Materials life costs

How whole-life costing helps business Private finance initiative bidders estimate that construction accounts for only 15% of the cost of a building over 25 years. Little wonder, then, that the construction industry is keen to find out how much buildings cost to run.

In January, Building published the first of a series of whole-life cost models, in which facilities managers provide breakdowns on the cost of running specific building types. To help specifiers and clients further, Building is launching a monthly series of materials life costs for building elements.

The series kicks off with a look at timber windows. Based on information provided by consultant Building Performance Group, materials life costs takes a number of typical softwood and hardwood window specifications and compares initial costs with costs over 60 years. This includes maintenance and replacement.

BPG also offers advice on issues that affect durability, such as the type of preservative or adhesive to use. In addition, the consultant provides information on typical ways that timber windows fail, tips on how to extend durability, and references for further reading.

How materials life costs are calculated

Materials life costs use generic descriptions contained in the Housing Association Property Mutual's Component Life Manual and from component life manuals prepared for the Ministry of Defence.

The generic specifications chosen by BPG are intended to show distinct levels of performance. For instance, a softwood window that is treated with preservative and jointed with waterproof glue will last longer than a window that has not had the same treatment.

Initial costs are based on manufacturers' and suppliers' prices for each building component. The life costs are based on a 60-year life and are discounted to a net present value using the Treasury's standard 6% discount rate. Where relevant, the life costs allow for the removal and replacement of the building element at the end of its life and necessary maintenance over the 60-year period.

Life assessments are based on assumptions about the building element: that it is appropriate for its intended use; properly detailed; and installed and maintained in accordance with recognised good practice. Further guidance on these issues is provided in the Component Life Manual.

Who calculated the materials life costs

BPG compiled the costs exclusively for Building. The consultant specialises in assessing the durability and whole-life performance of buildings. Formed in 1982 by senior staff from the National Building Agency, it employs 60 professionals, including surveyors, architects, auditors and technicians.

Through its 10-year involvement with HAPM's latent defects insurance, BPG has compiled a database of feedback from 100 000 buildings in use. It has also produced a Defects Avoidance Manual published by the Building Research Establishment, and is drafting parts of a forthcoming international standard on service life planning of buildings.

Materials life costs: timber windows

Softwood windows

Redwood double-vacuum impregnated with an organic solvent preservative to British Wood Preserving Association schedule V1 or V2 after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in weather and boil-proof quality adhesive or D4 adhesive to British Standard EN 204 to seal end grain. Timber quality at least class J50 frames, class J40 sashes, class J30 beads.

Heartwood only of untreated softwood of a species designates as SW or SWC (suitable for external use without preservation) in table NA1 of BS EN 942 (such as Douglas fir and western red cedar). Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in WBP quality adhesive or D4 adhesive to B5 EN 204 to seal end grain.

Selected redwood double vacuum impregnated with an organic solvent preservative to BWPA schedule V1 or V2 or selected whitewood/hemlock to V3 after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints coated in moisture-resistant adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Selected redwood treated with a three-minute immersion in an organic solvent preservative after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in moisture-resistant quality adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Redwood or whitewood untreated or whitewood/hemlock immersion-treated. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in moisture-resistant quality adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Hardwood windows

Permeable hardwood (such as opepe, English elm) double-vacuum impregnated with organic solvent preservative to BWPA schedule V1 or V2 or non-permeable (idigbo, keruing, luan, mahogany, meranti, European oak, utile) to schedule V3, after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in weather and boil-proof quality adhesive or D4 adhesive to BS EN 204 to seal end grain. Timber quality at least class J50 frames, class J40 sashes, and class J30 beads.

Heartwood only of untreated hardwood of a species designated as SW or SWC, for example afrormosia, iroko, mahogany (but not Philippine mahogany), European oak, sapele. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in weather and boil-proof quality adhesive or D4 adhesive BS EN 204 to seal end grain.

