Whole life costing: Rainwater drainage

Peter Mayer

Keeping rainwater off roofs and away from buildings is essential to
avoid inevitable damage and exorbitant repair costs. Peter Mayer
of Building LifePlans considers the costs and options for eaves
gutters and rainwater downpipes.

The lifetime cost of a suitable rainwater drainage system is modest compared with the cost of repairs to the building that may be necessary if there were a leak. If specified incorrectly, eaves gutters are vulnerable to premature failure, owing to the effects of rainwater and debris and the stress placed on joints and seals by temperature changes.

The supporting documents to the Building Regulations give basic design guidance for the size of gutters. A more sophisticated approach is detailed in BS EN 12056-3. Installation guidance is in BS 8000-13. Regular cleaning, removal of debris, inspection and servicing is essential to ensure problem-free operation of rainwater systems. In wooded areas, gutter protection systems can prevent leaf blockage.

Collecting rainwater can provide long-term cost benefits, providing annual savings of up to £0.50-£2.50 for each square metre of roof.

Here are some materials commonly used:

 

Metals


• Aluminium gutter systems to BS 2997, now obsolescent, ensure minimum product thicknesses – for example, for gutters, cast: 3.2mm, extruded: 1.6mm, heavyweight sheet: 2mm and lightweight sheet 0.9mm.
Where gutters are visible or enhanced performance is required, finishes are applied – typically polyester powder coating or anodisation. The expected life of an aluminium rainwater product is more than 40 years. Coating life is less, depending on colour, environment and aspect.

High thermal movement needs to be catered for in joint design to prevent leakage. Direct contact with copper or cement products should be avoided to prevent accelerated corrosion.

• Cast iron. Modern pipes to BS EN 877 are centrifugally spun and lighter than historic cast-iron. Faster installation is possible with push-fit joints and enhanced corrosion resistance is provided by epoxy coatings. The high cost is offset by resistance to damage and a life of more than 60 years.

• Galvanized steel eaves gutters tend to be found on industrial buildings clad in metal sheeting. The gutters are made from similar materials finishes. Steel, to BS EN 10327 and no less than 1.6mm thick, can be coated in galvanized zinc at a minimum of 275g/m2 or zinc aluminium at 255g/m2. Alternatively it can be hot dipped to BS EN ISO 1461 after fabrication. Plastic coatings include polyvinyl chloride at 100-200 microns, or polyvinylidene fluoride at 25 microns.

Other metal eaves gutter products with expected lives in excess of 60 years include:
• Copper to BS EN 1172 designation Cu-DHP
• Zinc-copper-titanium alloy to BS EN 988
• Stainless steel: austenitic grade 1.4301 to BS EN 10088 (UK type 304).

BS EN 612, the European standard for metal sheet eaves gutters and rainwater downpipes has limited applicability for UK products.

 

Plastics


BS EN 607 is the product standard for all unplasticised polyvinyl chloride eaves gutters. A complete system also requires reference to BS EN 1462 for brackets and BS EN 12200-1 for downpipes.

The current British Standard for half round gutters and round downpipes is scheduled for withdrawal in September 2007.

BS EN 607 provides assurance of performance by means of tests for strength, performance at high temperatures, artificial ageing and water-tightness.

Plastics rainwater systems can perform well but are vulnerable to damage. Colour fades with time, particularly south-facing components, and a common failing is leakage from joints owing to insufficient allowance for expansion or grit damaging the O-ring seal. Short life expectancies of 20-25 years are offset by low capital costs.

 

 

Specification options

 

Eaves gutter and downpipe rainwater drainage systems Capital cost
£/m
Net present value for 60 years
£/m
Expected service life (years)
Cast iron      
Cast iron 100mm half round eaves gutter to BS 460 28 85 >60
Cast iron 63mm round downpipe to BS 460 44 76 >60
Aluminium      
Cast aluminium 100mm half round eaves gutter 30 89 40 - 60
Cast aluminium 63mm round downpipe 36 72 40 - 60
Plastics      
PVC-U 114mm half round eaves gutters to BS EN 607 17 88 20 – 30
PVC-U 68mm round downpipe to BS EN 12200-1 18 65 20 – 30

 

 

Table notes


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

• Linear meter rate based on simple eaves gutter run to include stop ends and outlet, downpipes with shoe and offset bends, all fixings included.

• Life cycle costs include servicing, inspection and allowance for repairs. Redecoration for cast iron and aluminium systems.

• Average service lives used in calculations.

• 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

 

 

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