The voice of the UK's rooflight industry
Design Details

Assembly and Accessories – In Plane Rooflights
Fixings
The mechanical properties of plastic rooflights differ from metal and fibre cement sheets. They are more flexible and can have a lower fastener pull through value (i.e. Suction loadings which pull the rooflights over their fasteners).

The pull through performance values of fastener assemblies should be determined in accordance with Annex B: BS5427: Part 1: 1996.

Fasteners are required to be watertight and to restrain the rooflights without damage when subjected to wind loads determined in accordance with BS6399: Part 2: 1997 – Code of Practice for Wind Loads, and support the design snow loadings described in BS6399: Part 3: 1988 – Code of Practice for Imposed Roof Loads.

When required, rooflight manufacturers can provide guidance for calculating wind and snow loads covered by the above Code of Practices. Load calculations outside the scope of the above documents should be provided by a structural designer.

Assembled rooflights are also required to meet the HSE non-fragility requirements as detailed in Section 5. The number of fixings, the size of washer, purlin centres and location of fixings will have a bearing on the non-fragile performance of the rooflights.

Fig.1 – Site Assembled Triple Skin – Cross Section To meet the above design loadings and the non-fragility requirements, washers of at least 29mm diameter should be used in conjunction with 5.5mm diameter primary fasteners. The preferred location of the fasteners is usually in the bottom flat troughs of profiles (see below), except for continuous sinusoidal profiles which have no flat area where crown fixings should be employed.

To prevent build up of rainwater behind the fasteners, the washer diameters should be at least 10mm less than the trough width. Wide troughs may require more than one fixing in each trough.

When sheets are fixed through the crown of the corrugation, rigid profile shaped supports are required between rooflights and supporting members to enable the fasteners to be correctly tightened without distorting the profile.

Roof purlins must have a level face parallel to the roof plane, otherwise if twisted the rooflight liners will deform.

NB: With the new Thermal Performance Regulations, the additional weight of insulation and accessories may be an issue regarding roof purlin design.

Fig.2 – Site Assembled Triple Skin – Longitudinal Section

Where buildings are in non-exposed locations, less than 10 metres high, and have limited permeability, wind loading is usually less than 1.2kN/m2 in general roof areas. GRP rooflights in 32mm deep trapizoidal profiles of weight 1.83kg/m2 and 2.44kg/m2 can be used at purlin centres of 1.8 metres and 2.0 metres. Similarly profiled polycarbonate rooflights of thickness 1.2mm can be used at purlin centres of 1.5 metres. In all cases the rooflight should be fixed at all purlins with 29mm diameter washers on fasteners, and a minimum of five fasteners across the sheet width.

Heavier or thicker rooflights, or reduced purlin centres will be required when rooflights are located in areas of high local suction wind loading adjacent to roof verges and ridge.

Provided that rooflights, located in the general roof area, are installed to meet the design wind and impact loadings, they will support the snowloads likely to occur in the UK. When rooflights are used in zones where:

  • exceptional high loadings may occur
  • on high buildings
  • adjacent to abutments
  • where valleys abut parapet walls
  • other obstructions where snow drifts are likely; then heavier weight rooflights will probably be needed.
Plastic rooflights are more flexible than metal and fibre cement sheets. Whilst this allows these sheets to deflect to a greater extent without damage the following criteria should be adhered to:
  • Limit wind load deflection to 1/15th span or up to 100mm total deflection, to prevent excessive wear around the fasteners.
  • Snow loadings should not deflect the rooflights to more than 1/15th span or never more than 50mm, to avoid disruption of sealants which may cause end laps to birdmouth.
On built up site assembled rooflights, it is recommended that the liners and the top sheet assembly is fitted progressively across the roof. If lining out only, contractors must be fully aware of CDM non-fragility requirements for both rooflights and opaque sheets. To prevent any distortion of liners, always fix progressively from one end. Do not secure each end prior to fixing at intermediate purlins.

Stitch side laps at centres not exceeding 450mm. On exposed sites and roof pitches below 10°, reduce centres to 300mm. Stitch rooflight to rooflight with roofing bolts or proprietary fasteners, which provide adequate support on the undersides. Where rooflights overlap metal sheets, self tap screw fasteners may be used.

When drilling for side lap fasteners, where the rooflight underlaps care must be taken not to push down the underlap with the drill. When the drill bursts through the outer sheet, the drill should be lifted to allow the liner to recover and then continue drilling with care.

Primary fasteners should not be fixed within 50mm of the end of the rooflight, after allowing for on site tolerances, unless provision is made to reinforce the edge of the rooflight, (a typical example is the built up/end upstand on factory assembled units).

Where rooflights extend to the bottom of the downslope (e.g. at eaves or valley) the overhang should not exceed 150mm.

