Specifying GRP: Delivering Natural Daylight

The need for daylighting in buildings is driven by the demands for providing a healthy and safe environment for the occupants combined with delivering the most energy efficient building possible with the lowest operational carbon consumption.

The provision of daylight into a building, also reduces the demands on the internal lighting system. No matter how efficient modern LED lighting systems are, the truth of the matter is that the longer they burn and the brighter they burn, the sooner they will need to be replaced. This is neither helpful from a financial and operational aspect, nor an environmental one.

We are now seeing an increasing number of undeliverable specifications written for rooflights in industrial, commercial and warehouse type buildings. The problem arises because of high light transmittance levels being specified hand in hand with requirements for low thermal (U-value) and total solar transmittance (g-value) values being specified that are not deliverable. These performance properties are either being entered into building energy modelling software independently of any product specification or the performance data from high performance glazing systems used. These glazed systems are inappropriate for use in self-supporting profiled metal roofs whether the roof is a composite panel type roof, or a built-up site assembled system.

Profiled sheet or panel GRP rooflights are the default and best option. They are produced to match the surrounding metal roof covering and therefore are installed and sealed in a very similar way and without any interference to the drainage of the roof. Depending upon specification they can have long service life guarantees and lengthy periods of expected non-fragile performance.

The nature of GRP means that it has a similar co-efficient of expansion to the roof into which it is installed and delivers naturally diffused light due to the composite resin and glass formulation. This is important in avoiding glare and hot spots from materials that deliver more direct light transmittance such as glass and polycarbonate. It is also important to consider that the big metal shed buildings will usually have racking, machinery, manufacturing equipment etc. installed that will rise significantly higher above the floor than the working plane. The light distribution offered by large panels over larger areas means that more of the space can be lit much more evenly and to greater levels while avoiding shadows, gloom and glare compared to smaller, higher light transmitting units.

Polycarbonate is not favoured for ‘in-plane’ applications for self-supporting metal roof systems. When compared to GRP, expected service life and non-fragile periods are reduced, light blocking additives are required to create any degree of diffusion and maintaining waterproof sealing is more difficult due to the very high coefficient of thermal expansion compared to the surrounding roof construction. Accordingly special installation measures are generally required.

Where roof pitches are very shallow and near horizontal, GRP used in the form of long run barrel vault rooflights mounted on kerbs tend to be the preferred option for the reasons discussed.

Small single kerb mounted rooflights are not considered appropriate for this sector of the construction industry. The primary reasons are because the high numbers required to deliver a healthy, evenly and well-lit building interior without the gloom, glare and hotspots results in many roof penetrations. Each upstanding rooflight kerb must be waterproofed using a different method, usually liquid applied insitu and not always delivering the durability of the rest of the roof.

Creating these penetrations in a roof with a profiled trapezoidal, sinusoidal or standing seam roof that require rooflight kerbs or upstands, unless installed immediately below a ridge flashing, can create a dam to prevent the discharge of rainwater unless more complicated site detailing with higher maintenance requirements is used. Rooflights located only along the ridge line of a large footprint building only increases the contrast between well lit spaces along the centre of the building and dark, shadowy and gloomy spaces along the sides.

There is also a significant cost uplift due to both the rooflight itself and increased labour time. For this reason, where these rooflights tend to be found in profiled metal roofs, there are generally very few and they would not be used as the primary source of lighting a building during the daytime.

These small upstand mounted rooflights are much better suited to different applications such as flat roofs in health and education where room heights are lower and long-term waterproofing is easier to achieve without disrupting the discharge of rainwater from the roof.

For further information please get in touch with Hambleside Danelaw via the contact form, or email marketing@hambleside-danelaw.co.uk.