Whole life costing: Pitched roof coverings

Peter Mayer

Reducing wind damage to roofs is one key to meeting the challenge
of climate change.
Peter Mayer of Builidng Life Plans considers the whole-life costs of materials.

Introduction


Anyone almost blown away by the strong winds at the beginning of 2007 or who witnessed the damge they did to roofs will be wondering about the stability of the structures above their own head. The bad news is that forecasts predict more frequent high winds in the future.

The effects of these gales on the long-term performance and whole-life costs of roofs is mixed:
• For roofing materials such as clay, concrete and slate, which fail from freeze/thaw cycles, the milder temperatures may result in longer service lives and lower whole-life costs.
• Higher wind speeds may result in damage to vulnerable roof areas, in particular edges. Repairs will mean higher whole-life costs.

The recent BRE digest Designing Roofs for Climatic Change, recommends a 10% increase in the fixing frequency of roof coverings to prevent wind damage, which will raise capital costs but lower whole-liofe costs.

The code of practice for slating and tiling, BS 5534:2003, covers fixing location and frequency, but is quite complex. A simplified and approach is The Zonal Method for Roof Tile Fixing Specifications.

 

Roof covering options


Pitched roof coverings should be specified to European standards or have third-party assurance.

Clay roofing tiles and fittings


These are covered by BE EN 1034:2005. Durabiltiy is confirmed by tests for flexural strength, impermeability and frost resistance. When importing clay tiles, ensure compliance to frost resistance test method D to EN 539–2.
Some clay interlocking tiles have third party assurances for pitches below 20°, but check for use in severe exposure zones.
Handmade clay tiles may not meet all the requirements of BS EN 1034, but durability can be assured by testing for flexural strength, impermeability and frost resistance.

Concrete tiles


BS EN 490: 2004 considers mechanical resistance, water impermeability and freeze/thaw resistance.

Artificial slates


Fibre cement slates are covered by BS EN 492:2004 which includes tests for bending resistance, water impermeability, freeze/thaw and heat–rain cycling. Service life guarantees between 20 – 30 years are available.
Alternative material include resin-based reconstituted slates and polyethylene composite moulded slates with service lives of over 20 years. Colour fading and edge-curling may occur.

Natural slates


BS EN 12326–1:2004 tests slates for bending strength, water absorption, freeze/ thaw resistance, thermal cycling and sulfur dioxide exposure.
The thermal cycling test allocates one of three codes based on appearance, surface oxidisation and changes in metallic minerals. T1 and T2 indicate no structural change. T3 code indicates risk of water penetration.
The sulfur dioxide test also results in three codes, which determines the minimum thickness of the slate.

Wood shingles and shakes


Shingles are sawn, shakes split. Western Red Cedar offers a roof covering from a renewable source so scores on the sustainable agenda. Although it is naturally durable, timber preservative treatment is usually applied. Fire retardant treatment may also be required. Fixings should be stainless steel and care should be taken with acidic run–off from cedar.

Design and installation should follow the codes of practice BS 5534 and BS 8000–6.

 

Specification options

 

  Capital cost
£/m²
Net present value for 60 years£/m² Service life
Years
Pitched roof coverings      
Plain clay tiles to BS EN 1304. Flexural strength 600N minimum. Frost resistance: pass frost test D to BS EN 539–2; 265 x 165mm 67 81 50 - 70
Concrete interlocking tiles to BS EN 490, interlocking tiles; 420 x 330mm 30 43 50 - 60
Concrete plain tiles to BS EN 490, double lap; 265 x 165mm 53 70 50 - 60
Fibre cement slates to BS EN 492 with third party approval; 600 x 300mm 43 71 20 - 30
Natural slate to BS EN 12326–1; 500 x 250mm 87 92 75 - 100
Western Red Cedar shingles Grade No 1 “Blue Label”, preservative treated; 400 x 75 – 350mm 65 88 30 - 50

 


Table notes


• Service lives represent average lives. In practice there is evidence of clay tiles with over 100 years of service life and natural slates extant after 250 years. There are also examples of premature failures.

• Capital costs are based on an inclusive rate for a simple 35° duo–pitched roof area of 250m2 for a row of terraced houses with gable ends, no protrusions through the roof. Costs include tiles and detailing at eaves, verge and ridge; fixings, underlay and battens. Lap suited to a sheltered environment.

• A discount rate of 3.5% is used to calculate net present values.

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

First published in Building 2007

 


Further information


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

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

• Pitched roofing materials compared

 

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