Due to high thermal expansion coefficient of PVC and polycarbonate rooflights, over sized holes are required around the primary fasteners to accommodate the thermal movement without stress. On such rooflights up to 3 metres long over size holes should be 10mm diameter. On sheets up to 4 metres long over size holes should be 12mm. Due to high thermal movement, the length of PVC and polycarbonate rooflights should not exceed 6 meters and at this length a very high standard of workmanship at installation is required.

GRP rooflights do not normally require any special provision to allow for the thermal movement.

Application
To comply with the statutory requirements discussed in the Compliance section, rooflights used on insulated and heated buildings must be of triple skin construction. They may be assembled on site as a built up system or fabricated as a single component under factory conditions.

Use site assembled rooflights with in situ insulated double skin roofing systems. Factory assembled rooflights are used in conjunction with composite panels or under purlin lining systems.

Rooflights assembled on site consist of top sheets and liners to match the profiles of the adjacent opaque roofing systems with proprietary profiled sheets or other insulating layers installed between the top sheets and the liners.

On factory manufactured insulating units, flat or profiled liners with upstands to form a box are bonded to the underside of the external sheet,with additional insulating components between the sheet skins.

Sealants
Seal end laps on external weather sheets with two runs of preformed sealants applied within 15mm on each side of the primary fasteners. Ensure that sealants are well bedded into the corrugations prior to the application of the overlapping sheets.

When rooflights overlap rooflights or overlap metal, an additional seal close to the end of the lap will restrict dirt and moisture ingress.

Seal weather sheet side laps with at least one strip of preformed sealant tape located out board of the side lap stitchers (sealant laid in line with side lap fasteners can twist and become distorted when drilled through).

On built up assemblies, translucent liners form an integral part of the vapour sealed lining system. It is recommended that each side of the translucent liners should overlap the metal liners, and be sealed with 50mm wide film backed butyl tape applied over the joints between the translucent and metal liners. Seal end laps with a similar tape or a single run of sealant fixed above the fasteners.

Where the vertical upstands of factory assembled rooflights abut composite panels, they may be effectively sealed with closed cell, foam plastic strip.

Although adequate sealing will control moist air entering the rooflight in new build, some temporary misting may occur on the underside of the external sheet, particularly on cold, clear, frosty nights. This is normal and the misting will disappear as the structure dries out.

Polycarbonate rooflights should not come into contact with plasticisers, and barrier tape (not PVC) should be used to prevent contact with plastisol coatings on steel sheets.

Assembly and Accessories – Out of Plane Individual and Continuous Rooflights

Fixing Requirements and Weather Tightness
Fixing requirements vary slightly between rooflight manufacturers but the general curb/dome arrangement remains the same. However, the curb installer must follow the instructions supplied with each particular type of rooflight.

When using a preformed metal, plastic or GRP curb – Figure 3, this must be fixed squarely to the roof structure which surrounds the rooflight opening using appropriate fixings e.g. wood screws in the case of a timber structure.

Fig. 3
An allowance will need to be made within the roof construction for the height of the roof insulation, in order that a 150mm clearance can be achieved from the top of the finished roof weatherings to the top of the rooflight curb. It is important to continue the roof weatherings to the top of the preformed curb, thus providing a continuous weathertight seal. Where vents are incorporated into the side of the curb, the clearance must be at least 150mm to the underside of the vents before a break in the weatherings.

If no allowance is being made within the roof construction for the thickness of the roof insulation, an extra high preformed curb should be specified as necessary in order to maintain the 150mm minimum installation of the dome above the roof surface.

When domes are supplied complete with preformed curbs, the fixing holes in the domes are normally pre-drilled. Should it be necessary to drill fixing holes, these must be oversized to allow for thermal movement.

Care should be taken when bonding torch applied membranes and flashings to a preformed curb, and this should be completed prior to the installation of the dome. Many single ply membranes can be cold bonded to the preformed upstand, therefore, is possible to apply these following installation of the dome.

Prior to fitting the dome, it is important to fit a sealing strip around the entire perimeter of the fixing flange and fixing washers must be compressed onto dome, again maintaining a weather tight seal.

Many intermediate sections are available for fitting between the preformed curb and dome, such as ventilators, access hatches and smoke vents. These are normally factory fitted to the preformed curb, however, should site assembly be necessary, the installer must follow the particular manufacturers instructions.

Where a dome is to be installed directly to a builders timber curb – Figure 4, an allowance must be made within the roof construction in order that a 150mm clearance is maintained between roof weatherings and the top of the finished curb. It is advisable to continue the flashings over the top edge of the curb.
Fig. 4
Many intermediate adaptor sections, vents, etc., are available for installation between the builders timber curb and dome, and these should be fixed in accordance with the manufacturers instructions.

The sealing strip, which must be continuous, is applied to the top of the builders curb prior to the installation of the dome, which will normally allow for overlap of the flashings assuming the curb is level and fixed squarely.

