10.1.9Garage roofs

Garage roofs shall satisfactorily resist the passage of rain and snow to the inside of the building, support applied loads and self-weight, and transmit the loads to the walls safely and without undue movement. Issues to be taken into account include:

  1. holding down
  2. bracing
  3. detailing at abutments
  4. movement
  5. adequate disposal of rainwater.

Garage roofs will be acceptable where they are in accordance with:

  • Chapter 7.1 ‘Flat roofs and balconies’, or
  • Chapter 7.2 ‘Pitched roofs’.

Holding down

To prevent uplift, flat roofs and, where necessary, pitched roofs should be provided with holding-down straps at not more than 2m centres where the roof members bear on the supporting wall. Straps should have a minimum cross-section of 30mm x 2.5mm, be at least 1m long and have three fixings to the wall.


The building designer should specify all bracing. Trussed rafter roofs should be braced in accordance with Chapter 7.2 ‘Pitched roofs’, unless the roof is designed and braced in accordance with PD 6693-1.

All timber bracing to trussed rafters should be at least 100mm x 25mm in section and nailed twice to each trussed rafter. Nailing should be 3.35mm (10 gauge) x 65mm long galvanized round wire nails.

Detailing at abutments

Precautions should be taken at abutments between a garage roof and the main building or between stepped garages, including:

  • flashings and weatherproofing that allow for differential movement
  • cover flashings formed from metal or other approved material
  • cavity trays that divert water from inside the cavity to the external surface of the roof.


Movement joints in foundations and the structure should be continued through roof coverings and be provided with appropriate weather protection.

Adequate disposal of rainwater

The provision of rainwater should be in accordance with building regulations.

Individual roofs, or combinations of roofs that drain from one to another, with a total area greater than 6m², should have a rainwater drainage system.

Where rainwater from a large roof surface discharges onto a garage roof, precautions should be taken to prevent premature erosion of the lower surface.

Rainwater should not discharge from the roof directly to a drive or path.

For details on the design of rainwater disposal systems, reference should be made to the following chapters, as appropriate:

  • 7.1 ‘Flat roofs and balconies’
  • 7.2 ‘Pitched roofs’.