Thermal Movement occurs in most buildings with any type, size or shape of roof and regardless of the covering. If a "cracking" sound is heard, it is simply owing to thermal movement of the covering including the substrate and should not be a cause for concern. It is however important to understand the properties of GRP

The Coefficient of Linear Expansion (C.L.E.) of a product, dictates the amount a given product, will expand or contract under changes of temperature. The C.L.E. of cured resin (GRP) devoid of an added glass mat is 60-80(x10 ˜6) unit of measurement / °C This equates to a figure approximately twice that of steel. However, once a 450 gm mat is added, the figure is reduced by approximately 65%.in other words much less than steel. Any movement of a GRP roof is therefore negligible. More important perhaps is the elongation to break of resin ( the amount resin will stretch before breaking ) normal resin has an elongation to break of 1.5% Topseal is double that at 3%. Therefore with UK temperature ranges of perhaps -10°C to 50°C some movement can occur that might in cases, result in noises being generated. These noises are perhaps more common in PVC gutters and cladding when subjected to increases and decreases of temperature.