FRP Purlins for Corrosive Roofs: Selection Guide

In short: In fertilizer stores, chemical plants, salt barns, smelters, aquaculture halls and coastal buildings, the air itself attacks steel. Galvanized and painted steel purlins rust from the inside of the roof, where nobody looks until sheets start to sag. Pultruded FRP purlins do not rust. They are much less stiff than steel, though, so they are usually sized by deflection and wind uplift, not by strength, and long-term creep must be allowed for. Design follows ASCE/SEI 74-23 in the US or CEN/TS 19101 in Europe, with profiles meeting EN 13706 grades. FIBROVX makes FRP purlins to order: section, length, hole pattern and resin are customized to your structural design.

Why purlins fail first in corrosive buildings

Purlins sit just under the roof sheets, in the warmest, most humid part of the building. In a fertilizer warehouse or a chemical plant, that zone collects ammonia, chlorides, acid mist and condensation. Steel purlins in that air corrode at the laps, bolt holes and cut edges where coatings are thinnest. The damage is hard to see from the floor, and replacing purlins means stripping the roof.

Common problem buildings include:

• Fertilizer and salt stores, with hygroscopic dust and high chloride or nitrate levels.
• Chemical, plating and pickling plants, with acid fumes.
• Smelters and refineries, with sulfur compounds.
• Aquaculture and fish processing halls, with warm, salty, wet air.
• Wastewater plants and composting halls, with hydrogen sulfide and ammonia. See our guide to FRP in wastewater treatment plants.
• Coastal buildings, with salt spray.

In these buildings, a non-metallic roof frame can last far longer than coated steel. Pairing FRP purlins with FRP transparent roofing sheets and non-corroding fasteners keeps the whole roof free of rusting parts.

How FRP purlins differ from steel

The headline is stiffness. An FRP section of the same shape bends roughly ten times more than steel under the same load. That is why FRP purlins are often deeper or more closely spaced than the steel they replace. Strength is rarely the limit; deflection and stability are. Our FRP vs steel and aluminum profiles guide explains the comparison in more depth.

The design checks that govern

A structural engineer should design the purlins for your building. These are the checks that usually decide the section:

1. Deflection under service load. Roof sheet manufacturers and codes limit purlin deflection, often as a fraction of span such as L/150 or L/200. Use the limit in your project specification. FRP's low modulus makes this the most common governing check.

2. Shear deformation. FRP is weak in shear stiffness compared with its bending stiffness, so shear adds to deflection, especially on short, deep spans. ASCE/SEI 74-23 and manufacturer data include it.

3. Creep. Permanent loads, such as roof dead load, solar panels or hanging services, make FRP deflect more over time. Designers use reduced long-term stiffness or creep factors. Our FRP structural design basics covers this.

4. Wind uplift and stability. Under wind suction, the bottom flange of a purlin goes into compression, often with no sheeting to restrain it. Thin-walled FRP sections can twist (lateral-torsional buckling) or buckle locally. Sag rods, bracing or anti-sag bars may be needed. Wind loads come from your building code, for example ASCE 7 in the US.

5. Connections. Bolted connections in FRP are designed for bearing, shear-out and net tension at the holes. Use stainless steel or other corrosion-resistant fasteners, and seal drilled holes and cut ends with compatible resin.

6. Temperature. FRP stiffness drops as temperature rises toward the resin's limit. Roof voids can run hot in sunny climates. Tell the designer the maximum service temperature.

7. Fire. Check what your building code requires for the roof structure. Fire-retardant resins can improve flame spread performance, but FRP is still combustible.

Standards and data

• ASCE/SEI 74-23 gives load and resistance factor design (LRFD) rules for pultruded FRP structures in the US.
• CEN/TS 19101:2022 covers the design of fiber-polymer composite structures in Europe.
• EN 13706 specifies pultruded profiles and defines two grades, E17 and E23, named for their minimum longitudinal modulus of 17 and 23 GPa.

Design should use the actual section properties and material data for the profile you buy. FIBROVX provides drawings and test data for the specific item quoted. We supply mechanical values only from confirmed test data, and we do not claim certifications we do not hold. You can read more about the standard in our pultruded profiles guide.

Choosing the section and resin

Section shape. C, Z, I and hat sections are all used as purlins in the wider industry. C and Z shapes lap or nest at supports. I sections are stiffer for their weight but need cleats. Tell us the shape and size your designer wants. If it needs new tooling, we will say so and quote it.

Depth and wall thickness. Driven by span, spacing and load. Deeper sections cut deflection most effectively.

Length. Pultrusion makes continuous lengths, cut to size. Long lengths reduce joints but need to fit your transport.

Resin. Isophthalic polyester suits many mildly corrosive buildings. Vinyl ester resists stronger acids and alkalis. Fire-retardant formulations can be quoted on request. See polyester vs vinyl ester vs phenolic resin.

