Some owners and designers might want ground and paving levels that are close to floor levels to avoid having steps between the inside and the outside of a building, but this can have a serious effect on material durability and occupant health.
It’s vital for builders to observe ground clearances – the minimum distances that claddings and floor levels must be from the ground levels outside or under the building – to keep the building materials and interior as dry as possible.
If ground clearances aren’t observed:
- water may splash onto absorbent claddings
- cladding, framing or flooring may absorb water
- paint finishes may fail prematurely when claddings are continuously damp (for example, when soil is in contact with the cladding)
- drainage from E2/AS1 20 mm cavities may become blocked
- drainage slots for veneer cladding may become blocked – water will then build up in the cavity, causing rot, dampness and mould inside the building
- drainage slots for aluminium joinery may become blocked – water will then drain inside, damaging floors and floor coverings
- water may flow inside from paving, paths and gardens that have been built up against the wall cladding to level with or above floor levels
- subfloor ventilation openings in suspended timber floors may become blocked – dampness will then cause premature deterioration of timber framing and flooring materials
- when suspended timber floors are too close to the ground, moisture from the ground may be absorbed by framing, flooring and cladding, causing rot, dampness and mould inside the building
- there may be inadequate space for maintenance access under suspended timber floors
- there may be increased humidity levels inside the building
- surface water may enter the foul water drainage system because the gully trap is installed below ground level.
To avoid these problems, builders should:
- ask for accurate drawings from the designer and follow them – these should show correct site levels (including contours and slopes) and minimum slab to ground clearances
- ensure the set-out on site works from the datum point that the designer has used
- make allowance for future landscaping, particularly where extra soil will be brought on site or where paving will be added
- remove surplus soil from the area immediately around the building
- make sure drainage slots to veneer claddings and aluminium joinery are clear
- ensure that subfloor vents are not obstructed with framing, plaster or paint
- ensure basement walls that are retaining soil are tanked to prevent moisture entry
- advise the owner not to build up soil, mulch, paths, driveways or paving around the base of the building (unless the top surface will be below the bottom edge of the cladding or any subfloor vents or veneer drainage slots)
- advise the owner not to place any planting in front of subfloor vents
- slope the area around the building away from it so that surface water does not drain towards the building.
Minimum permitted ground clearances
The rules for minimum clearances are set out in Building Code Acceptable Solution E1/AS1 and NZS 3604.
|Top of concrete slab on ground – veneer cladding – above paving||100 mm|
|Top of concrete slab on ground – veneer cladding – above soil||150 mm|
|Top of concrete slab on ground – other cladding – above paving||150 mm|
|Top of concrete slab on ground – other cladding – above soil||225 mm|
|Top of timber pile above finished ground level||300 mm or 150 mm with DPC|
|Top of concrete pile above finished ground level||150 mm with DPC|
|Top of foundation wall above finished ground level||225 mm|
|Suspended timber floor construction – bottom of cladding||200 mm|
|Suspended timber floor construction – underside of joists (i.e. crawlspace)||450 mm|
|Bottom of cladding to paving – except masonry veneer||100 mm|
|Bottom of cladding to unpaved ground – except masonry veneer||175 mm|
Getting the floor level wrong can lead to on-going moisture problems. It can also bring serious Building Code compliance problems and potential drainage problems due to an insufficient height of the floor above ground and lack of fall to the drains.
Building Code requirements for floor levels are the absolute minimum – designers can be conservative and provide a bigger difference.
The Building Code states that houses must be built so that a 50-year flood will not enter the building. The achieve this, Acceptable Solution E2/AS1 says that, on near-level sites, the floor level must be no less than 150 mm above the crown of the road or the lowest point of the boundary. For steep sites, specific design and consent as an Alternative Solution is required.
E2/AS1 gives minimum heights of finished floor levels above ground for concrete slab floors and suspended timber floors, the measurements depending on whether there is grass or paving outside.
NZS 3604 sets a minimum height above ground for wood-based products used for flooring. E2/AS1 also gives a minimum threshold height of 100 mm at an opening onto a waterproof deck.
Updated: 9 September 2014