What is thermal expansion

Taking the expansionwith the contraction

All materials used in roof systems expand and contract with changes in temperature on a daily and seasonal basis. Even a cooling breeze, a cloud passing overhead or a sudden rain shower can cause roof systems to contract. If you don’t take this into account, premature roof failure could occur.

Thermal expansion and contraction issues are responsible for a large percentage of roofing leaks. In the long term, a properly designed weather-tight roof will cost less than an improperly designed leaky low-cost roof.

Thermal expansion occurs when heat is added to most materials, causing the material to expand. Consequently removing the heat energy (cooling) causes the material to contract. Each different material type expands at a set rate unique to that material for one degree in material temperature change.

The nature of the way we occupy the buildings under a roof will determine the amount of roof movement over the structural system. In the past, most buildings with metal roofs were used for shade and shelter or utilitarian space – meaning the temperature inside the building was nearly the same as the outside.

Elongation at break

Measuring flexibility

Elongation at break (EAB) measures how much a material can flex before it fractures, products with lower EAB percentages will tend to be suitable for smaller projects and may require additional products to increase flexibility. Materials with high EAB are generally more flexible and are able to undergo more deformation.


Elongation at break

Flexibility Rating

Standard GRP system

Resin technology is made for strength rather than flexibility and is best suited to small roofing projects.

Rating: Low flexibility

Standard GRP roofing systems are recommended for roofs under 50m², any higher and you would need to install expansion joints.

Flexible GRP system

The modern version of the standard GRP system adds slightly more flexibility and the ability to overlay onto substrates.

Rating: Low flexibility

Even with the additional flexibility expansion joints are still recommended for roofs under 50m².


This incredibly versatile roofing system with a high flexibility rating is suitable for any roofing project and will not require expansion joints.

Rating: High flexibility

Fibrecoat can be applied across multiple surfaces and is incredibly flexible removing the need for expansion joints.


A product that goes beyond repair, Overcoat is incredibly flexible and can be applied to many different surfaces without the need for expansion joints.

Rating: High flexibility

The hybrid polymer technology in Overcoat makes it incredibly flexible and is the perfect choice to overlay onto virtually any roofing surface.