FRP vs Wood: Where Pultruded Fiberglass Replaces Timber

The short answer

Choose pultruded FRP over wood where the member stays wet, sits in salt or brackish water, is attacked by fungi, insects or marine borers, or needs to keep the same dimensions and stiffness for decades without treatment. Keep wood where the member is dry and protected, is cut, nailed and modified on site, carries large sections cheaply, or where the look and feel of timber is the point.

FRP vs wood: property comparison

The wood values below are for small, clear, straight-grained specimens of two common structural softwoods. Graded lumber with knots and sloping grain has lower design values, and wet wood is weaker and less stiff than dry wood. The FRP values are EN 13706 grade minimums for structural pultruded profiles; the ordered profile's values come from its own test reports.

Decay, moisture and treatment

Wood decays when it stays wet. The Wood Handbook puts the threshold for serious decay at the fiber saturation point, about 30% moisture content, and treats air-dried wood at 20% or less as a reasonable margin of safety. Rain, condensation and ground contact take wood past that point; water vapor alone does not. Outdoors and in wet process areas, wood therefore depends on naturally durable heartwood, preservative treatment or both, and on keeping water away from joints, cut ends and checks.

Pultruded FRP has no comparable moisture threshold. Glass fiber and thermoset resin are not a food source for fungi or insects, and the profile does not swell and shrink with the seasons. Its own durability questions are the resin system for the chemicals present, UV protection of exposed surfaces and sealing of cut ends and holes. State these in the RFQ the same way you would state a wood treatment specification.

Turn water chemistry, temperature and UV exposure into a resin proposal. FRP resin selection: specify the service environment

Cooling towers

Cooling towers are the classic case. The Wood Handbook notes that soft rot, favored by wet conditions and by repeated wetting and drying, is most damaging to thin pieces of wood such as cooling tower slats, and that a mix of bacteria and fungi can accelerate decay of treated cooling tower slats. Pultruded FRP columns, beams, braces, fill supports and drift eliminator frames avoid that failure mode.

The Cooling Technology Institute publishes CTI STD-137, a specification for fiberglass pultruded structural shapes used in cooling towers. It covers classification, materials, tolerances, defects, workmanship, inspection and mechanical and design properties. If your project or tower supplier calls for it, say so in the RFQ and ask how the offered profiles are tested against it. Fire protection for the tower is a separate requirement set by the applicable code and the insurer; name it, because fire-retardant resin is a different formulation.

Waterfront: piles, decking and marine borers

In salt and brackish water, marine borers are the main cause of damage to wood piles and other wood marine structures. The Wood Handbook states that along the Pacific, Gulf and South Atlantic coasts of the United States untreated pilings may be completely destroyed in a year or less; colder New England waters slow the attack but still require protection where long life is needed. Treated timber relies on preservatives whose use is regulated, and the handbook advises reviewing those regulations when choosing a treatment.

FRP sheet piles, structural profiles, deck boards and railing components remove the borer and decay problem from the water and splash zone. Deck boards are usually hollow multi-cell or ribbed sections; specify span, support spacing, load, deflection limit, surface texture for slip resistance and color. Wood-grain surfaces are available where appearance matters.

Bending, interlocks, driving and how to write a fiberglass sheet pile into the specification. FRP sheet piles vs steel sheet piles

Stakes, handles and small components

Solid fiberglass rods and small tubes replace wooden and bamboo stakes in vineyards, orchards and nurseries, and wooden handles on tools, because they do not rot at the soil line and keep the same diameter and straightness from batch to batch. Specify diameter, length, tip or end shape, color and the bending or breaking test the product must pass. For handles, the assembled tool, not the bare rod, is what gets qualified.

Fastening and working on site

Carpentry habits need adjusting. FRP is not nailed; members are bolted, screwed with suitable fasteners into pre-drilled holes, or bonded. Use stainless steel or FRP fasteners in wet and marine service. Cutting and drilling use carbide or diamond tools, and site cutting needs dust extraction and respiratory protection. Seal cut ends and holes with resin to keep moisture out of exposed fibers. Where accuracy matters, put holes on the drawing so they are drilled in the factory.

Cost: compare service life, not the first purchase

A meter of FRP profile costs more than the softwood member it replaces. The comparison changes when treatment, inspection, replacement of decayed or borer-damaged members, downtime and the remaining life of the structure are counted. Compare both materials on the same installed scope over the same design life. pultrx does not publish prices, because profile price depends on the section, resin, surface, quantity and machining; the quotation for your drawing is the price that applies.

How to specify an FRP replacement for a wood member

Our factory supplies structural shapes, tubes, rods, flat strips and boards including wood-grain deck boards, grating and sheet piles, and makes custom sections with a separately quoted die. The structural design and its approval stay with the project engineer; the quotation states the profile, grade, tests and inspection records supplied with the order.