The short answer

Treating FRP grating as steel grating that will not rust is the most common mistake. It is light, non-conductive and does not bleed red rust, but its modulus is roughly a tenth that of steel, and its temperature and fire behaviour are different.

Four questions come first: how long the span is, how heavy the load is, how aggressive the environment is, and whether there is a high-temperature or fire requirement. If any answer falls outside the FRP range, steel or a mixed steel and FRP scheme may fit better.

For the point-by-point data and the limits of each material, see the FRP versus steel grating comparison

Weight: more than easier lifting

Steel has a density near 7.85 grams per cubic centimetre; FRP usually sits between 1.5 and 2.0, so by volume it is roughly a quarter to a fifth of steel. FRP grating is normally deeper for the same stiffness, however, so the saving per square metre is smaller than the density ratio suggests.

Less weight does more than ease lifting. Support beams, posts, foundations and lifting equipment all see lower loads, and on a retrofit of an existing structure that often shapes the scheme more than the panel price does.

Lightness has a cost too. Low self-weight means wind uplift, flotation and vibration must be resisted by fixings and hold-down plates. Roofs, tank tops and open platforms need a check against wind and maintenance loads, not a clip chosen from panel weight alone.

Stiffness is the other point that gets missed. FRP modulus is typically 20 to 30 GPa against about 200 GPa for steel, nearly an order of magnitude apart. At the same section depth FRP deflects far more, so long spans need a deeper panel or closer supports.

Corrosion: steel's real weakness

Steel grating is protected by galvanizing, coatings or a switch to stainless. A zinc layer lasts many years in ordinary atmosphere, but in acid, salt spray, fertiliser, seawater or washdown duty it is consumed quickly, and cut edges and welds fail first.

Stainless grating resists more, but it can still pit and suffer stress corrosion cracking in chloride, and it costs more to buy and to weld. FRP has no electrochemical corrosion cell, so its surface does not bleed rust in acid, alkali or salt spray.

FRP is not immune. Resins are attacked by some solvents, strong oxidisers and hot concentrated alkali, and UV exposure causes chalking and colour shift. The resin has to be selected against the actual medium, concentration and temperature, not against the word corrosion.

For how resin and glass each behave against different media, see the FRP material properties and benefits

Conductivity and spark risk

Steel conducts. Near electrical equipment, in substations, electrolysis plants and static-prone powder areas, steel grating needs bonding and earthing, or it becomes a path for potential difference.

FRP is an insulator with high volume resistivity. It does not carry mains current and does not throw sparks when struck by metal, which is a genuine advantage in flammable, dust and solvent environments.

Insulation is not a licence for live work. FRP absorbs moisture and a contaminated surface can conduct, so it must never be treated as an insulating platform for work on live equipment. Where electrical isolation matters, earthing and proving dead still follow the electrical safety rules.

Spark risk works the same way. FRP against metal tools generally does not spark, but cutting and grinding produce dust, and that dust may itself be combustible. In explosive atmospheres, use non-sparking tools and control the dust.

Installed cost and life-cycle cost

FRP grating usually costs more per panel than hot-dip galvanized steel grating, and that is where many projects reject it at first sight. Unit price is not total cost: installation, lifting, corrosion maintenance and lost production all belong in the sum.

Because it is light, two people can carry and set a panel without a crane, and it cuts and drills with ordinary woodworking tools. Steel grating needs lifting gear, welders and galvanizing repair, so its installed labour and programme are usually higher.

Maintenance separates them further. Galvanized steel in a corrosive site is inspected, touched up or replaced every few years; FRP needs no corrosion maintenance, only periodic washing and a check for cracks, wear and loose fixings.

A life-cycle comparison should state the service life, the discount rate and the cost of downtime. Where a project cares only about capital cost and the life is short, steel can be cheaper; where corrosion is severe or shutdowns are expensive, the higher FRP price is usually recovered within a few years.

Temperature and fire

Steel grating does not burn; it loses strength at high temperature but will not sustain combustion. FRP is an organic material, and ordinary polyester resin softens and distorts under sustained heat, so service temperature is normally kept below the resin heat deflection temperature.

Fire-retardant resins can bring FRP to a stated flame spread class, but retardant does not mean non-combustible. Where a fire certificate is required, read the flame spread, smoke development and burning drip data rather than the word retardant.

Hot duty also brings thermal movement. FRP expands little along the fibre and much more across it, so long panels change length with temperature and need an expansion gap instead of being fixed hard at both ends.

When steel is still the right answer

Heavy load, impact and high temperature are steel's home ground. Forklift traffic, heavy maintenance platforms, furnace areas and anything that must take a falling impact still rely on steel's strength, stiffness and toughness.

Where the span is long and supports cannot be added, steel's high modulus keeps deflection down. FRP would need a deeper panel or more supports to meet the same limit, and the installed cost can end up higher.

Strict fire requirements, a need for conductive earthing, a short service life and a tight budget are all reasons to choose steel. Corrosive duty, electrical isolation, low weight and difficult maintenance are where FRP earns its place.

Comparing without distorting the result

Compare the two on the same span, the same load and the same deflection limit, converted for the actual support conditions. Pitting an FRP failure load against a steel allowable load, or comparing tables with different deflection limits, produces the wrong answer every time.

Wording for depth, bearing bar height and deflection limit can be aligned with the FRP grating specification guide

• Fix span, load and deflection limit before comparing price.
• Include installation, lifting and corrosion maintenance in the total.
• Confirm temperature, fire and earthing requirements first.
• State the medium, concentration and temperature for corrosive sites.