Now that we understand the different types of cement-based products, we can look at how a common strip foundation is created. Whilst we’ve seen that there are many different types of foundations that are used for a range of scenarios, strip foundations are the most commonly used by far in residential construction. As such, we will take the time to look in depth at how they are poured, and how the different styles of ground floor assemblies are made.  

Having marked out the correct areas for excavation in relation to the drawings, the machinery can begin to excavate the trenches and oversight. A regular 600mm wide strip footing is centred around the location of the intended masonry of a building. The trenches can be marked accurately using layout rods, string lines, ground marking paint, and large folding squares.  

It is typical for the area inside the trenches, known as the oversight, to be reduced in height first before the trenches are excavated to their full depth. The machinery will methodically remove material from the oversight down to the correct height for the buildup of the floor, before proceeding to excavate the trenches. With the height of the finished floor in mind, as well as the specified ground floor build up, we can calculate the appropriate depth for the excavation of the oversight. The layers of the oversight often include some type of compacted aggregate, insulation, concrete, and screed, and so the specific depth can be calculated as such. In the context of carrying out extension works, the total depth of the oversight to the excavated soil should be calculated backwards from the height of the existing finished floor. We will look at the exact layers involved in establishing the oversight shortly.  

With the correct height for the oversight buildup established, the surrounding trenches are excavated to the correct depth to accommodate the poured foundations. Bearing in mind the bottom of the trench needs to reach a minimum of 1m below the common ground, an excavating machine will pull material out of the trench, maintaining the straight lines of the layout markings. Using a 600mm wide bucket, the correct width of trench can be maintained during the entire excavation process. The excavation continues until good ground is reached, at which point, if all of the other requirements are met, an operative can clean out the bottom of the trench using a shovel and begin to prepare for the concrete.  

To indicate the height of the top of the footing during the pour, rebar pegs are driven into ground at regular intervals along the trench. As the concrete is poured into the trench, the tops of the pegs can be used to reference the correct height for the top of the footing. To calculate the correct height of these pegs, we must once again reference the buildup of the oversight. Ideally, for the ease of pouring the concrete layer of the oversight, the internal course of masonry should line up with the height of the concrete. Provided that the laid masonry is level around the perimeter of the oversight, this course can be used to reference the correct height of the concrete. With the floor buildup in mind, we can work backwards from the top of the concrete slab to determine a suitable height for the concrete footing. The minimum thickness for the footing must be 300mm to meet the current building regulations and structural requirements. That being said, we can increase this depth of concrete, or the depth of the trench, to create a suitable height difference between the top of the concrete footing and the top of the concrete pad. Each course of blockwork including a bed of mortar should be 225mm in height. Using this knowledge, we can step backwards in increments of 225mm from the top of the concrete slab height to find a suitable height for the footing. 

The diagram displays a perfect scenario in reference to the above. We can see the different layers that will be constructed as part of the oversight slab, inside the perimeter of the foundation trenches. By stepping down from this, we can locate the perfect height for the concrete. With this height established, we can peg out the bottom of the trench with rebar stakes at regular intervals, using a rod and a laser level to ensure that the tops of the pegs are even around the trench for a flat pour.   

Before the footings are poured, regardless of the style of foundation, the building inspector will want to perform a site visit and observe the ground at the bottom of the trench so as to ensure good ground. Pictures will be taken and added to a portfolio relating to the undertaking, with a written description being added too. Once the trench is pegged out and the inspector has given the go ahead, the footings are ready to be poured.