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GRP Roofing Common Misconceptions Explained

wet-lay

Typical wet lay method

Having long witnessed disputes and arguments regarding various aspects of GRP roofing, we would like to assist in clarifying common misconceptions about the way that GRP roofing performs.

Traditional ‘wet’ laid GRP roofs, where liquid resin is applied directly to a timber substrate before having the glass reinforcement incorporated insitu can provide extremely tough, reliable and durable roofs that will outlast any thermoplastic or elastomeric membrane.

In this method of application, resin is absorbed into the surface of the timber substrate or decking, usually an OSB3 or roofing grade WBP plywood. Where there is sometimes a noise problem experienced with these type of roofs, it is usually due to the expansion and contraction of the decking through the seasons due to the boards dimensionally reacting to thermal and moisture content changes.

It is generally recommended that 8’ x 4’ (2.40m x 1.20m) roof decking boards are fitted with a minimum of a 3mm gap around all of the edges, and 10mm at an abutment wall to allow for this movement. In ‘wet-lay’ GRP practice, this movement at the board edges should be restrained or eliminated by the application of additional glass reinforcing layers over the board joint. This can generally be perfectly successful if undertaken by properly trained operatives and large roof areas avoided as this introduces a requirement for expansion joints within the system.

CGI

The Dryseal is a GRP component based flat roofing system

Unfortunately, some might argue, roofing resins and roof edge trims are available to anyone who wishes to purchase and install them, often with insufficient knowledge or training. Board joints can be intentionally laid tight to minimise the movement at the edges, and the reinforcement at these joints can often be inadequate.

As the boards expand and contract, stresses can be induced at the edges due to movement, and just like a PVC fascia or gutter, creaking and cracking or banging noises can be heard. The movement at the board edges can also lead to excessive loads being transferred to the fully bonded GRP layer resulting in cracks that pass all of the way through.

Unlike fully bonded GRP systems, Dryseal was developed as a pre-cured single layer membrane that is supplied on a roll and typically fully mechanically fixed through side edge overlaps at 300mm centres. This allows the GRP membrane to be secured to the roof construction, but still be allowed to move independently of the layers below. This capability allows for the differential rates of expansion between the differing roof layers and an installation that can reasonably be expected to last longer than a thermoplastic or elastomeric single ply system, and without the need for the inclusion of expansion joints unless required to accommodate specific areas of building movement.

Dryseal has carried BBA certification since 1995 including ‘zero falls’ applications and is only available to trained and approved installers, with perhaps one of the best insurance backed guarantees in the construction sector.

Dryseal is generally one of the most universal systems available and can accommodate almost all applications except where the requirement for the waterproofing layer is to be ballasted only.

John-Godley-cropped

John Godley, Technical Manager

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