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Chapter 7.210.1.9 Garage 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:
- holding down
- bracing
- restraint
- detailing at abutments
- movement
- adequate disposal of rainwater.
Garage roofs will be acceptable where they are in accordance with:
- Chapter 7.1 ‘Flat roofs, terraces 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.
Holding down straps should be:
- provided where the roof pitch is below 15° or the roof has a lightweight covering with a laid mass of less than 50 kg/m2
- where local experience suggests there is a risk from wind gusts
- provided in accordance with the design
- a minimum cross-section of 30mm x 2.5mm (galvanised steel straps are generally acceptable) or proprietary straps and fixings
- at least 1m long and have three fixings to the wall, or turned into a bed joint.
Bracing#
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.
Restraint#
For garages of masonry construction, the gable should be restrained at rafter level with restraint straps. Restraint should also be provided at ceiling level where the height (h) exceeds:
- 16 x thickness of the wall for single leaf walls
- 16 x sum of thickness of leaves + 10mm for cavity walls.
Note: ‘h’ should be measured from the top of foundations or from the underside of the floor slab where this provides effective lateral restraint.
The restraint at rafter level may be provided by restraint staps or by a gable ladder.
Where straps are used they should have a minimum cross section of 30mm x 5mm or be proprietary straps that have been designed for lateral restraint purposes. Straps should be fixed to rafters with solid blocking (or be fixed to the longitudinal bracing (see Clause 7.2.8).
For cavity walls the straps should be at 2m maximum centres, with the top strap near the apex of the gable. For solid walls it will normally be necessary to fix the straps to the inside face with at least two 6mm x 30mm plug and screw fixings per strap.
Unless the design states otherwise, the restraint straps should be provided at the following positions:
- walls constructed of solid bricks – two restraint straps per roof slope at maximum 2m centres plus one extra strap near the apex of the gable
- walls construction of perforated bricks/blocks or concrete/aerated concrete blocks – three restraint straps per roof slope at maximum 1.5m centres.
Note: For the purpose of lateral restraint, a ‘perforated’ brick/block is where the perforations (vertical holes) represent more than 25% of the volume of the brick/block.
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#
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, terraces and balconies’
- 7.2 ‘Pitched roofs’.
Last updated: 2nd January 2024