Barrel vault rooflights are available in all rooflight materials to suit standing seam systems, secret fix systems, flat and curved roofs. There are numerous designs which employ different methods of construction although all types are normally fixed to a curb support structure or similar.

The manufacturers fixing and sealing recommendations must be followed to ensure that weather tightness, impact resistance, durability and insulation requirements are maintained.

Fig.5 illustrates a typical cross section of a barrel vault rooflight. These are available in a range of widths to match the system that the rooflights are used with. Barrel vault rooflights can provide varying lengths and widths as required.
Fig.5 Barrel Vault Rooflight – Cross Section
Durability
Durability is the ability of a building and its parts to perform its required function over a period of time (BS7543). Virtually all materials will change physically when subject to UV radiation, moisture and atmospheric pollution. This change may well affect both their performance and appearance. The designer must therefore ensure that, not only will the materials and details used be suitable initially, but also that they will have a satisfactory life if the necessary maintenance requirements are met.

Materials
When considering in plane rooflights, the materials selected for both the roof cladding and rooflight can have a significant effect on the durability of the rooflights, and the amount of maintenance that will be necessary during their life. Components which are exposed to the weather and sunlight are particularly important.

The type of rooflight materials and roof sheeting colour must both be considered. Generally light coloured roof sheets are preferable because they do not absorb as much sunlight as dark colours, and they are therefore cooler. This means they will have less effect on the rooflight laps, which tend to deteriorate more quickly at higher temperatures. Similarly light coloured seals and fillers should always be used. This is particularly important with thermoplastic rooflights, and generally it is not an issue for GRP thermosets. Lighter roof sheet colours also have the best life and they optimise the thermal performance of the roof. The performance might also depend on the shape and orientation of the building and the environment.

Out of plane rooflights are generally not affected by the surrounding and adjacent materials, being isolated from them by the upstands, curbs and isolating systems. They are however, similarly subject to the same rules regarding fillers, seals and other components. Normally however, the rooflight will be delivered in a condition such that it can be incorporated directly into the roof assembly.

All rooflights are subject to gradual deterioration which will cause fading, discolouration and embrittlement, with some PVC being particularly susceptible. Plastic rooflights are generally resistant to normal pollution inthe atmosphere, provided the products have been protected with UV light inhibitors, and suitable surface protection.

With the use of special coatings and films the products can be used in aggressive chemical environments. Resistance to discoloration, surface degradation and embrittlement depends, to a large extent, on the surface protective treatment used by the manufacturer.

GRP
Most GRP rooflights will remain structurally sound for 30 years or longer. UV light and weathering could cause discolouration and surface erosion (thinning), but does not cause embrittlement or weakening of the sheets. Long term performance depends on environment, quality of sheets and surface protection, and maintenance. Discolouration of unprotected sheets can begin within 5 years, but good quality sheets incorporating UV absorbing surface protection (as supplied by all NARM members) will usually prevent significant discolouration for at least 20 years with the right maintenance program, and can virtually eliminate UV discolouration throughout their life. Higher fire resistant sheeting discolours more quickly when exposed to UV light due to the effect of the fire retardant additives.

Polycarbonate
The current generation of polycarbonate rooflighting products are manufactured from high quality extruded sheet material. With these materials, not only is there a high level of basic UV inhibitor but also co-extruded protective layer on both faces of the sheet. This is known as enhanced UV protection and always carries a manufacturers warranty. Additionally, the sheet manufacturer often warrants the performance of the material, even after thermo-forming.

Polycarbonate rooflights can be expected to be fit for the purpose, in excess of 15 years, with a slow (but documented) deterioration of light transmission and strength. Some enhanced UV protected high performance polycarbonate products have a life of 15 – 20 years. As with many high performance materials, care must be exercised with regard to compatibility with adjacent materials. Some roofing sheet finishes (for example plastisol coated steel) can, over time, affect the mechanical performance of the product and an appropriate isolating system should be applied.

PVC
Most PVC darkens and embrittles under UV radiation providing a useful life of 5-10 years. Specially formulated and protected grades are available and, if properly fixed, can last over 20 years. PVC, especially in cold conditions, should always be treated as a fragile material and should therefore not be used on industrial/commercial buildings, unless additional means are provided in the design to prevent falls through the rooflight.

Fasteners

In Plane Rooflights
Both plated carbon steel and stainless steel fasteners are available. Plated steel fasteners provide acceptable performance as long as their heads are protected from the elements. However, where 25 year non-fragility is required, stainless steel fasteners must be used. Stainless steel fasteners must also be used when fixing to aluminium sheets, to prevent bi-metallic corrosion.

The durability of the fixings will affect the non-fragile status of the rooflights, and care must be taken to ensure the fixings’ durability is compatible with the specified or stated non-fragile life of the roof and rooflights.

