7.2 Pitched roofs

Also see:

3.3

7.2.25 Roof cassette systems

Roof cassette systems shall form the roof structure and habitable space beneath and safely transmit loads to the supporting structure without undue movement or deformation. Issues to be taken into account include:

  1. provision of information
  2. structural performance
  3. thermal and acoustic performance
  4. fire 
  5. condensation and ventilation
  6. moisture protection
  7. durability
  8. installation.

Roof cassette systems are building systems consisting of prefabricated roof panels which may also be supplied with prefabricated wall panels, beams and other supporting structure. Systems can be open or closed panel.

Provision of information#

Clear and fully detailed drawings should be available on site to enable work to be carried out in accordance with the design. Design and specification information should be issued to site supervisors, relevant specialist subcontractors and other appropriate personnel, and include the following:

  • a full set of drawings
  • material specifications
  • fixing schedules
  • junction details eg steps and staggers in both the horizontal and vertical plane
  • the position and material specification for cavity barriers and fire-stopping
  • manufacturer’s requirements relating to ancillary items eg chimneys, dormers, etc
  • site installation manual.

Structural performance#

The structure of the roof cassette system should be adequately designed to support dead, imposed and wind loads in accordance with the requirements of Clause 7.2.3.

Compound deflection of the roof cassette system should be designed to acceptable limits, particularly where they are supported by roof purlins.

Thermal and acoustic performance#

The roof and walls of roof cassette systems, shall be insulated in accordance with the Building Regulations.

The roof and walls to roof cassette systems, shall be constructed to ensure that sound transmission is adequately limited between homes, in accordance with the Building Regulations.

Fire#

The roof cassette system should meet the fire performance requirements of the Building Regulations, including all critical junctions eg where walls and roof cassettes intersect.

The roof cassette system should:

  • have adequate structural fire protection
  • provide adequate compartmentation between dwellings
  • have cavity barriers and fire-stopping in accordance with the Building Regulations.

Condensation and ventilation#

The elements of the roof cassette system shall adequately limit the risk of interstitial condensation and be constructed in accordance with BS 5250. Air and vapour control layers should be provided in accordance with manufacturer's recommendations.

Rooms formed by the roof cassette system shall be adequately ventilated in accordance with the Building Regulations.

Moisture protection#

Walls and roofs should be adequately protected from moisture, using appropriate breather membranes and roofing underlays.

Wall and roof membranes shall form a continuous barrier to moisture and be adequately lapped.

Roof coverings should be in accordance with this Chapter.

Wall cladding should be in accordance with the relevant NHBC Chapter(s).

Durability#

The structure of the system shall have a life expectancy of at least 60 years. Timber members should be preservative treated or have adequate natural durability in accordance with Chapter 3.3 ‘Timber Preservation (natural solid timber)’.

Installation#

The system shall be erected in accordance with the manufacturer’s recommendations and provide satisfactory performance. Issues to be taken into account include:

  • competence of installers
  • preparation 
  • design tolerances
  • structural connections
  • sealing of joints.

The manufacturer should provide and make available on site a set of clear instructions in the form of a site installation manual and relevant accompanying drawings, detailing the assembly and installation for the system as appropriate.

Systems should be installed by operatives who:

  • are competent
  • are familiar with the system being installed and
  • hold a certificate (or similar confirmation) confirming that they have been trained by the system manufacturer.

Ancillary components

Ancillary components ie, dormers, chimneys, roof windows, should be capable of integrating with the roof cassette system to ensure structural integrity and weathertightness is maintained.

Last updated: 2nd January 2024

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