Outdoor Thermal Design
Designing Exterior FRP Grating for Temperature
Outdoor FRP grating moves more than steel and heats up in direct sun. The design job is to give it calculated room to move and to rate the surface temperature, not the air reading. Get the gaps right and summer buckling never appears; get them wrong and the first hot day is a repair call.
Surface vs. Air Temperature
A dark FRP deck in direct sun can run 20-30 C above the air temperature. Rate the design surface temperature (e.g. air 35 C then deck 60-65 C) for both thermal movement and UV service life. Light colours and UV-stabilized resin run cooler and chalk more slowly; see UV weathering.
CTE by Direction (indicative)
| Resin | Longitudinal (along bars) | Transverse (across width) |
|---|---|---|
| Isophthalic polyester | 12-18 x 10^-6 / C | 25-35 x 10^-6 / C |
| Vinyl ester | 10-16 x 10^-6 / C | 22-30 x 10^-6 / C |
| Phenolic / FR | 8-14 x 10^-6 / C | 20-28 x 10^-6 / C |
Per ASTM D696. The transverse (width) direction governs gap design because it expands most. Worked example and gap rules are in the thermal expansion chapter.
Install at the Midpoint
Install panels near the midpoint of the seasonal temperature range (spring or autumn), so the calculated gap accommodates both summer expansion and winter contraction. Install on the hottest day and you leave too little expansion room; on the coldest day you risk over-closed gaps that chatter under traffic. One end fixed, one slotted; dividers every 6-9 m on long runs.
Rate deck, not air
Add 20-30 C to air temperature for sun-exposed dark decks.
Size transverse gap
Transverse CTE governs; use 6-8 mm where movement is large.
UV-stabilized grade
HALS/UV-absorber + C-veil for 15-25 yr outdoor life.
Calculating Thermal Movement
Thermal movement is length × CTE × temperature range. For a 3.66 m pultruded panel in isophthalic polyester with a longitudinal CTE of ~15 × 10⁻⁶/°C and a 50 °C seasonal range, the movement is 3.66 m × 15 × 10⁻⁶ × 50 ≈ 2.7 mm — small, but real over long deck runs and fixed structures. The design response is to provide calculated gaps: leave a nominal 10 mm gap at panel ends and around fixed columns and penetrations, drill fastener holes rather than forcing clips, and use clips that permit movement instead of rigid connections. Getting the gaps right at design stage is what prevents the summer buckling call.
Solar Loading and Surface Temperature
A dark FRP deck in direct sun can run 20–30 °C above air temperature because the laminate absorbs solar radiation and has low thermal mass. Rate the design surface temperature — not the air reading — for three purposes: thermal movement calculation, UV service life and surface-touch comfort in pedestrian areas. Light colours and UV-stabilised resin run cooler and chalk more slowly; on architectural and pedestrian decks, colour choice is therefore part of thermal design.
CTE by Direction
| Resin | Longitudinal (along bars) | Transverse (across width) |
|---|---|---|
| Isophthalic polyester | 12–18 × 10⁻⁶/°C | 25–35 × 10⁻⁶/°C |
| Vinyl ester | 14–20 × 10⁻⁶/°C | 28–38 × 10⁻⁶/°C |
The transverse direction moves more than the longitudinal — the reason expansion provision matters most across the panel width and at long runs of adjoining panels. Pultruded sections, with aligned bars, are more directional still; orient and gap them with the CTE in mind.
Design Checklist for Outdoor Decks
- Calculate the design surface temperature from solar load, not air temperature.
- Specify UV-stabilised resin for all permanently exposed panels.
- Allow ≈ 10 mm gaps at panel ends and around fixed members.
- Use clips that permit thermal movement; never over-tighten.
- Choose light colours where surface-touch comfort or solar movement matters.
- Calculate the design surface temperature from solar load, not air temperature.
- Specify UV-stabilised resin for all permanently exposed panels.
- Allow ≈ 10 mm gaps at panel ends and around fixed members.
- Use clips that permit thermal movement; never over-tighten.
- Choose light colours where surface-touch comfort or solar movement matters.
Need an Outdoor Gap Calculation?
Send panel size, installation temperature and min/max service temperature - ZeAllgrate returns the calculated gap and fixed/sliding support schedule.
Source: ZeAllgrate (Fiberglass Grating Manufacturers Council / ACMA) design guidelines; ASTM International; ISO 14122; OSHA 29 CFR 1910. Indicative data for specification guidance only — final design verified by ZeAllgrate engineering against certified load/span charts.