Plans and specifications
A key part of the process of designing and constructing weathertight homes is having a solid base of detailed plans, specifications and documents. They are a means of communicating between parties, and of ensuring that the appropriate designs, materials and construction techniques are used.
You’ll find more information in Bulletin 622 Good plans and specifications. Here, we just focus on some of the elements that relate to weathertightness.
Good documentation can reduce:
- time delays in consent applications
- inaccuracies in prices and quotes
- disputes between the builder and the owner/designer
- the need for extras (the cost of carrying out work not originally included is usually greater than that if it had been there at the start)
- the need for amendments during construction.
Defining plans and specifications
Plans and specifications are defined by the Building Act 2004. They include:
- the drawings, specification and other documents (see below) from which the building is to be constructed, altered, demolished or removed
- the proposed procedures for inspection during construction
- the definition of the intended building use
- details of specified systems and procedures for their inspection and maintenance.
Requirements for plans and specifications
A good set of documents:
- accurately represents the extent and content of the project by defining:
- the scope of work to be done
- the materials and products to be used by product name and manufacturer identification number or reference
- acceptable standards of workmanship
- levels of finish required by the client
- shows sufficient detail so that the main contractor or subtrades do not have to guess what is required
- is presented:
- clearly and concisely
- neatly and legibly
- in a logical sequence
- with consistency between drawings and specifications
- with the drawings of different consultants coordinated to prevent conflicts, ambiguity or contradictions
- with all dimensions shown and drawn to scale
- where required to prove compliance with the Building Code, has supporting information such as producer statements, CodeMark certification, manufacturers’ literature, BRANZ Appraisal certificates and recommendations/drawings from BRANZ publications:
- that are relevant to the project or material being considered
- with the relevant Building Code clauses clearly stated – for example, a producer statement may just cover the durability of a product or may cover the structural adequacy of a proprietary building system.
Producer statements (including structural design certificates, durability assessments, weathertightness opinions, thermal design calculations and specific fire engineering design) must:
- be made by suitably qualified, independent, competent persons – a BCA may decline to accept a producer statement if the credibility of the person supplying it cannot be established, and specific structural design can only be submitted from engineers with relevant experience and skills, such as a Chartered Professional Engineer
- confirm that material quality, design standards or construction standards comply with the Building Code
- confirm design assumptions as the work proceeds, where the work is an alteration.
Common omissions from plans and specifications
Some sets of drawings show the easy, straightforward aspects of construction (which both the designer and builder could reasonably be expected to know) and do not include the complicated or non-standard parts of the building. Where a particular construction detail is required, it must be shown in sufficient detail so that the builder (and, where subject to Building Code compliance, the BCA) can understand what is intended.
Areas where specification and drawing detail are sometimes lacking are:
- appropriate referencing of current standards, and Acceptable Solutions
- roof structure, and roof and deck falls with adequate drainage
- flashing details, particularly:
- at the intersection and ends of flashings
- miscellaneous penetrations in walls and roofs
- cladding junctions and movement control joints
- window/door joinery installation including airseals and sill supports
- provision for service installation and replacement and access for maintenance
- trade literature specific to the materials and construction methods chosen
- details that incorporate allowance for construction tolerances.
A BRANZ study released in 2016 looked at 52 sets of drawings for building projects underway in four locations. While the majority of documents were found to be good or acceptable, over 30% of houses had documents that did not include all the necessary details – some drawings just referred to standards for details – or a list of materials. One in five had details shown that were not applicable to the build, which may be the result of older drawings being reused.
Updated: 4 August 2020