Specification is mainly about the quality of building work, both in terms of materials and workmanship. This is a very important subject because in the building industry there is such a huge range of material choices and standards of workmanship can vary enormously. A specification is a document which pins down these variables as far as is reasonable. It is then sent out with the drawings and other documents so that builders can tender accurately for the job. During the progress of the building work it can be referred to in order to check whether standards are being met.
prescriptive and/or performance
There are fundamentally two types of specification, prescriptive and performance based. The former tells the builder exactly which materials and techniques to use; the latter leaves it up to the builder to achieve a certain standard of performance any way they like (within the limits set by the drawings and schedule of work).
Which is best?
Well this takes a degree of judgement and there is often a sliding scale. At one end of the scale is something very specific you might want to prescribe. For instance you might want a clause which goes ” All kitchen worktops to be solid sweet chestnut with.. …. ….”. At the other end of the scale you might not care how the drains are constructed as long as they conform to building regulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations approved standards, so you would have a standard clause to that effect. In this way you can allow the builder to use methods and materials they are accustomed to for drains whereas you still get exactly the worktop you want.
Prescriptive standards ensure a more predictable result but take a lot more time for the designer to produce. They also tie builders down in several ways:
- they may have to use materials and methods which they are not used to working with. They will probably increase their pricing to reflect this.
- they may have to order hard-to-find materials when they could use a convenient local equivalent. Ditto on price.
- they may have to deal with a range of suppliers where they have no discounts set up. Ditto on price.
- they have less creative input. Often builders spot better ways of doing things than a designer behind a desk will, particularly on smaller renovation and extension jobs where the on-site situation is less predictable.
This all adds up to higher prices and a better job.
Performance standards allow builders to use their initiative, experience and existing skill set and utilise their established trade contacts to the fullest. However they do have to do more of the design work to ensure they meet the performance and this extra cost will partly offset the other gains. You still need to do some prescriptive specifying. (It’s no good writing a performance clause which goes “Bathroom tile work to be nice and bright and happy looking…….”. You do need to specify the tiles). There are a couple of other potential problems:
- a builder may be tempted to do everything the cheapest way they can providing it achieves the basic standard and this may not always be to your advantage.
- it may be difficult to establish whether the quality of the work does actually meet the performance standard specified
a greener spec
Specification has always been integral to a building contract but aspects of ‘green specification’ have been developing rapidly on several fronts:
- the sustainability of building materials. See the section on Building Materials. For instance timber specification now often includes the sourcing of the timber
- the embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy of materials and products including the extraction, processing and transport elements.
- new greener technologies coming on the market. E.g. high efficiency boilers, heat pumps, controls etc. are improving rapidly
- the health aspects of building materials and construction techniques. For instance this can be seen in the move away from petroleum based paint solvents to water based/natural oils etc.
- working practices such as the quality of building regarding air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard, building waste management etc.
You can buy / download standard specifications which cover the full scope of a building contract and then tailor them to your own needs by adding or striking out clauses.
One source of such a standard document is the NBSNational Building Specification. This is a company specializing in specification writing. It's owned by the RIBA Domestic specification which can be purchased on line for a little over £50. It is designed specially for small domestic jobs.
Greenspec is an excellent web site which many building professionals turn to when they are ‘greening up’ their specification writing.
With Uniclass, just go for the search box and start typing words you think might be applicable. A set of filters will guide you to the various categories
Whichever approach you choose you may want to build up check lists of ‘everything’ to make sure you don’t miss anything out. A useful (but dauntingly large) tool for assembling check lists is Uniclass where you can search for practically anything to do with buildings and activities. It can be very useful if you don’t know the proper description for something. The full (enormous) list is available here.
A useful document is the Technical Manual produced by Premier
There is more information on all this at Construction Project Information. Although this web site is aimed at the building industry as a whole there is considerable information useful to the self builder.
A large data base of building materials and suppliers is available at SpecifiedBy