Landscaping and Floor Levels
The Building Act 2004 sets minimum floor level requirements for building work to ensure adequate drainage and sub-floor ventilation, preventing moisture damage to houses and other habitable buildings.
After the final inspection of your building has been made, a building consent officer will issue a Code Compliance Certificate (CCC) if the building meets the requirements of the Building Code. Any subsequent raising of ground levels for gardens, landscaping or paving around the house may result in moisture problems and your home no longer complying with minimum floor level requirements.
What are the minimum floor levels above ground?
A building’s floor must be above the surrounding ground to prevent outside water seeping in under the walls. It is very important to ensure that the building floor level allows sufficient clearance for you to add landscaping, topsoil or paving slabs later on.
|Minimum ground clearance requirements post landscaping|
|House type||Unpaved surfaces||Paved surfaces|
|Timber framed house||225mm||150mm|
|Home with masonry veneer||150mm||100mm|
What are the problems caused by inadequate floor heights?
Low floor levels, or raised ground levels surrounding your house, can result in excess sub-floor moisture. If there are obstructions to the sub-floor ventilation, moisture may eventually cause damage to the floor structure and contribute to:
- Mould or fungi in the ground that is hazardous to human health.
- Visible signs of dampness such as water droplets on sub-floor timber, particularly on the colder, southern side of the house.
- Decay or borer attack of sub-floor timber.
- Water stains on the timber.
What can I do to reduce sub-floor moisture?
Sub-floor ventilation depends on openings at regular intervals around the entire building perimeter. It is recommended that:
- Sub-floor vents be cleared of all obstructions, or additional vents be added to compensate.
- The ground area under the house be covered with a polythene sheet to stop moisture rising.
- Stored items under the house be removed to create a clear air flow.
- Surface water be intercepted with channels or sub-surface drainage into a stormwater system.
- Ground levels around the building be altered so they are lower than the vents.
Reduce run-off from your property
Aim for a surface with low percentage run-off or high soakage. The table below will help you make an informed decision.
|Surface type||% run-off|
|Sandy and volcanic soil with bush cover||15%|
|Gardens and lawns||25%|
|Heavy clay soil type with bush cover||35%|
|Heavy clay soil type with grass cover||40%|
|Unsealed roads e.g. loose stone surface||50%|
|Stone, brick and paving panels with open joints||60%|
|Stone, brick and paving panels with closed joints||80%|
|Asphalt and concrete||85%|
Landscaping your property with stormwater management in mind will help prevent flooding and pollution.
If you use a high percentage run-off surface (such as concrete), consider how you can capture that water within your property. A soakhole will usually accommodate 50-60sq.m catchment of hard surface.
Gardens and porous paving with a free draining sub-base, as opposed to concrete and hard paved surfaces, will allow water to be absorbed by the ground. This reduces run off and filters out contaminants before stormwater finds its way to our streams and beaches. Heavy clay sub-grade layers act as a water barrier and can prevent the proper function of a permeable surface treatment.
- Planting gardens and trees can reduce flooding and pollution of our beaches and streams significantly as well as enhancing the visual character of your property.
- Avoid planting vegetation near manholes or over drains on your property.
- Maintain higher floor levels. Once ground levels have been raised with extensive landscaping, poor drainage can be a no-fix or very expensive problem.
- Keep under-floor vents clear from topsoil, shrubs, plaster and concrete paths.
- Do not allow wall cladding to come into contact with the ground as this will allow water to soak up into the cladding on the inside of the building.
- Make provisions for drainage when concreting or paving. Sloping paths away from the house will direct excess surface water away from the building. Concrete on the ground adjoining the building should fall away from the foundations for at least 1m, at a minimum slope of 1:25.
- Stormwater must be drained to a stormwater drain or appropriate soak hole.
- Ensure a secondary flow path exists to provide an alternative in the event of a drain blockage.
- Make sure you are not directing a flow path onto a neighbouring property.