Performance Specification vs Full Design


James Finnie

March 31st, 2020
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You’re a developer building 10-20 residential units per year and are always looking for ways to streamline the building process. For starters, you realise that you need to pay more attention to the specification mix you use on your projects.

Hopefully by considering the following aspects, your decision will be made easier. Each type of spec has its own merits, and they can be used successfully in combination provided that your allocation is appropriate for the end result.

What is a performance specification?

This is a specification where the designer (architect / engineer) sets the criteria for the contractor to deliver a package of work within a set of parameters. This allows the contractor to consider several ways of providing the required package to achieve a cost-effective solution as per the design intent drawings.

For example, a good illustration of where a performance specification is beneficial could be an external aluminium sliding door system with clear glass and aluminium panel infills with an integral sliding louvred screen. Since this sliding door system is made up of several elements, a performance specification is appropriate as it’s unlikely that an off the shelf system would cover the intended design.

In order to achieve the architect’s desired outcome, the specification should identify a minimum U-value, state that it must be waterproof, not rust, not buckle, be wind resistant, prevent internal condensation, drain, allow for tolerance, and meet a minimum deviation for unevenness. It will then be up the contractor to achieve this in the most economical way.

The architect can also add additional criteria to ensure they get the result they are looking for by being more specific, setting rules, and adding compliance with certain regulations.

A typical specification document will comprise of:

  • A general introductory section where standards are set
  • A products section describing the materials to be used
  • The execution, laying out parameters for preparation, testing and installation

What is a prescriptive specification?

Also known as a full specification, it essentially identifies each and every item, right down to the spacing and number of screws required to fix plasterboard for example, as well as the size, type and material of the screw. This gives the client more certainty when the work is tendered and priced by the contractor. There should be at least two or three different manufacturers of each item in the market place, which is where savings in costs can be achieved when contractors source the most competitive price.

Typical examples would be drywall partitions, doors and brickwork.

Drywall systems, for example, can become quite complex with fire-proofing, acoustic, and structural stability affecting their design. Drywalling is a system, consisting of several products, which when arranged in various configurations can suite your specific fire, acoustic, and structural requirements. These are typically summarised in a table that covers most conditions likely to be encountered on a residential project.

So, for a one-hour fire rated wall which requires a 35dB acoustic rating and needs to span up to 3.2m in height, a quick reference to the appropriate tables will provide the required drywall build up as specified by the architect.

Drywall systems could also be tendered using a performance specification, particularly on large complex projects when there are many variations.

How should each one be set up?

There are master specification systems like the NBS (National Building Specification) which are prepopulated with the latest regulation and product information, you choose what your project requires. The NBS has become a bedrock of the profession, endorsed by the industry for years in the UK and overseas. The person preparing them must go carefully through them to ascertain the correct data pertinent to their projects.

Ideally, you should use a master specification system. However, not all architects choose this route as it can be costly, particularly for smaller practices and builders. Large practices may have their own systems, and some even have their own specification writing teams. Certain smaller practices have put together their own set of specifications over the years. There are also independent consultants whom you could appoint to compile your specification for you.

There are useful guidelines for writing specifications, although it can take years of experience to prepare your own with confidence.

Some good rules to follow when preparing a specification document are:

  • Use plain English in short sentences
  • Say what needs to be done • Refer to standards where required
  • Only state information once – this is a golden rule across all contract information
  • Think of this as a complete document, set out neatly and in logical sequence

After the preliminaries and general section, there should be a series of trade sections set roughly in the sequence in which work will occur on site.

Some basic principles to follow are:

  • The specifier is solely concerned with ‘outcome’ … the contractor with ‘process’
  • Start each new project from scratch – don’t copy a spec from a previous project which ‘sort of fits’ your needs. Regulations change and mistakes can happen, so repeated specifications may contain out of date clauses as well as hidden mistakes. A specification is assumed to be up to date, and no matter how good the checking, errors can slip through
  • All too often specifications are started late and there isn’t enough time to give them the attention they deserve, and this results in a compromised contract document
  • Double-checking the specification document is good practice and also ticks your quality assurance box

Your spec should be compiled by someone with intimate knowledge and well versed in detail as well as the latest construction methods and regulations.

What happens if a specification document is poorly administered or created?

The first point is that it’s a contractual document and should get as much attention as the other elements making up the contract. The industry is increasingly realising its importance as a single document containing essential details of the project.

