Building a home is a huge project for anyone. For many, it’s the biggest project they’ll ever carry out. Starting with the idea to planning, construction and moving into your brand new home, every little detail is important.
If you’re still somewhere between the idea and planning, you need to get started with the basics. The first and the most basic element of any building is the house slab types .
Slabs are mass concrete structures that are used to create flat surfaces such as floors, ceilings or roofing decks. Slabs are reinforced with steel rods and mesh and can be supported by the ground, beams, columns or walls. The thickness of slabs depends on the application but most house slabs founded on natural ground are 100mm thick.
Deciding on the right house slab type
You have to make many decisions when you’re about to build a new home. How many stories is it going to be? Will it be a acreage home or a modern concrete and glass architecture? Who is going to build it? What about the exterior and interior?
Choosing the house slab is not going to be one of those many decisions. An engineer will evaluate the site and soil conditions, slope of the block and specify the slab required for the building.
The site and soil conditions are evaluated by a Geotechnical Engineer with their findings passed onto a Structural Engineer. The type of soil on your land, slope and presence of trees are the main factors in determining which house slab types are suitable for your project.
Some house slab types are stronger than others and are therefore more appropriate for reactive soils. If you want to understand the process of choosing the right house slab, then the preceding text has set just the right foundation for it.
In Australia, soils are classified according to the Australian Standard AS 2870/2011, Residential slabs and footings. The different soil types are based on their reactivity and range from Class A for non-reactive soils to Class P for problem sites.
Geelong and Torquay are both coastal and rural areas so the soil type in the region varies considerably. This means an engineer has to come to the site, evaluate the soil and design the slab most suitable for the soil type.
Soil can react in all sorts of ways to a structure being built upon it. In this case, saying “reactive” refers to how the soil reacts to changes in moisture levels.
Clay-based soils are prone to changing their volume based on the amount of moisture reaching the soil. Because of the reaction with the moisture, the soil is called reactive.
As a rule, too much moisture causes the soil to expand. Too little moisture, for example after a long dry period, causes the soil to compact, which may result in the soil withdrawing from the footing system.
Some soils have more potential to change their volume. The changes are measured in millimetres and the result determines their class according to AS 2870/2011. By assigning the proper class to the soil, the engineer can plan the foundations and slab accordingly to avoid issues with shrinkage or heaving.
Abnormal moisture conditions
In some conditions, having the right type of house slab may not be sufficient to protect you from the reactions of the soil. These abnormal conditions cause moisture changes and may lead to serious and very expensive damage to the building. Most of these conditions can be prevented if you know what to look out for.
Prolonged dry periods
As mentioned, when the ground dries out for a long period, the soil shrinks and may pull away from the footing system. This issue is easily preventable by moisturising the soil around your house during droughts.
Prolonged wet periods
Long wet periods may also make the soil under your house react to the moisture. To prevent water from building up and drenching the ground, make sure to incorporate good drainage and invest in a reliable stormwater system.
Landscaping may sometimes also cause issues with houses built over reactive soils, seeing how different plants require a different level of moisture. While your lawn requires extensive daily watering, the shrubs on the other side of the house may only need to be watered once a week.
Uneven watering can lead to an uneven reaction in the soil which may result in soil changes and costly damages. To avoid potential issues, make sure to keep an eye on your whole garden and pay attention to the watering.
Sewage and stormwater pipes are now required by law to use flexible mechanical joints for ease of maintenance. This is enforced to help prevent structural damage related to leaking pipes near reactive soils.
When the pipes are located under the house, leaks may create a wet spot that can damage the house’s slab and walls within as little as two weeks. To prevent such potential issues, repair any leaking pipes as soon as you notice them.
Air conditioners and hot water systems
Air conditioners and hot water systems tend to create wet spots and lift the ground, producing the same effect as sewage and stormwater pipe leaks. The risk is easily avoided by directing the flow into the stormwater system.
Soil types in Geelong & Torquay
Every soil of the world belongs to a certain soil type – Wikipedia
Two soil types are not found in Geelong and Torquay. The first one is class A, consisting of mostly rocks and sand that have little to no ground movement due to moisture changes. The other one is Class E, meaning that the site is extremely reactive, which may result in extreme ground movements of over 75mm.