Permeable hardwood double-vacuum impregnated with organic solvent preservative to BWPA schedule V1 or V2 or non-permeable species to schedule V3, after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in MR adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Non-permeable hardwoods, or "mixed" species, double-vacuum impregnated with an organic solvent preservative to BWPA schedule V1 or V2 after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in moisture-resistant adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Permeable hardwood, treated with a three-minute immersion in an organic solvent preservative after machining. Joints combed or morticed, held with plated/non-ferrous mechanical connection aids. Joints fully coated in moisture-resistant quality adhesive or type D3 adhesive to BS EN 204 to seal end grain.

Whole-life cost over 60 years – what factors are included

Capital cost; installation cost; painting of softwood every five years or staining of hardwood every three years; replacement of ironmongery at 10-year intervals; replacement of weatherstrips at 10-year intervals; replacement of window frame at the end of its service life; costs are discounted at a rate of 6% a year.

Durability tips

• Set windows back from the face of brickwork as far as possible. This will reduce exposure and can dramatically improve durability. Better still, set the window behind the brick outer leaf (regular practice in Scotland). You will, however, need to think carefully about the sill detail to avoid over-wide thin sill sections.

• Specify "run-up" blocks or wedges to take the weight of the sash while in the closed position. These will considerably reduce stress on the sash joints.

Modes of failure Decay With the more general use of preservative treatment of timber for windows, timber decay is no longer the problem it used to be in the 1960s and 1970s. Failure to treat cut ends on site or in the factory can lead to decay in vulnerable areas such as glazing beads and sills.

Joint failure Inadequate adhesives – frequently used, even by leading manufacturers – lead to joints failing, even under usual weather exposure. The problem is exacerbated by the widespread use of timber stains. Joints in opening sashes can be stressed by the omission of setting and location blocks when fitting heavy double-glazed units. The glass in traditional putty glazing used to contribute enormously to the rigidity of the sash.

Factors affecting durability

Adhesives The most durable adhesives designated weather- and boil-proof (BS 1204) or D4 (BS EN 204) are not widely used in timber window manufacture, yet adhesive failure is one of the most common limits on durability. Specify and insist on the use of a durable adhesive.

Exposure Exposure to sunlight and rain will result in cyclical movement and lead to adhesive failure and the breakdown of protective paint coatings.

Preservation Most softwood timber windows are preservative-treated after machining and cutting to length, but this is only the start. All cut ends should be site-treated with preservative. Usual specification is organic solvent to BWPA table 4 type F/N (fungicidal and insecticidal).

Timber species Remember that some hardwoods are non-durable and some softwoods are naturally durable without preservation. Refer to tables NA1 and NA2 of BS EN 942.

Timber quality Quality is measured in terms of knot size and location, straightness of grain, and width of growth rings. Five timber qualities are listed in BS EN 942 Part 1. Classes J40 and J50 are most commonly used in window construction. Class J30 is used for smaller sections such as glazing beads.

Protection Paints offer better protection than stains – this is especially important where less durable (D3) adhesives have been used. All sharp arrises should be pencil-rounded in order to maintain a full thickness paint film.

Maintenance Windows should be repainted at a maximum of five-year intervals and restained at three-year intervals. Glazing compounds, weather stripping and ironmongery may need replacing before the rest of the window as a whole.

Joinery design All horizontal surfaces should be adequately weathered to shed water efficiently. Part one of BS 644 suggests a minimum slope of four degrees.

Further information

Authored by Construction Audit Limited, the technical audit arm of Building Performance Group, the HAPM Component Life Manual provides lifespan assessments of more than 500 building components. It is updated twice a year to reflect industry feedback and changes to standards and codes of practice. Published by E&FN Spon, the manual is available in loose-leaf format, price £175, or on CD-ROM from June 1999.

The BPG Building Fabric Component Life Manual and BPG Building Service Component Life Manual will be available from E&FN Spon, in loose leaf and in CD-ROM formats, from summer 1999.

BS Handbook HB 10141: 1997: Buildings - Service Life Planning: Part 1: General Principles is available from the British Standards Institution on 0181-996 9000.