grating
Pultruded grating is manufactured by pulling continuous glass fiber rovings through a resin bath and then through a heated die, producing individual bearing bars with unidirectional fiber orientation. These bars are then assembled mechanically, using cross-rods and spacers, to form a grating panel. The unidirectional fiber alignment gives pultruded grating substantially higher flexural strength and stiffness in the bar direction compared to an equivalent depth of molded grating — typically 30% to 50% higher ultimate flexural strength for the same panel thickness. This makes pultruded grating the economical choice for longer spans, generally 1.2 m to 1.8 m between supports, where molded grating would require either a thicker panel or closer support spacing.
However, pultruded grating is strongly orthotropic: it is stiff and strong along the bars and comparatively weak in the transverse direction. Cutting or notching a pultruded panel across the bearing bars can sever the primary load path and must be accounted for in the design. Pultruded grating also has a different chemical resistance profile; while the resin matrix may be identical to that used in molded grating, the exposed cut ends of the pultruded bars can wick chemicals along the glass-resin interface if not properly sealed during fabrication — a detail worth confirming with the manufacturer for aggressive chemical service.
Phenolic Grating
Phenolic resin grating is a specialized product for fire-critical applications. Unlike polyester and vinyl ester, which are thermoset polymers that will burn when exposed to sustained flame, phenolic resin chars and forms a carbonaceous insulating layer with very low smoke and toxic fume emission. Phenolic grating meets the International Maritime Organization (IMO) FTP Code for low flame spread, smoke density, and toxicity — the criteria required for offshore helideck access, muster-point walkways, and primary egress routes on oil and gas platforms.
The engineering trade-off is mechanical: phenolic grating typically achieves 60% to 75% of the flexural strength of an equivalent isophthalic or vinyl ester molded grating. It is also more brittle, with lower impact resistance. The specification is therefore selective — phenolic for the walkway sections where fire performance is non-negotiable, molded or pultruded in polyester or vinyl ester for the general grating elsewhere on the same facility. The two should not be mixed on a single walkway run without accounting for the stiffness difference at the transition joint.
Resin Selection for Chemical Environments
The resin system in FRP grating is not simply a "chemical-resistant coating" — it is the continuous polymer matrix that binds the glass fibers and determines the grating's resistance to chemical attack throughout its cross-section. The three most common resin families in industrial grating service are:
- Isophthalic polyester: suitable for general chemical plant atmospheres, including most organic acids, aliphatic solvents, and oxidizing agents at ambient to moderately elevated temperatures. Not recommended for strong alkalis (pH > 12) or aromatic solvents at elevated temperatures.
- Vinyl ester: specified for strong acid service (sulfuric, hydrochloric, phosphoric), strong alkali service (sodium hydroxide, potassium hydroxide), and environments where alternating acid and alkali exposure occurs. Vinyl ester's molecular structure has fewer ester linkages susceptible to hydrolysis, which gives it roughly twice the chemical resistance lifespan of isophthalic polyester in aggressive wet acid environments.
- Fire-retardant (FR) formulations: available in both isophthalic and vinyl ester bases, achieved by adding brominated compounds or alumina trihydrate to the resin. These formulations achieve ASTM E84 flame spread index ≤ 25 but typically reduce mechanical strength by 10% to 20% compared to the non-FR version.
The most common mismatch observed in the field is using isophthalic grating in areas with intermittent strong alkali exposure — the ester linkages in the resin hydrolyze, and the surface begins to show fiber bloom within 2–3 years. A switch to vinyl ester eliminates the degradation mechanism entirely.
For product-specific data on molded grating configurations, see our Molded FRP Grating and Phenolic Grating pages. For pultruded grating span tables, refer to FRP Grating Load Tables.