The other four soil types are found in the area, so we’ll look at them in more detail.
Class S soils
Class S stands for a slightly reactive or stable clay site. Such sites are prone to experiencing minor ground movements due to moisture changes. Soils whose movement varies between 0mm and 20mm belong to this class.
Most Class S sites only require a basic slab with concrete beam footings at the edges of the slab.
Class M soils
Class M means that the soil found on your clay or silt site is moderately reactive. The movement expected from this type of soil is 20-40mm.
Class M sites require a stiffened slab with cross beams in the slab. The stiffening prevents the slab from twisting and bending as the clay soil moves.
Class H soils
H stands for highly reactive soil. Highly reactive soils are differentiated into two separate classes. Class H1 defines soils with high ground movement, measured from 40mm to 60mm.
Class H2 corresponds to clay soils with very high ground movement of 60-75mm.
Any construction projects over Class H and more reactive soils need to be done in tandem with a structural engineer. The engineer will assess the risks and give recommendations on how to safely continue with the construction.
Most commonly, this includes adding concrete piers under the footing beams on the slab. The piers may need to go so deep into the ground that they reach soils less prone to moisture movements. It’s worth mentioning here that the movements tend to be lower in intensity for soils deeper into the ground.
Class P soils
The Class P soil is the one you don’t want on your land. Class P stands for problem sites. Class P sites are expected to have severe ground movements due to moisture changes. If you intend to build over them, you should consult a structural engineer.
Such soils cannot bear the load evenly and are usually the result of abnormal moisture conditions caused by dams, poor drainage and even trees.
Sites that are usually rated as Class P include:
- cut and/or filled sites
- soft soils with a lack of proper bearing, including loose sands or soft clay and silt
- mine subsidence
- landslips and collapsing soils
- soil that has suffered erosion
- reactive sites with unusual moisture conditions
- sites that don’t fit into other classes
Class P soil types don’t have a defined range of movement; they can sit perfectly still but be a problem for structural integrity of the building on top of it.
House slab types
Now that you’re acquainted with the different types of soil and how the environment affects them, it’s time to look at the house slab types that could be built into your house.
Among the most popular house slab types is the waffle slab. It is normally used in mass-produced homes where the costs are kept to a minimum and the soil is classified as Class A or S, and in some cases can be used on sites with Class M.
Just like reactive soils, waffle slabs in Australia are regulated by the AS 2870/2011.
In general, the waffle slab is a system of reinforced concrete footing and a slab. It is constructed on the land and sits about 400mm above the surrounding ground level when finished. When the construction work on the house is done, the builders usually raise the surrounding ground to hide the slab partially or completely.
The slab consists of perimeter footing, also known as edge beam, and multiple strip footings, or internal beams, that are situated one metre away from each other. The voids are filled with polystyrene. When looked at from above, the slab looks like a waffle, which is how it got its name.
Waffle slabs are not recommended on sites where the clay soil is classified as Class H or higher because of the limited drainage possibilities. Another factor that affects whether waffle slabs are suitable for your project is the site terrain. Waffle slabs can be used on mostly flat sites, sites with natural soils or sites with a controlled fill with good surface strength.
The force with which the waffle slab resists moisture-related ground movements depends on its stiffness. If the slab is raised higher over the ground then the beams are positioned deeper into it. The deeper the beams go into the ground, the more stiffness the system offers, hence the better protection.
Waffle slabs have their advocates and their naysayers. You might have heard stories about people who jacked up their car in the garage and the jack broke into the slab or where rats, toads, or snakes sheltered under the concrete.
Unfortunately, such failures do happen, but only when the slab was built by careless contractors who refuse to follow the engineering guidelines. Taking shortcuts to cut on building costs isn’t worth it and most often results in expensive repairs later on. What’s more, once the slab has been poured, there is no way for the customer to know what size the rebar is or how thick the slab really is.
Waffle slabs are not recommended for cyclonic areas because the system is rather light. It may not be able to resist the force of a cyclone that might lift the slab and crack the walls.