UV protection. Purlins under translucent sheets see daylight. A surface veil and UV-stabilized resin help prevent fiber bloom.

Hole pattern. Factory drilling to your connection drawing saves site work and keeps holes accurate.

Life-cycle view: when FRP pays back

FRP purlins usually cost more per meter than galvanized steel, and the deeper sections needed for stiffness add to that. The case for FRP rests on what happens after year five or ten in an aggressive building:

• No recoating. Steel purlins in a corrosive roof void need inspection and touch-up painting, often from scaffolding or lifts. FRP needs neither.
• No early replacement. Replacing corroded purlins means stripping the sheets, shutting the area below and re-roofing. In a working fertilizer store or process hall, the downtime can cost more than the material.
• Lighter handling. At roughly a quarter of the weight of steel, FRP lengths can often be lifted by hand or with light equipment, which speeds erection on roofs with limited crane access.
• No rust staining. Products stored below, such as fertilizer, salt or food, are not contaminated by falling rust flakes.

The comparison depends on how aggressive your environment is. In a dry, clean warehouse, coated steel is usually the economic choice. Where steel purlins have already failed once, FRP is worth pricing. Ask your engineer to compare both options over the design life of the building, including access and downtime costs.

Installation tips

1. Design first. Have the purlins designed for your loads, spans and deflection limit.
2. Confirm details. Agree section, lengths, laps and hole pattern on a drawing before production.
3. Cut and seal. Cut with diamond or carbide tools, wear dust protection, and seal every cut end and site-drilled hole with compatible resin.
4. Fix and brace. Use corrosion-resistant fasteners. Install sag rods or bracing as designed, before loading the roof.
5. Sheet the roof. Fix sheets to the sheet maker's instructions. Avoid overtightening fasteners, which can crush FRP walls; washers spread the load.

Customization from FIBROVX

FIBROVX supplies FRP purlins by quotation. Every order is made to your requirements. Please send:

• Section profile and dimensions, or the designer's drawing
• Span and spacing from your structural design
• Length and quantity
• Connection details and hole pattern
• Service environment, temperature and any fire requirement
• Any engineering data you need with the quote

We confirm dimensions, resin and data in writing with each quotation. Browse related products in Grating, Profiles & Access, including FRP pultruded profiles.

Common mistakes

• Swapping steel for FRP at the same size. FRP is far less stiff. Redesign the section and spacing.
• Ignoring uplift. The free bottom flange needs restraint under wind suction.
• Forgetting creep. Permanent loads keep adding deflection over time.
• Using plain steel fasteners. They corrode and stain, and can loosen the roof.
• Leaving cut ends unsealed. Exposed fibers wick moisture and chemicals.

Key terms

• Purlin: A horizontal roof member spanning between rafters or frames that supports the roof sheets.
• Pultrusion: Continuous molding of constant-section FRP profiles. See what is pultrusion.
• Creep: Slow increase in deformation under constant load.
• Lateral-torsional buckling: Sideways twisting of a beam when its compression flange is unrestrained.

FAQ

Can FRP purlins directly replace steel purlins? Rarely at the same size. FRP is about one-tenth as stiff, so an engineer usually specifies a deeper section or closer spacing.

What spans can FRP purlins reach? It depends on section, load and deflection limit. Your structural engineer calculates it from the section data we provide.

Are FRP purlins fire-rated? FRP is combustible. Fire-retardant resins can be specified, but check your code requirements; we confirm in writing what we can supply.

Which resin should I choose? Isophthalic polyester for mild environments, vinyl ester for stronger chemicals. Tell us the chemicals in the air.

Can you make a custom section? Often yes. New sections may need new pultrusion tooling, which we will quote separately.

Related guides

• FRP Structural Design Basics
• FRP Profiles vs Steel and Aluminum
• FRP Pultruded Profiles Guide
• FRP Transparent Roofing Sheets
• Industries we serve

Sources

• ASCE/SEI 74-23, Load and Resistance Factor Design for Pultruded FRP Structures: https://ascelibrary.org/doi/book/10.1061/9780784415771
• CEN/TS 19101:2022, Design of fiber-polymer composite structures (EU JRC Eurocodes): https://eurocodes.jrc.ec.europa.eu/publications/design-fibre-polymer-composite-structures
• BSI / CEN, EN 13706, Specifications for pultruded profiles: https://knowledge.bsigroup.com/products/reinforced-plastics-composites-specifications-for-pultruded-profiles-specific-requirements
• A European pultruder, European Standard EN 13706 (E23 and E17 minimum properties)
• American Institute of Steel Construction, steel properties: https://www.aisc.org/
• ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures: https://www.asce.org/publications-and-news/asce-7