Whole life costing: Rooflights

Peter Mayer

Rooflights are a great way to get light into a building.
Peter Mayer of Building LifePlans 
considers the specification, durability
and whole life costs.
 

Introduction


Roof lights can save energy on lighting and provide evenly distributed light that enhances the internal environment. Roof lights with kerbs are also known as out-of-plane rooflights are used on flat or low-pitched roof. They can also provide ventilation either through the light opening or by incorporating ventilation grilles in the kerb or frame. Dome rooflights come in various shapes, including pyramidal, dome or trapezoid. Other types of out-of-plane rooflight included:.
• Vault rooflights. Curved units available as modular units or complete system formats to create much larger areas of roof lighting.
• Pitched rooflights. Rooflights configured to create a variety of pyramid, hipped, gable and lantern shapes with glazing bars and posts.

 

Specification options: glazing


The glazing material may be single, double or triple skin. The more layers, the greater the thermal efficiency. The extra costs for multi–layer glazing may be compensated by reduced heating costs. Glazing may be clear or translucent and include anti-glare coatings or tints.

• Polycarbonate has excellent light transmission and impact resistant qualities. Its expected service life is 15 – 25 years.

PVC is mainly used for domestic applications as it does not meet the non-fragility requirements for industrial applications. Its expected service life is 15-20 years. Glass options include safety glass such as Georgian wired, laminated or toughened glass. The service life of insulated glass units is 10 – 25 years; for single glazing it is 60 years.

Plastics sheets are generally 3mm thick, glass is at least 7mm.

Specification options: kerbs

Kerbs may be part of the unit or constructed on site. Factory-made kerbs are manufactured from aluminium, galvanized steel, glass-reinforced plastics or PVCu. Metal kerbs may require additional insulation to minimise the risk of condensation.

The service life of plastic kerbs is 20 – 30 years. The service life of aluminium is related to its thickness, whereas the service life for steel is related to the galvanizing thickness. Initially durability will relate to the decorative finish. Surface coating type, orientation and environment will determine the period before redecoration is needed, which can range from 10 to 30 years.

 

Durability


The durability of glazing material is mainly a function of its resistance to solar radiation and loading, especially impacts.

Specifiers must identify what tests the plastics should meet to confirm resistance to the detrimental effects of ultraviolet light. “UV-protected” means little unless backed up by evidence. Some rooflights have a statement of expected durability from a third party.

BS EN 1873 for rooflights

The recent European standard for plastic rooflights BS EN 1873:2005 describes various tests and classes for the glazing material. Performance is judged by how the glazing material changes in response to artificial ageing under exposure to the energy from a xenon–arc lamp.

 

Light transmission


Two light transmission properties are measured: ‘total luminous transmittance’ and ‘yellowness index’. Durability is a measure of the change of each property due to artificial ageing. There are nine durability classes for light transmission: from A (most resitant) to I (least resistant).

 

Mechanical performance


Mechanical properties that influence the impact and loading performance of the glazing are also classified by the degree of change after artificial ageing. Reduction in tensile strength and elasticity are classified in 10% bands from 0% to greater than 30%. Plastics are exposed to other stresses, including damp, temperature fluctuations and pollutants, which influence durability.

 

Maintenance


The maintenance regime is related to the complexity of the rooflight assembly, use and environmental factors. Opening rooflights are more likely to need expensive maintenance. Typical requirements include cleaning, inspection of seals and ancillary components. Some parts, including seals and fixing caps, may need replacing during the life of the rooflight.

 

 

Specification options

 

  Capital cost
(£/unit)
Net present value for 60 years (£/unit) Service life
Years
Polycarbonate      
3mm thick polycarbonate pyramid rooflight single skin 120 386 15-25
3mm thick polycarbonate pyramid rooflight double skin 180 510 15-25
3mm thick polycarbonate rooflight triple skin 200 551 15-25
PVC      
3mm thick PVC domed rooflight single skin 120 402 15-20
3mm thick PVC domed rooflight double skin 180 533 15-20
Glass      
Safety glass to BS 6206, flat surface, double glazed with built-in kerb. 310 777 10-25
Georgain wired glass circular domed glazing, 600mm diameter, 7mm thick, glass to BS EN 572-3 290 488 60

 

 

Table notes


• A discount rate of 3.5% is used to calculate net present values. Average service lives are used to calculate net present values.

• Costs are based on 600mm x 600mm rooflights and for glazing only unless otherwise noted.

• Installation, replacement, maintenance and cleaning are allowed for.

• A cost analysis based on project-specific information is essential for a realistic best value appraisal.

First published in Building 2006

 

Further information


BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450

 

 

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