To avoid such issues, hire only reputable contractors with experience, that value quality and are willing to follow the engineering guidelines.
Raft slabs, also known as mat slabs, are another popular option for your foundation system. The raft slab can take a very heavy load so it’s usually constructed when building commercial buildings and large family homes.
This type of slab is most suitable when you need to cover a large area over soil with low-bearing capability. Still, raft slabs are also a good option for sites with normal soil, in which case it is usually more cost effective than stumps or strip footings, easy to install and does not require too much digging.
Raft slab designs are also regulated by the AS 2870/2011. This type represents a reinforced concrete slab with integrated concrete beams positioned in both directions. The minimum thickness of the slab is 100mm and could vary up to 150mm when a heavier load is expected or the top slab is suspended over fill.
The depth of the strip footings is determined by the soil type, so a structural engineer has to be involved.
Constructing a raft slab usually involves several stages:
- The outer boundaries of the building are marked.
- The edge beams and internal stiffening beams are dug into the ground at the specified depth leaving pads in between. Unless the soil requires otherwise, beams are usually 300mm thick
- Sand is compacted on the pads – and damp-proof membranes are laid on the beams and pads.
- The steel reinforcement of the edge beams is designed by the structural engineer based on the soil type. The reinforcement is held away from the ground using plastic supporters to avoid potential rust issues.
- The top slab is reinforced using steel rods and mesh. The reinforcements again need to be set higher up so they don’t touch the damp proof membrane.
- The concrete for the edge beams is poured into the reinforcement up to the ground level.
- The concrete for the top slab is poured, smoothed and levelled. It is recommended the slab be left to properly cure over the course of at least 5 days to have the bearing capacity to build on.
Bondek is a trademarked steel formwork that is used mainly when you want to reduce the slab thickness.
In essence, Bondek is a metal sheet that can be laid as formwork or as a structural component. The concrete is poured over the sheet and hence the sheet is not removed. It acts as a permanent formwork and remains in the slab for the entire lifetime of the building.
Bondek sheets are considered to be very strong and are able to carry heavy loads of concrete. They are quick and easy to install and, because of the thin nature of the slab, they provide savings on both the reinforcement and concrete costs.
Another benefit of the Bondek sheets is that they are a safe work platform because of the slip-free embossments and ribs that are 200mm apart.
The Bondek formwork is usually laid over prepared propping that is later removed once the concrete has set. The propping is prepared from steel bearers held by vertical props or from solid timber. The propping should be strong enough to hold both the steel sheets and the wet concrete.
Not all house slab types require a formwork. Infill slab is a type of slab where the concrete is poured between already existing walls. Infill slabs are commonly used between masonry walls that have been built on a footing, for example in a garage.
It is important to note that the concrete must not be in direct contact with the walls because concrete and masonry expand and shrink at different rates. A 12mm impermeable separation strip is used between the slab and the walls. Failing to separate them poses a potential risk for developing cracks in the slab along the wall edges.
Infill slabs are also successfully combined with an underfloor heating system, adding to the energy efficiency of your home. Infill slabs can also be smoothed and/or polished, leaving you with a reliable and good-looking floor.
Suspended slabs are amongst house slab types however used mainly for commercial purposes.
As the name suggests, the suspended slabs are not in direct contact with the ground. Instead, they are used for floors or roofs above ground level.
There are two main types of suspended slabs:
- One way slabs are supported only on two sides in the direction of the shortest side. This type of suspended slab is suitable when the long to short side ratio is 2:1 or higher.
- Two way slabs are supported on all four sides. They are suitable when the side length ratio is about 1:1.
Suspended slabs may be prefabricated in a factory where they are cast and transported to the site. Alternatively, they could also be cast on the site by pouring the wet concrete over a framework. In that case, rebars may be set within the framework to reinforce the slabs.
Trust Pivot Homes For All Your Custom Home Building Needs
Deciding between the different house slab types for your new build isn’t easy. If you’re looking for a professional team with years of experience building in Geelong and Torquay, contact Pivot Homes today.