Building on a Sloping Site
Building on a sloping building site can be fraught with challenges. It is always more expensive than building on a dead flat site and so it pays to take an ‘eyes wide open' approach and learn as much as you can as early as you can to find the most appropriate solution.
If you are considering or have already purchased a lifestyle section on which you'll build your dream home then there's a fairly strong possibility that the building site won't be dead flat.
Option 1: Building 'Into the Slope'
Where the site is only gently sloping the most prudent and cost effective approach is often to build ‘into the slope'. This involves excavating or ‘cutting' into the hillside to create a flat building platform on which a concrete foundation and slab floor can be designed. Depending on the amount of slope involved it is possible that a retaining wall will need to be constructed to support the ground on the high side above the finished floor level of the building.
Note: The retaining wall will need to be designed by a qualified Engineer, be clearly detailed in the building consent application and be inspected by Council during construction or certified with a Producer Statement from a qualified Engineer.
One of the considerations in this approach is that you will have to dispose of the spoil that is ‘cut' from the hillside. A small amount can often be stockpiled or retained on the site however if the ‘cut' is more significant a Resource Consent is required and this may involve trucking the spoil off site to a permitted dumping site which can be expensive.
Option 2: Building 'Out of the Slope'
It is difficult to be specific because there are many variables but as the steepness of a site increases there generally comes a point where it is more cost effective to consider a timber foundation and floor system and build ‘out of the slope' rather than excavate into the hillside. The advantage with this method is that much less soil is disturbed on the site compared to the cutting in method described above. Note that there is still a requirement to remove vegetative matter from under the building which basically involves scraping off and removing the topsoil. Using this approach the timber floor is supported by timber piles or poles and is specifically designed by an Engineer who will take into account the soil structure described in your Geo-technical report.
Poles can be ‘driven' (or ‘rammed') into the ground or set into drilled holes and surrounded by concrete. The advantage of driven poles is that there is no spoil to be removed as with drilling holes. The decision to drive poles or drill holes requires a discussion with the Engineer who will consider all the variables such as the soil structure and accessibility.
Depending on how high the poles are out of the ground (ie the steepness of the site) there may be requirement for cross bracing between the poles under the floor. Once again this is a question for the Engineer in the conceptual stage of your project.
Option 3: A Combination of the Two
Another option is to use a combination of the above two approaches. For example you could cut into the slope to create a flat building platform for a concrete floor for your garage and then have the rest of the building designed on piles or poles for a timber floor.
This discussion has just scratched the surface at a basic level. There are many other factors to consider which could have an impact on your choice of design. One of the best things you can do is to talk with a builder who has plenty of experience in designing for and building on sloping sites – and do it as early in your project as you can!
Neville Dennis, Manager
Econobuilt - Lifestyle Homes