Before preparing a specification for a roof, establish the employer's requirements or design brief for exactly what type of roof the building requires – such as:
- Single-ply membrane
The design intent will largely dictate the choice of roof system and materials, taking into consideration significant roof factors such as fall, profile and degree of exposure.
After the roof system has been chosen, determine the type of specification required. Generally, the responsibility for the detailed design of the roofing system should be placed with the contractor (or the specialist subcontractor), based upon the following minimum-performance criteria:
- Thermal performance (providing U-values to meet the requirements of the new Part L of the Building Regulations)
- Solar performance (particularly for glazed roofs)
- Acoustic performance
- Fire rating
- Wind-load requirements
- Live and dead loads, and project-specific loads
- Psychrometric data
The above should be collated from information provided by relevant consultants. The contractor should be asked to confirm that all materials proposed, including build-ups and thicknesses, satisfy the specified performance criteria. Alternatively, some roof systems may be specified prescriptively (depending upon the consultant's design responsibility and form of building contract). The preferred route should always be based on performance specification, with a proviso that the contractor provides a single-source warranty/guarantee for the complete roofing system against failure of materials and/or workmanship.
Design and specification should always be prepared with due regard to health and safety, CDM requirements, and guidance provided by specialist and approved bodies, such as the Advisory Committee for Roofwork.
To minimise the risk of falls, risk assessment should be made to determine what constitutes a fragile roofing material. This will require the specification of suitable safety systems such as fall-arrest safety systems or edge guard-rails.
Whenever possible, all roofing materials should have a British Board of Agrément or CEN certification as evidence that the materials have been suitably tested for:
- Fire resistance
- Impact resistance
- Thermal resistance
Written evidence of these should be submitted with the tender. Where this evidence cannot be provided, roofing systems should be independently tested by a competent person or testing authority for suitability and compliance with the specification.
When making specification decisions, be sure to consider the project constraints of budget, form of construction, programme and, most importantly from an architectural perspective, appearance. The quality and workmanship of the intended design should not be compromised. Work should always be carried out by experienced and certified installers of the particular type of roof system being specified. This will prevent disputes at a later date by minimising the risk of failure of materials or bad workmanship.
The specification should include suitable clauses for samples, mock-ups, prototypes, benchmarks and on-site testing. This is particularly important when choosing a system that has a bespoke or particular visual impact. Mock-ups and prototypes should be specified to include all specific interface details including roof-to-cladding junctions, drainage details, penetrations for services, maintenance equipment and supporting structure and fixings.
Finally, the specification of the roofing system should be written to ensure complete protection of the inside of the building from weather.