10.1.3Garage foundations

Garage foundations shall transmit all loads to the ground safely and without undue movement. Issues to be taken into account include:

  1. hazardous ground
  2. type of foundation required for integral/attached garages
  3. type of foundation required for detached garages and blocks of garages
  4. adjacent structures
  5. underground services
  6. provision for movement.

Garage foundations should adequately support the imposed loads, taking account of ground conditions.

Further guidance is given in Chapter 4.3 ‘Strip and trench fill foundations’.

Hazardous ground

For foundations on hazardous ground, the following chapters are relevant:

  • 4.1 ‘Land quality – managing ground conditions’.
  • 4.2 ‘Building near trees’.
  • 4.4 ‘Raft, pile, pier and beam foundations’.

Any existing fill on the site of the garage should be examined and identified. Where any potential health hazard or risk of damage is indicated, appropriate precautions should be taken, as described in the following chapters:

  • 4.1 ‘Land quality – managing ground conditions’.
  • 5.1 ‘Substructure and ground-bearing floors’.

Type of foundation required for integral/attached garages

Foundations for integral or attached garages should be the same as those for the home, unless proper consideration is given to each foundation, and the possibility of differential movement between them.

Type of foundation required for detached garages and blocks of garages

Foundations for detached individual garages or blocks of garages should avoid damage caused by differential loads and uneven settlement.

Where the ground is uniform and provides a satisfactory foundation bearing, an unreinforced edge thickened concrete slab may be used.

Unreinforced concrete slabs should:

  • have a minimum thickness of 100mm
  • have a minimum downstand thickening of 350mm below ground level around the whole perimeter of the slab
  • have a minimum width of edge thickening of 300mm
  • be constructed on 100mm minimum of properly compacted hardcore
  • have dimensions not exceeding 6m in any direction – for dimensions greater than this, movement joints should be provided.

Adjacent structures

Foundations for garages should not impair the stability of the home or any other adjacent structure.

Underground services

Garage foundations that are to be above or near services should be constructed so that no excessive settlement of foundations or damage to services occurs (see Chapter 5.3 ‘Drainage below ground’).

Provision for movement

Movement joints in foundations should be provided:

  • between homes and attached garages where there is a change of foundation type or depth
  • at 6m intervals where unreinforced concrete slab foundations are used.