Slab foundations and their pros and cons explained
A slab foundation is the fastest foundation to install on flat surfaces. In New Zealand, concrete slab floors became more common in the late 1970s (3). The reason for the slabs becoming more popular is that they were cheaper on flat sites, lasted longer and were safe against fires and draughts (3).
As the slab foundation cannot easily be removed, electrical services and water pipes going from the house down into the soil need to be installed before the foundation is poured. The slab material is most of the time concrete, but can also be fly ash concrete or limecrete.
Installation of a slab foundation:
In the slab foundation, first, the topsoil is removed. Then a beam that is approximately 60cm deep is dug around the outer edges of the slab. A 12-15cm layer of gravel is added onto the slab soil. Add a layer of sand on top of the gravel and a thin, often plastic membrane on top of the sand to ensure that moisture stays out. Steel and wire mesh is added on top of the membrane to make the foundation stronger. In most cases, a concrete layer of 12-15 cm is then added as the slab foundation.
Important to note is that in New Zealand it is as of the beginning of 2020 not required to have a fully insulated slab on the side. We recommend that if you use a concrete slab, to insulate the sides. The insulation will reduce your heat loss through the floor.
Type of Slab materials:
Concrete is the most common slab material. However, due to the high release of CO2 into the atmosphere during the manufacture of a key ingredient- cement, it is not very popular with many people wanting a more environmental friendly product. Cement accounts for about 5% of global CO2 emissions (7).
What is fly ash? Fly ash is a by-product of burning coal for electricity. Fly ash is not a replacement of cement but is used as an additive. By adding fly ash into the concrete mix, less cement is needed. It reduces the amount of CO2 put into the atmosphere by the concrete manufacture (saving depends on the percentage of fly ash added) (4).
Limecrete or lime concrete has instead of cement, lime as the ingredient. This has the immediate environmental benefit as lime doesn't require a temperature as high as cement to be made into limecrete, saving CO2 emissions. Further, lime carbonates and absorbs its own weight in CO2 emissions in time while helping other building materials to regulate moisture throughout the building (5).
Advantages of a slab foundation:
A slab foundation has a very long lifespan
Faster to construct
More energy efficient as no cold comes through the bottom of the floor if properly insulated
A concrete slab adds a lot of thermal mass to your house; depending on what floor covering you chose, this can help you save on heating costs
Concrete can be recycled but is currently often not recycled in NZ
Disadvantages of a slab foundation:
Tree roots can damage the foundation
Pipes and wiring are difficult to access
Can crack with shifting soil- more prone to damage in earthquakes
After the building is removed the soil can only recover if the foundation has been taken out
The way we build houses evolves over time and changes in the foundation are more difficult to carry out
As the concrete dries out cracks can form in the foundation (6)
Environmentally slab foundations are more damaging to the site as even if the foundation does get removed it takes considerable effort and energy to do so.
(1) Computers and Geotechnics, Volume 55, January 2014, Pages 172 - 186; Authors: Aslan S. Hokmabadi, Behzad Fatahi, Bijan Samali, 'Assessment of soil–pile–structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations'
(2)' Foundation Engineering', Publisher: Wiley, 1974, Chapter 12, Page 203, Authors: Ralph B. Peck, Walter E. Hanson, Thomas H. Thornburn
(3) 'Concrete slab floor construction', renovate BRANZ,https://www.renovate.org.nz/1970s/foundations-and-subfloors/foundations-original-details/concrete-slab-floor-construction/ , Reviewed November 2019
(4) Cement and Concrete Research, Volume 124, October 2019, Author: Zbigniew Giergiczny, 'Fly ash and slag'
(5) Wikipedia.org, 'Types of concrete', 2nd November 2019
(6) BRANZ/ Ministry of Business, Innovation and Employment, Study Report Revisiting concrete ground floor slabs, Author: R. H. Shelton, 2015
(7) Columbia University, Earth Institute, 'Emissions from the Cement Industry', Author: Madeleine Rubenstein, 09.05.2012