Poor administration could mean issuing the document incomplete or late, so the QS either doesn’t have time to fully co-ordinate or makes the wrong assumptions in the bill of quantities. If the specification is not updated or completed for construction issue, the architect / design team may not have a document which legally forces the contractor to meet the expected standards.

During construction, poor knowledge of what’s contained in the specification could also be an issue, as it’s a powerful tool to hold the contractor to deliver a compliant installation and fulfil their contractual obligation.

With performance specifications there should be a rigorous shop drawing approval system to ensure that everything is checked and signed off during the process, or defective or substandard work could go uncorrected. There are several reasons a contractor may wish to install inferior materials, and once covered up, substandard work could go unnoticed.

If a project is poorly administered, imagine the compound effect if it’s also poorly created. Even using a master specification system, a spec can be poorly created with sections missing or incomplete, or information is inputted incorrectly. It can also sow confusion on the contracting side if it’s not made clear from the beginning what’s required to do the job.

What are the consequences if there are errors?

Many things can go wrong with specification writing. The consequences can be minor if errors are picked up through quality checking and by a QS working closely with the contractor, or pointed out by the contractor. However not all contractors are honest enough to point out errors and may exploit a poor specification to your and the architect’s detriment – and more importantly, that of the client.

The most serious consequence could involve the client suing the design team, as errors on large projects could amount to large sums of money, as well as delays to the project as non-compliant work is removed and replaced as per the contract. Equally there could be multiple errors on smaller projects, also leading to serious consequences.

What are the pros and cons of each?

The benefits of a well-written performance specification are that once you have decided on the required outcome, the onus is on the contractor to produce a compliant proposal. The spec should allow the contractor some latitude to source and propose a cost-effective option. Once contractors are on board and signed up, it should trigger a comprehensive “shop drawing” submission process. The designer ensures that the original design vision is maintained while continuing to fine-tune details.

Detailing large complex buildings has become very demanding, so it makes complete sense to approach known specialist contractors. They have intimate knowledge of detailing and how to source materials. Prescriptive specifications also give clients better cost certainty as materials, products and system costs are generally known.

Elements such as façades can be covered in a prescriptive specification, though a specialist façade engineer would need to be employed to design the system in collaboration with the design team. The benefit is that the end product can be closer controlled and developed earlier in the design process.

On the other hand, using a performance specification and appointing a specialist contractor to prepare a design could save on consulting costs and typically deliver a more cost effective solution.

Are there certain projects where one is more suitable than the other?

Having had experience with both systems, for me they have equal merits, though some projects are better suited to one or the other.

In general, larger more complex building projects are best suited to performance. These would include offices, hospitals, hotels and retail. Where buildings are simpler in nature and less bespoke, such as housing, industrial and storage buildings, a prescriptive specification would fit well.

The majority of projects have a mix of both. Typical performance packages could be glazed and panelized façade systems, lifts, steel frame and heating and ventilation systems.

One particular article stated that where items crucial to the design will be specified prescriptively (such as external cladding), less critical items or items requiring specialist design are specified only by performance (such as service lifts). I am not so sure about this statement, as I feel this is very much project dependent and should be what each client /developer/design team feels comfortable with.

If so, can you give a scenario?

As a developer, you’re looking for increased quality, delivery and profit in each successive development with a view to enhancing your reputation in the market. To date you have been using a set of prescriptive specifications which have worked well and enabled your company to deliver better developments.

However, the last few projects saw no opportunity to increase profit. You are aware of other competitors in the market who use performance specifications which have been reasonably successful. You’re concerned about appointing the correct contractor, concerned that the performance specification won’t be as robust as a prescriptive one, and will the quality match what you’re currently delivering to the market?

I would suggest that with sufficient preparation, you should test the waters and try appointing one or two contractors using a performance specification on your next development. Asking around should quickly provide a short list of potential companies. Discuss pros and cons with other developers if possible. Appoint the contractors earlier in the design stage to ensure that the scope is clearly understood and proposals reviewed and signed off well in advance. Samples, calculations, certificates, mock-up and test mocks could all be undertaken and signed off prior to releasing the full package for production.

Possibly start with two packages which would have less impact on your programme should problems occur with delivery and installation. These could be basement blockwork and internal doors.

Ensure that detailed records are kept during the process so they can be compared with the prescriptive route to establish which is the preferred one. Should this approach be successful, you can build on it for future projects.

Image credit: Smigielski

About James Finnie

James has been based in Cape Town for the last 14 years, trained in Scotland, and has worked with some prodigious practices over the years. He recently started his own architectural practice where the philosophy of Kind Design puts kindness at the centre of the creative process.

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