Fixings with integral plastic heads are more reliable than push on caps, and the use of poppy red heads for rooflight fasteners is recommended.

Modular and Vaulted Rooflights
Always use stainless steel fixings, grade A2 to BS6105, with the fixing type being chosen to suit the supporting substrates.

Design Details for In Plane Rooflights
Rooflights must be assembled correctly in order to achieve the maximum durability. Avoiding water and dirt traps, by ensuring satisfactory slopes and end laps, is particularly important with in plane systems.

The frequency of fixings and the size of the washers, needed for rooflights and rooflight liners, will generally be different to that of the surrounding metal sheets.

Rooflights will also require side lap stitching. A full fixing specification must be obtained from the rooflight manufacturer to ensure long term durability and non-fragility.

Design Details for Out of Plane Rooflights
Generally these rooflights are delivered in a format such that they can be incorporated directly into the roof construction. If site assembly is required, the component parts are prefabricated from suitable materials.

Maintenance

The durability of any rooflight, regardless of the material from which it is made, is always dependent on regular maintenance. Maintenance regimes vary from manufacturer to manufacturer, and each should be approached for their specific recommendations according to their warranty, but in general terms, the requirements can be described as follows:

Cleaning
Clean regularly to maintain the highest levels of light transmission, usually every 12 months. As well as affecting light transmission, surface contamination can affect the heat absorption of many glazing materials, and this in turn can affect the long term physical and optical properties.

The cleaning process is generally uncomplicated, consisting of washing down with warm water and mild detergent. Abrasive, caustic and chemical treatments are unnecessary, and may actually cause damage to the exposed surfaces of the rooflight. A soft cloth or brush may be used to remove persistent contamination. In the case of paint or bitumen splashes, white spirit or alcohol applied with a soft cloth may be used with care. A final rinse with clean water will complete the process. Pressure hoses should not be used as the high pressure water can penetrate the sealing systems.

Inspection
Rooflights should be inspected at least once a year. This is often best combined with a cleaning process. The surface of the rooflights should be checked for damage, and any found should be repaired in accordance with the manufacturers’ instructions. Any damage which penetrates the surface protection of the units will, in time, affect the ability of the unit to resist impact, and with the advent of non-fragile systems, this is particularly important.

Finally, all fixings should be checked for tightness and corrosion. Many non-fragile systems rely on the security of the fixings to achieve their impact performance potential. Any fixing found to be inadequate should be replaced.

Every second or third inspection should include a check of the sealing systems, replacing any that are showing signs of failure.

Note:
Obviously, the frequency of inspection and maintenance must be tailored to suit the local environment conditions on the roof in question, with higher levels of aggressive atmospheres requiring shorter inspection periods.

General
Although rooflight degradation can be minimised by careful specification, attention to detail during construction, inspection/repair and frequent cleaning, the rooflights are only likely to provide adequate daylighting for 20 to 25 years. Replacement must be anticipated during the life of the building. More detailed information can be obtained from individual manufacturers. Long Term Non-Fragility
Provided rooflight products are fixed in accordance with the manufacturers recommendations, rooflights manufactured by NARM members will be designed and produced to be non-fragile when installed, unless stated to the contrary. As with most other roof cladding materials, it must not be assumed that the non-fragility status will last the life of the building.

Good quality GRP rooflights have a service life in excess of 25 years, and polycarbonate 15 - 20 years. However, resistance to impact relies heavily on the quality of the installation, such as method and condition of the fixings, and any deterioration of 'external' aspects of the installation can jeopardise the non-fragility classification of the rooflight assembly, even when there is no deterioration of the rooflight itself.

It is likely that most rooflights will remain non-fragile for between 5 and 20 years, but the exact time at which an assembly will become fragile cannot be determined.

For good quality GRP in-plane rooflights as supplied by NARM members, it is possible to increase the weight of the rooflight sheets, increasing the margin of safety sufficiently so that typical deterioration of an assembly will not jeopardise the non-fragility classification, so that it can be expected that non-fragility will be retained for 25 years, although this should not be guaranteed.

Detailed recommendations on the minimum weights of GRP rooflights which are required to ensure non-fragility when new, and the increases in sheet weights for expected 25 year non-fragility, are given in Tables 1 and 2 of NARM Guidance Note 2006/1. This document also explains in detail durability of good quality GRP sheets, factors which can affect non-fragility, maintenance requirements, and other conditions in order for the recommendations to be valid (for example, for expected 25 year non-fragility all fasteners should be stainless steel). Note that these recommendations cannot be assumed to apply to products not supplied by NARM members.

There are much stronger and safer rooflight options available which may retain their non-fragility classifications for longer periods. The designers, in line with their design responsibility, should determine the risks, the required life and period of non-fragility, and the extra margins to include in order to maintain longer term safety.